patch, 4 partsSideband the whole fetch v2 response

pkt-line.c, pkt-line.h, upload-pack.c

diff --git a/pkt-line.c b/pkt-line.c
index e70ea6d88f..9d3e402d41 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -129,12 +129,14 @@ static void set_packet_header(char *buf, const int size)
 	#undef hex
 }
 
-static void format_packet(struct strbuf *out, const char *fmt, va_list args)
+static void format_packet(struct strbuf *out, const char *prefix,
+			  const char *fmt, va_list args)
 {
 	size_t orig_len, n;
 
 	orig_len = out->len;
 	strbuf_addstr(out, "0000");
+	strbuf_addstr(out, prefix);
 	strbuf_vaddf(out, fmt, args);
 	n = out->len - orig_len;
 
@@ -145,13 +147,13 @@ static void format_packet(struct strbuf *out, const char *fmt, va_list args)
 	packet_trace(out->buf + orig_len + 4, n - 4, 1);
 }
 
-static int packet_write_fmt_1(int fd, int gently,
+static int packet_write_fmt_1(int fd, int gently, const char *prefix,
 			      const char *fmt, va_list args)
 {
 	static struct strbuf buf = STRBUF_INIT;
 
 	strbuf_reset(&buf);
-	format_packet(&buf, fmt, args);
+	format_packet(&buf, prefix, fmt, args);
 	if (write_in_full(fd, buf.buf, buf.len) < 0) {
 		if (!gently) {
 			check_pipe(errno);
@@ -168,7 +170,7 @@ void packet_write_fmt(int fd, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	packet_write_fmt_1(fd, 0, fmt, args);
+	packet_write_fmt_1(fd, 0, "", fmt, args);
 	va_end(args);
 }
 
@@ -178,7 +180,7 @@ int packet_write_fmt_gently(int fd, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	status = packet_write_fmt_1(fd, 1, fmt, args);
+	status = packet_write_fmt_1(fd, 1, "", fmt, args);
 	va_end(args);
 	return status;
 }
@@ -211,7 +213,7 @@ void packet_buf_write(struct strbuf *buf, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	format_packet(buf, fmt, args);
+	format_packet(buf, "", fmt, args);
 	va_end(args);
 }
 
@@ -486,3 +488,36 @@ enum packet_read_status packet_reader_peek(struct packet_reader *reader)
 	reader->line_peeked = 1;
 	return reader->status;
 }
+
+void packet_writer_init(struct packet_writer *writer, int dest_fd)
+{
+	writer->dest_fd = dest_fd;
+}
+
+void packet_writer_write(struct packet_writer *writer, const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	packet_write_fmt_1(writer->dest_fd, 0, "", fmt, args);
+	va_end(args);
+}
+
+void packet_writer_error(struct packet_writer *writer, const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	packet_write_fmt_1(writer->dest_fd, 0, "ERR ", fmt, args);
+	va_end(args);
+}
+
+void packet_writer_delim(struct packet_writer *writer)
+{
+	packet_delim(writer->dest_fd);
+}
+
+void packet_writer_flush(struct packet_writer *writer)
+{
+	packet_flush(writer->dest_fd);
+}
diff --git a/pkt-line.h b/pkt-line.h
index d7e1dbc047..023ad2951d 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -183,4 +183,18 @@ extern enum packet_read_status packet_reader_peek(struct packet_reader *reader);
 #define LARGE_PACKET_DATA_MAX (LARGE_PACKET_MAX - 4)
 extern char packet_buffer[LARGE_PACKET_MAX];
 
+struct packet_writer {
+	int dest_fd;
+};
+
+void packet_writer_init(struct packet_writer *writer, int dest_fd);
+
+/* These functions die upon failure. */
+__attribute__((format (printf, 2, 3)))
+void packet_writer_write(struct packet_writer *writer, const char *fmt, ...);
+__attribute__((format (printf, 2, 3)))
+void packet_writer_error(struct packet_writer *writer, const char *fmt, ...);
+void packet_writer_delim(struct packet_writer *writer);
+void packet_writer_flush(struct packet_writer *writer);
+
 #endif
diff --git a/upload-pack.c b/upload-pack.c
index 08b547cf01..60a26bbbfd 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -613,13 +613,14 @@ static void check_non_tip(struct object_array *want_obj)
 	}
 }
 
-static void send_shallow(struct commit_list *result)
+static void send_shallow(struct packet_writer *writer,
+			 struct commit_list *result)
 {
 	while (result) {
 		struct object *object = &result->item->object;
 		if (!(object->flags & (CLIENT_SHALLOW|NOT_SHALLOW))) {
-			packet_write_fmt(1, "shallow %s",
-					 oid_to_hex(&object->oid));
+			packet_writer_write(writer, "shallow %s",
+					    oid_to_hex(&object->oid));
 			register_shallow(the_repository, &object->oid);
 			shallow_nr++;
 		}
@@ -627,7 +628,8 @@ static void send_shallow(struct commit_list *result)
 	}
 }
 
-static void send_unshallow(const struct object_array *shallows,
+static void send_unshallow(struct packet_writer *writer,
+			   const struct object_array *shallows,
 			   struct object_array *want_obj)
 {
 	int i;
@@ -636,8 +638,8 @@ static void send_unshallow(const struct object_array *shallows,
 		struct object *object = shallows->objects[i].item;
 		if (object->flags & NOT_SHALLOW) {
 			struct commit_list *parents;
-			packet_write_fmt(1, "unshallow %s",
-					 oid_to_hex(&object->oid));
+			packet_writer_write(writer, "unshallow %s",
+					    oid_to_hex(&object->oid));
 			object->flags &= ~CLIENT_SHALLOW;
 			/*
 			 * We want to _register_ "object" as shallow, but we
@@ -662,7 +664,7 @@ static void send_unshallow(const struct object_array *shallows,
 	}
 }
 
-static void deepen(int depth, int deepen_relative,
+static void deepen(struct packet_writer *writer, int depth, int deepen_relative,
 		   struct object_array *shallows, struct object_array *want_obj)
 {
 	if (depth == INFINITE_DEPTH && !is_repository_shallow(the_repository)) {
@@ -680,7 +682,7 @@ static void deepen(int depth, int deepen_relative,
 		result = get_shallow_commits(&reachable_shallows,
 					     depth + 1,
 					     SHALLOW, NOT_SHALLOW);
-		send_shallow(result);
+		send_shallow(writer, result);
 		free_commit_list(result);
 		object_array_clear(&reachable_shallows);
 	} else {
@@ -688,14 +690,15 @@ static void deepen(int depth, int deepen_relative,
 
 		result = get_shallow_commits(want_obj, depth,
 					     SHALLOW, NOT_SHALLOW);
-		send_shallow(result);
+		send_shallow(writer, result);
 		free_commit_list(result);
 	}
 
-	send_unshallow(shallows, want_obj);
+	send_unshallow(writer, shallows, want_obj);
 }
 
-static void deepen_by_rev_list(int ac, const char **av,
+static void deepen_by_rev_list(struct packet_writer *writer, int ac,
+			       const char **av,
 			       struct object_array *shallows,
 			       struct object_array *want_obj)
 {
@@ -703,13 +706,14 @@ static void deepen_by_rev_list(int ac, const char **av,
 
 	close_commit_graph(the_repository);
 	result = get_shallow_commits_by_rev_list(ac, av, SHALLOW, NOT_SHALLOW);
-	send_shallow(result);
+	send_shallow(writer, result);
 	free_commit_list(result);
-	send_unshallow(shallows, want_obj);
+	send_unshallow(writer, shallows, want_obj);
 }
 
 /* Returns 1 if a shallow list is sent or 0 otherwise */
-static int send_shallow_list(int depth, int deepen_rev_list,
+static int send_shallow_list(struct packet_writer *writer,
+			     int depth, int deepen_rev_list,
 			     timestamp_t deepen_since,
 			     struct string_list *deepen_not,
 			     struct object_array *shallows,
@@ -720,7 +724,7 @@ static int send_shallow_list(int depth, int deepen_rev_list,
 	if (depth > 0 && deepen_rev_list)
 		die("git upload-pack: deepen and deepen-since (or deepen-not) cannot be used together");
 	if (depth > 0) {
-		deepen(depth, deepen_relative, shallows, want_obj);
+		deepen(writer, depth, deepen_relative, shallows, want_obj);
 		ret = 1;
 	} else if (deepen_rev_list) {
 		struct argv_array av = ARGV_ARRAY_INIT;
@@ -741,7 +745,7 @@ static int send_shallow_list(int depth, int deepen_rev_list,
 			struct object *o = want_obj->objects[i].item;
 			argv_array_push(&av, oid_to_hex(&o->oid));
 		}
-		deepen_by_rev_list(av.argc, av.argv, shallows, want_obj);
+		deepen_by_rev_list(writer, av.argc, av.argv, shallows, want_obj);
 		argv_array_clear(&av);
 		ret = 1;
 	} else {
@@ -834,8 +838,10 @@ static void receive_needs(struct packet_reader *reader, struct object_array *wan
 	int has_non_tip = 0;
 	timestamp_t deepen_since = 0;
 	int deepen_rev_list = 0;
+	struct packet_writer writer;
 
 	shallow_nr = 0;
+	packet_writer_init(&writer, 1);
 	for (;;) {
 		struct object *o;
 		const char *features;
@@ -892,9 +898,9 @@ static void receive_needs(struct packet_reader *reader, struct object_array *wan
 
 		o = parse_object(the_repository, &oid_buf);
 		if (!o) {
-			packet_write_fmt(1,
-					 "ERR upload-pack: not our ref %s",
-					 oid_to_hex(&oid_buf));
+			packet_writer_error(&writer,
+					    "upload-pack: not our ref %s",
+					    oid_to_hex(&oid_buf));
 			die("git upload-pack: not our ref %s",
 			    oid_to_hex(&oid_buf));
 		}
@@ -923,7 +929,7 @@ static void receive_needs(struct packet_reader *reader, struct object_array *wan
 	if (depth == 0 && !deepen_rev_list && shallows.nr == 0)
 		return;
 
-	if (send_shallow_list(depth, deepen_rev_list, deepen_since,
+	if (send_shallow_list(&writer, depth, deepen_rev_list, deepen_since,
 			      &deepen_not, &shallows, want_obj))
 		packet_flush(1);
 	object_array_clear(&shallows);
@@ -1102,6 +1108,8 @@ struct upload_pack_data {
 	int deepen_rev_list;
 	int deepen_relative;
 
+	struct packet_writer writer;
+
 	unsigned stateless_rpc : 1;
 
 	unsigned use_thin_pack : 1;
@@ -1125,6 +1133,7 @@ static void upload_pack_data_init(struct upload_pack_data *data)
 	data->haves = haves;
 	data->shallows = shallows;
 	data->deepen_not = deepen_not;
+	packet_writer_init(&data->writer, 1);
 }
 
 static void upload_pack_data_clear(struct upload_pack_data *data)
@@ -1136,7 +1145,8 @@ static void upload_pack_data_clear(struct upload_pack_data *data)
 	string_list_clear(&data->deepen_not, 0);
 }
 
-static int parse_want(const char *line, struct object_array *want_obj)
+static int parse_want(struct packet_writer *writer, const char *line,
+		      struct object_array *want_obj)
 {
 	const char *arg;
 	if (skip_prefix(line, "want ", &arg)) {
@@ -1149,9 +1159,9 @@ static int parse_want(const char *line, struct object_array *want_obj)
 
 		o = parse_object(the_repository, &oid);
 		if (!o) {
-			packet_write_fmt(1,
-					 "ERR upload-pack: not our ref %s",
-					 oid_to_hex(&oid));
+			packet_writer_error(writer,
+					    "upload-pack: not our ref %s",
+					    oid_to_hex(&oid));
 			die("git upload-pack: not our ref %s",
 			    oid_to_hex(&oid));
 		}
@@ -1167,7 +1177,8 @@ static int parse_want(const char *line, struct object_array *want_obj)
 	return 0;
 }
 
-static int parse_want_ref(const char *line, struct string_list *wanted_refs,
+static int parse_want_ref(struct packet_writer *writer, const char *line,
+			  struct string_list *wanted_refs,
 			  struct object_array *want_obj)
 {
 	const char *arg;
@@ -1177,7 +1188,7 @@ static int parse_want_ref(const char *line, struct string_list *wanted_refs,
 		struct object *o;
 
 		if (read_ref(arg, &oid)) {
-			packet_write_fmt(1, "ERR unknown ref %s", arg);
+			packet_writer_error(writer, "unknown ref %s", arg);
 			die("unknown ref %s", arg);
 		}
 
@@ -1220,10 +1231,11 @@ static void process_args(struct packet_reader *request,
 		const char *p;
 
 		/* process want */
-		if (parse_want(arg, want_obj))
+		if (parse_want(&data->writer, arg, want_obj))
 			continue;
 		if (allow_ref_in_want &&
-		    parse_want_ref(arg, &data->wanted_refs, want_obj))
+		    parse_want_ref(&data->writer, arg, &data->wanted_refs,
+				   want_obj))
 			continue;
 		/* process have line */
 		if (parse_have(arg, &data->haves))
@@ -1317,26 +1329,26 @@ static int process_haves(struct oid_array *haves, struct oid_array *common,
 	return 0;
 }
 
-static int send_acks(struct oid_array *acks, struct strbuf *response,
+static int send_acks(struct packet_writer *writer, struct oid_array *acks,
 		     const struct object_array *have_obj,
 		     struct object_array *want_obj)
 {
 	int i;
 
-	packet_buf_write(response, "acknowledgments\n");
+	packet_writer_write(writer, "acknowledgments\n");
 
 	/* Send Acks */
 	if (!acks->nr)
-		packet_buf_write(response, "NAK\n");
+		packet_writer_write(writer, "NAK\n");
 
 	for (i = 0; i < acks->nr; i++) {
-		packet_buf_write(response, "ACK %s\n",
-				 oid_to_hex(&acks->oid[i]));
+		packet_writer_write(writer, "ACK %s\n",
+				    oid_to_hex(&acks->oid[i]));
 	}
 
 	if (ok_to_give_up(have_obj, want_obj)) {
 		/* Send Ready */
-		packet_buf_write(response, "ready\n");
+		packet_writer_write(writer, "ready\n");
 		return 1;
 	}
 
@@ -1348,25 +1360,20 @@ static int process_haves_and_send_acks(struct upload_pack_data *data,
 				       struct object_array *want_obj)
 {
 	struct oid_array common = OID_ARRAY_INIT;
-	struct strbuf response = STRBUF_INIT;
 	int ret = 0;
 
 	process_haves(&data->haves, &common, have_obj);
 	if (data->done) {
 		ret = 1;
-	} else if (send_acks(&common, &response, have_obj, want_obj)) {
-		packet_buf_delim(&response);
+	} else if (send_acks(&data->writer, &common, have_obj, want_obj)) {
+		packet_writer_delim(&data->writer);
 		ret = 1;
 	} else {
 		/* Add Flush */
-		packet_buf_flush(&response);
+		packet_writer_flush(&data->writer);
 		ret = 0;
 	}
 
-	/* Send response */
-	write_or_die(1, response.buf, response.len);
-	strbuf_release(&response);
-
 	oid_array_clear(&data->haves);
 	oid_array_clear(&common);
 	return ret;
@@ -1379,15 +1386,15 @@ static void send_wanted_ref_info(struct upload_pack_data *data)
 	if (!data->wanted_refs.nr)
 		return;
 
-	packet_write_fmt(1, "wanted-refs\n");
+	packet_writer_write(&data->writer, "wanted-refs\n");
 
 	for_each_string_list_item(item, &data->wanted_refs) {
-		packet_write_fmt(1, "%s %s\n",
-				 oid_to_hex(item->util),
-				 item->string);
+		packet_writer_write(&data->writer, "%s %s\n",
+				    oid_to_hex(item->util),
+				    item->string);
 	}
 
-	packet_delim(1);
+	packet_writer_delim(&data->writer);
 }
 
 static void send_shallow_info(struct upload_pack_data *data,
@@ -1398,14 +1405,15 @@ static void send_shallow_info(struct upload_pack_data *data,
 	    !is_repository_shallow(the_repository))
 		return;
 
-	packet_write_fmt(1, "shallow-info\n");
+	packet_writer_write(&data->writer, "shallow-info\n");
 
-	if (!send_shallow_list(data->depth, data->deepen_rev_list,
+	if (!send_shallow_list(&data->writer, data->depth,
+			       data->deepen_rev_list,
 			       data->deepen_since, &data->deepen_not,
 			       &data->shallows, want_obj) &&
 	    is_repository_shallow(the_repository))
-		deepen(INFINITE_DEPTH, data->deepen_relative, &data->shallows,
-		       want_obj);
+		deepen(&data->writer, INFINITE_DEPTH, data->deepen_relative,
+		       &data->shallows, want_obj);
 
 	packet_delim(1);
 }
@@ -1467,7 +1475,7 @@ int upload_pack_v2(struct repository *r, struct argv_array *keys,
 			send_wanted_ref_info(&data);
 			send_shallow_info(&data, &want_obj);
 
-			packet_write_fmt(1, "packfile\n");
+			packet_writer_write(&data.writer, "packfile\n");
 			create_pack_file(&have_obj, &want_obj);
 			state = FETCH_DONE;
 			break;

pkt-line.c, pkt-line.h, sideband.c, sideband.h

diff --git a/pkt-line.c b/pkt-line.c
index 9d3e402d41..ebdc6c2530 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -439,6 +439,28 @@ ssize_t read_packetized_to_strbuf(int fd_in, struct strbuf *sb_out)
 	return sb_out->len - orig_len;
 }
 
+int recv_sideband(const char *me, int in_stream, int out)
+{
+	char buf[LARGE_PACKET_MAX + 1];
+	int retval = 0;
+	int len;
+
+	while (1) {
+		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX, 0);
+		retval = diagnose_sideband(me, buf, len);
+		switch (retval) {
+			case SIDEBAND_PRIMARY:
+				write_or_die(out, buf + 1, len - 1);
+				break;
+			case SIDEBAND_PROGRESS:
+				/* already written by diagnose_sideband() */
+				break;
+			default: /* flush or error */
+				return retval;
+		}
+	}
+}
+
 /* Packet Reader Functions */
 void packet_reader_init(struct packet_reader *reader, int fd,
 			char *src_buffer, size_t src_len,
diff --git a/pkt-line.h b/pkt-line.h
index 023ad2951d..a8e92a4b63 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -3,6 +3,7 @@
 
 #include "git-compat-util.h"
 #include "strbuf.h"
+#include "sideband.h"
 
 /*
  * Write a packetized stream, where each line is preceded by
@@ -120,6 +121,21 @@ char *packet_read_line_buf(char **src_buf, size_t *src_len, int *size);
  */
 ssize_t read_packetized_to_strbuf(int fd_in, struct strbuf *sb_out);
 
+/*
+ * Receive multiplexed output stream over git native protocol.
+ * in_stream is the input stream from the remote, which carries data
+ * in pkt_line format with band designator.  Demultiplex it into out
+ * and err and return error appropriately.  Band #1 carries the
+ * primary payload.  Things coming over band #2 is not necessarily
+ * error; they are usually informative message on the standard error
+ * stream, aka "verbose").  A message over band #3 is a signal that
+ * the remote died unexpectedly.  A flush() concludes the stream.
+ *
+ * Returns SIDEBAND_FLUSH upon a normal conclusion, and SIDEBAND_PROTOCOL_ERROR
+ * or SIDEBAND_REMOTE_ERROR if an error occurred.
+ */
+int recv_sideband(const char *me, int in_stream, int out);
+
 struct packet_reader {
 	/* source file descriptor */
 	int fd;
diff --git a/sideband.c b/sideband.c
index 368647acf8..842a92e975 100644
--- a/sideband.c
+++ b/sideband.c
@@ -1,7 +1,6 @@
 #include "cache.h"
 #include "color.h"
 #include "config.h"
-#include "pkt-line.h"
 #include "sideband.h"
 #include "help.h"
 
@@ -109,103 +108,94 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
 }
 
 
-/*
- * Receive multiplexed output stream over git native protocol.
- * in_stream is the input stream from the remote, which carries data
- * in pkt_line format with band designator.  Demultiplex it into out
- * and err and return error appropriately.  Band #1 carries the
- * primary payload.  Things coming over band #2 is not necessarily
- * error; they are usually informative message on the standard error
- * stream, aka "verbose").  A message over band #3 is a signal that
- * the remote died unexpectedly.  A flush() concludes the stream.
- */
-
 #define DISPLAY_PREFIX "remote: "
 
 #define ANSI_SUFFIX "\033[K"
 #define DUMB_SUFFIX "        "
 
-int recv_sideband(const char *me, int in_stream, int out)
+int diagnose_sideband(const char *me, char *buf, int len)
 {
-	const char *suffix;
-	char buf[LARGE_PACKET_MAX + 1];
+	static const char *suffix;
 	struct strbuf outbuf = STRBUF_INIT;
 	int retval = 0;
+	const char *b, *brk;
+	int band;
+
+	if (!suffix) {
+		if (isatty(2) && !is_terminal_dumb())
+			suffix = ANSI_SUFFIX;
+		else
+			suffix = DUMB_SUFFIX;
+	}
 
-	if (isatty(2) && !is_terminal_dumb())
-		suffix = ANSI_SUFFIX;
-	else
-		suffix = DUMB_SUFFIX;
+	if (len == 0) {
+		retval = SIDEBAND_FLUSH;
+		goto cleanup;
+	}
+	if (len < 1) {
+		strbuf_addf(&outbuf,
+			    "%s%s: protocol error: no band designator",
+			    outbuf.len ? "\n" : "", me);
+		retval = SIDEBAND_PROTOCOL_ERROR;
+		goto cleanup;
+	}
+	band = buf[0] & 0xff;
+	buf[len] = '\0';
+	len--;
+	switch (band) {
+	case 3:
+		strbuf_addf(&outbuf, "%s%s", outbuf.len ? "\n" : "",
+			    DISPLAY_PREFIX);
+		maybe_colorize_sideband(&outbuf, buf + 1, len);
+
+		retval = SIDEBAND_REMOTE_ERROR;
+		break;
+	case 2:
+		b = buf + 1;
 
-	while (!retval) {
-		const char *b, *brk;
-		int band, len;
-		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX, 0);
-		if (len == 0)
-			break;
-		if (len < 1) {
-			strbuf_addf(&outbuf,
-				    "%s%s: protocol error: no band designator",
-				    outbuf.len ? "\n" : "", me);
-			retval = SIDEBAND_PROTOCOL_ERROR;
-			break;
-		}
-		band = buf[0] & 0xff;
-		buf[len] = '\0';
-		len--;
-		switch (band) {
-		case 3:
-			strbuf_addf(&outbuf, "%s%s", outbuf.len ? "\n" : "",
-				    DISPLAY_PREFIX);
-			maybe_colorize_sideband(&outbuf, buf + 1, len);
-
-			retval = SIDEBAND_REMOTE_ERROR;
-			break;
-		case 2:
-			b = buf + 1;
-
-			/*
-			 * Append a suffix to each nonempty line to clear the
-			 * end of the screen line.
-			 *
-			 * The output is accumulated in a buffer and
-			 * each line is printed to stderr using
-			 * write(2) to ensure inter-process atomicity.
-			 */
-			while ((brk = strpbrk(b, "\n\r"))) {
-				int linelen = brk - b;
-
-				if (!outbuf.len)
-					strbuf_addstr(&outbuf, DISPLAY_PREFIX);
-				if (linelen > 0) {
-					maybe_colorize_sideband(&outbuf, b, linelen);
-					strbuf_addstr(&outbuf, suffix);
-				}
-
-				strbuf_addch(&outbuf, *brk);
-				xwrite(2, outbuf.buf, outbuf.len);
-				strbuf_reset(&outbuf);
-
-				b = brk + 1;
+		/*
+		 * Append a suffix to each nonempty line to clear the
+		 * end of the screen line.
+		 *
+		 * The output is accumulated in a buffer and
+		 * each line is printed to stderr using
+		 * write(2) to ensure inter-process atomicity.
+		 */
+		while ((brk = strpbrk(b, "\n\r"))) {
+			int linelen = brk - b;
+
+			if (!outbuf.len)
+				strbuf_addstr(&outbuf, DISPLAY_PREFIX);
+			if (linelen > 0) {
+				maybe_colorize_sideband(&outbuf, b, linelen);
+				strbuf_addstr(&outbuf, suffix);
 			}
 
-			if (*b) {
-				strbuf_addstr(&outbuf, outbuf.len ?
-					    "" : DISPLAY_PREFIX);
-				maybe_colorize_sideband(&outbuf, b, strlen(b));
-			}
-			break;
-		case 1:
-			write_or_die(out, buf + 1, len);
-			break;
-		default:
-			strbuf_addf(&outbuf, "%s%s: protocol error: bad band #%d",
-				    outbuf.len ? "\n" : "", me, band);
-			retval = SIDEBAND_PROTOCOL_ERROR;
-			break;
+			strbuf_addch(&outbuf, *brk);
+			xwrite(2, outbuf.buf, outbuf.len);
+			strbuf_reset(&outbuf);
+
+			b = brk + 1;
+		}
+
+		if (*b) {
+			strbuf_addstr(&outbuf, outbuf.len ?
+				    "" : DISPLAY_PREFIX);
+			maybe_colorize_sideband(&outbuf, b, strlen(b));
 		}
+		retval = SIDEBAND_PROGRESS;
+		break;
+	case 1:
+		retval = SIDEBAND_PRIMARY;
+		break;
+	default:
+		strbuf_addf(&outbuf, "%s%s: protocol error: bad band #%d",
+			    outbuf.len ? "\n" : "", me, band);
+		retval = SIDEBAND_PROTOCOL_ERROR;
+		break;
 	}
 
+cleanup:
 	if (outbuf.len) {
 		strbuf_addch(&outbuf, '\n');
 		xwrite(2, outbuf.buf, outbuf.len);
diff --git a/sideband.h b/sideband.h
index 7a8146f161..a56cd86287 100644
--- a/sideband.h
+++ b/sideband.h
@@ -3,8 +3,21 @@
 
 #define SIDEBAND_PROTOCOL_ERROR -2
 #define SIDEBAND_REMOTE_ERROR -1
+#define SIDEBAND_FLUSH 0
+#define SIDEBAND_PRIMARY 1
+#define SIDEBAND_PROGRESS 2
+
+/*
+ * buf and len should be the result of reading a line from a remote sending
+ * multiplexed data.
+ *
+ * Determines the nature of the result and returns it. If
+ * SIDEBAND_PROTOCOL_ERROR, SIDEBAND_REMOTE_ERROR, or SIDEBAND_PROGRESS, also
+ * prints a message (or the formatted contents of the notice in the case of
+ * SIDEBAND_PROGRESS) to stderr.
+ */
+int diagnose_sideband(const char *me, char *buf, int len);
 
-int recv_sideband(const char *me, int in_stream, int out);
 void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_max);
 
 #endif

Documentation/technical/protocol-v2.txt, fetch-pack.c, pkt-line.c, pkt-line.h, sideband.c, sideband.h, upload-pack.c

diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index 09e4e0273f..1b0633f59f 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -307,6 +307,16 @@ the 'wanted-refs' section in the server's response as explained below.
 	particular ref, where <ref> is the full name of a ref on the
 	server.
 
+If the 'sideband-all' feature is advertised, the following argument can be
+included in the client's request:
+
+    sideband-all
+	Instruct the server to send the whole response multiplexed, not just
+	the packfile section. All non-flush and non-delim PKT-LINE in the
+	response (not only in the packfile section) will then start with a byte
+	indicating its sideband (1, 2, or 3), and the server may send "0005\1"
+	(a PKT-LINE of sideband 1 with no payload) as a keepalive packet.
+
 The response of `fetch` is broken into a number of sections separated by
 delimiter packets (0001), with each section beginning with its section
 header.
diff --git a/fetch-pack.c b/fetch-pack.c
index 3f9626dbf7..b89199891d 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -1125,7 +1125,8 @@ static int add_haves(struct fetch_negotiator *negotiator,
 static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 			      const struct fetch_pack_args *args,
 			      const struct ref *wants, struct oidset *common,
-			      int *haves_to_send, int *in_vain)
+			      int *haves_to_send, int *in_vain,
+			      int sideband_all)
 {
 	int ret = 0;
 	struct strbuf req_buf = STRBUF_INIT;
@@ -1151,6 +1152,8 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 		packet_buf_write(&req_buf, "include-tag");
 	if (prefer_ofs_delta)
 		packet_buf_write(&req_buf, "ofs-delta");
+	if (sideband_all)
+		packet_buf_write(&req_buf, "sideband-all");
 
 	/* Add shallow-info and deepen request */
 	if (server_supports_feature("fetch", "shallow", 0))
@@ -1359,6 +1362,10 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 	packet_reader_init(&reader, fd[0], NULL, 0,
 			   PACKET_READ_CHOMP_NEWLINE |
 			   PACKET_READ_DIE_ON_ERR_PACKET);
+	if (server_supports_feature("fetch", "sideband-all", 0)) {
+		reader.use_sideband = 1;
+		reader.me = "fetch-pack";
+	}
 
 	while (state != FETCH_DONE) {
 		switch (state) {
@@ -1392,7 +1399,8 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 		case FETCH_SEND_REQUEST:
 			if (send_fetch_request(&negotiator, fd[1], args, ref,
 					       &common,
-					       &haves_to_send, &in_vain))
+					       &haves_to_send, &in_vain,
+					       reader.use_sideband))
 				state = FETCH_GET_PACK;
 			else
 				state = FETCH_PROCESS_ACKS;
diff --git a/pkt-line.c b/pkt-line.c
index ebdc6c2530..71b17e6b93 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -447,7 +447,7 @@ int recv_sideband(const char *me, int in_stream, int out)
 
 	while (1) {
 		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX, 0);
-		retval = diagnose_sideband(me, buf, len);
+		retval = diagnose_sideband(me, buf, len, 0);
 		switch (retval) {
 			case SIDEBAND_PRIMARY:
 				write_or_die(out, buf + 1, len - 1);
@@ -483,16 +483,42 @@ enum packet_read_status packet_reader_read(struct packet_reader *reader)
 		return reader->status;
 	}
 
-	reader->status = packet_read_with_status(reader->fd,
-						 &reader->src_buffer,
-						 &reader->src_len,
-						 reader->buffer,
-						 reader->buffer_size,
-						 &reader->pktlen,
-						 reader->options);
+	while (1) {
+		int retval;
+		reader->status = packet_read_with_status(reader->fd,
+							 &reader->src_buffer,
+							 &reader->src_len,
+							 reader->buffer,
+							 reader->buffer_size,
+							 &reader->pktlen,
+							 reader->options);
+		if (!reader->use_sideband)
+			break;
+		retval = diagnose_sideband(reader->me, reader->buffer,
+					   reader->pktlen, 1);
+		switch (retval) {
+			case SIDEBAND_PROTOCOL_ERROR:
+			case SIDEBAND_REMOTE_ERROR:
+				BUG("should have died in diagnose_sideband");
+			case SIDEBAND_FLUSH:
+				goto nonprogress;
+			case SIDEBAND_PRIMARY:
+				if (reader->pktlen != 1)
+					goto nonprogress;
+				/*
+				 * Since pktlen is 1, this is a keepalive
+				 * packet. Wait for the next one.
+				 */
+				break;
+			default: /* SIDEBAND_PROGRESS */
+				;
+		}
+	}
 
+nonprogress:
 	if (reader->status == PACKET_READ_NORMAL)
-		reader->line = reader->buffer;
+		reader->line = reader->use_sideband ?
+			reader->buffer + 1 : reader->buffer;
 	else
 		reader->line = NULL;
 
@@ -514,6 +540,7 @@ enum packet_read_status packet_reader_peek(struct packet_reader *reader)
 void packet_writer_init(struct packet_writer *writer, int dest_fd)
 {
 	writer->dest_fd = dest_fd;
+	writer->use_sideband = 0;
 }
 
 void packet_writer_write(struct packet_writer *writer, const char *fmt, ...)
@@ -521,7 +548,8 @@ void packet_writer_write(struct packet_writer *writer, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	packet_write_fmt_1(writer->dest_fd, 0, "", fmt, args);
+	packet_write_fmt_1(writer->dest_fd, 0,
+			   writer->use_sideband ? "\1" : "", fmt, args);
 	va_end(args);
 }
 
@@ -530,7 +558,8 @@ void packet_writer_error(struct packet_writer *writer, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	packet_write_fmt_1(writer->dest_fd, 0, "ERR ", fmt, args);
+	packet_write_fmt_1(writer->dest_fd, 0,
+			   writer->use_sideband ? "\3" : "ERR ", fmt, args);
 	va_end(args);
 }
 
diff --git a/pkt-line.h b/pkt-line.h
index a8e92a4b63..ad9a4a2cd7 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -162,6 +162,9 @@ struct packet_reader {
 
 	/* indicates if a line has been peeked */
 	int line_peeked;
+
+	unsigned use_sideband : 1;
+	const char *me;
 };
 
 /*
@@ -201,6 +204,7 @@ extern char packet_buffer[LARGE_PACKET_MAX];
 
 struct packet_writer {
 	int dest_fd;
+	unsigned use_sideband : 1;
 };
 
 void packet_writer_init(struct packet_writer *writer, int dest_fd);
diff --git a/sideband.c b/sideband.c
index 842a92e975..cbeab380ae 100644
--- a/sideband.c
+++ b/sideband.c
@@ -113,7 +113,7 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
 #define ANSI_SUFFIX "\033[K"
 #define DUMB_SUFFIX "        "
 
-int diagnose_sideband(const char *me, char *buf, int len)
+int diagnose_sideband(const char *me, char *buf, int len, int die_on_error)
 {
 	static const char *suffix;
 	struct strbuf outbuf = STRBUF_INIT;
@@ -144,6 +144,9 @@ int diagnose_sideband(const char *me, char *buf, int len)
 	len--;
 	switch (band) {
 	case 3:
+		if (die_on_error)
+			die("remote error: %s", buf + 1);
+
 		strbuf_addf(&outbuf, "%s%s", outbuf.len ? "\n" : "",
 			    DISPLAY_PREFIX);
 		maybe_colorize_sideband(&outbuf, buf + 1, len);
@@ -196,6 +199,8 @@ int diagnose_sideband(const char *me, char *buf, int len)
 	}
 
 cleanup:
+	if (die_on_error && retval == SIDEBAND_PROTOCOL_ERROR)
+		die("%s", outbuf.buf);
 	if (outbuf.len) {
 		strbuf_addch(&outbuf, '\n');
 		xwrite(2, outbuf.buf, outbuf.len);
diff --git a/sideband.h b/sideband.h
index a56cd86287..3786670694 100644
--- a/sideband.h
+++ b/sideband.h
@@ -16,7 +16,7 @@
  * prints a message (or the formatted contents of the notice in the case of
  * SIDEBAND_PROGRESS) to stderr.
  */
-int diagnose_sideband(const char *me, char *buf, int len);
+int diagnose_sideband(const char *me, char *buf, int len, int die_on_error);
 
 void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_max);
 
diff --git a/upload-pack.c b/upload-pack.c
index 60a26bbbfd..765b7695d2 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -71,6 +71,8 @@ static int allow_filter;
 static int allow_ref_in_want;
 static struct list_objects_filter_options filter_options;
 
+static int allow_sideband_all;
+
 static void reset_timeout(void)
 {
 	alarm(timeout);
@@ -1046,6 +1048,8 @@ static int upload_pack_config(const char *var, const char *value, void *unused)
 		allow_filter = git_config_bool(var, value);
 	} else if (!strcmp("uploadpack.allowrefinwant", var)) {
 		allow_ref_in_want = git_config_bool(var, value);
+	} else if (!strcmp("uploadpack.allowsidebandall", var)) {
+		allow_sideband_all = git_config_bool(var, value);
 	}
 
 	if (current_config_scope() != CONFIG_SCOPE_REPO) {
@@ -1284,6 +1288,11 @@ static void process_args(struct packet_reader *request,
 			continue;
 		}
 
+		if (allow_sideband_all && !strcmp(arg, "sideband-all")) {
+			data->writer.use_sideband = 1;
+			continue;
+		}
+
 		/* ignore unknown lines maybe? */
 		die("unexpected line: '%s'", arg);
 	}
@@ -1496,6 +1505,7 @@ int upload_pack_advertise(struct repository *r,
 	if (value) {
 		int allow_filter_value;
 		int allow_ref_in_want;
+		int allow_sideband_all_value;
 
 		strbuf_addstr(value, "shallow");
 
@@ -1510,6 +1520,12 @@ int upload_pack_advertise(struct repository *r,
 					 &allow_ref_in_want) &&
 		    allow_ref_in_want)
 			strbuf_addstr(value, " ref-in-want");
+
+		if (!repo_config_get_bool(the_repository,
+					 "uploadpack.allowsidebandall",
+					 &allow_sideband_all_value) &&
+		    allow_sideband_all_value)
+			strbuf_addstr(value, " sideband-all");
 	}
 
 	return 1;

fetch-pack.c, t/README, t/lib-httpd/apache.conf, t/t5537-fetch-shallow.sh, t/t5701-git-serve.sh, t/t5702-protocol-v2.sh, upload-pack.c

diff --git a/fetch-pack.c b/fetch-pack.c
index b89199891d..4618568fee 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -1362,7 +1362,8 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 	packet_reader_init(&reader, fd[0], NULL, 0,
 			   PACKET_READ_CHOMP_NEWLINE |
 			   PACKET_READ_DIE_ON_ERR_PACKET);
-	if (server_supports_feature("fetch", "sideband-all", 0)) {
+	if (git_env_bool("GIT_TEST_SIDEBAND_ALL", 1) &&
+	    server_supports_feature("fetch", "sideband-all", 0)) {
 		reader.use_sideband = 1;
 		reader.me = "fetch-pack";
 	}
diff --git a/t/README b/t/README
index 28711cc508..b275c883b8 100644
--- a/t/README
+++ b/t/README
@@ -358,6 +358,11 @@ GIT_TEST_MULTI_PACK_INDEX=<boolean>, when true, forces the multi-pack-
 index to be written after every 'git repack' command, and overrides the
 'core.multiPackIndex' setting to true.
 
+GIT_TEST_SIDEBAND_ALL=<boolean>, when true, overrides the
+'uploadpack.allowSidebandAll' setting to true, and when false, forces
+fetch-pack to not request sideband-all (even if the server advertises
+sideband-all).
+
 Naming Tests
 ------------
 
diff --git a/t/lib-httpd/apache.conf b/t/lib-httpd/apache.conf
index 581c010d8f..9a6d368247 100644
--- a/t/lib-httpd/apache.conf
+++ b/t/lib-httpd/apache.conf
@@ -78,6 +78,7 @@ PassEnv GNUPGHOME
 PassEnv ASAN_OPTIONS
 PassEnv GIT_TRACE
 PassEnv GIT_CONFIG_NOSYSTEM
+PassEnv GIT_TEST_SIDEBAND_ALL
 
 SetEnvIf Git-Protocol ".*" GIT_PROTOCOL=$0
 
diff --git a/t/t5537-fetch-shallow.sh b/t/t5537-fetch-shallow.sh
index 6faf17e17a..6caf628efa 100755
--- a/t/t5537-fetch-shallow.sh
+++ b/t/t5537-fetch-shallow.sh
@@ -243,7 +243,8 @@ test_expect_success 'shallow fetches check connectivity before writing shallow f
 	       "$(git -C "$REPO" rev-parse HEAD)" \
 	       "$(git -C "$REPO" rev-parse HEAD^)" \
 	       >"$HTTPD_ROOT_PATH/one-time-sed" &&
-	test_must_fail git -C client fetch --depth=1 "$HTTPD_URL/one_time_sed/repo" \
+	test_must_fail env GIT_TEST_SIDEBAND_ALL=0 git -C client \
+		fetch --depth=1 "$HTTPD_URL/one_time_sed/repo" \
 		master:a_branch &&
 
 	# Ensure that the one-time-sed script was used.
diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
index ae79c6bbc0..fe45bf828d 100755
--- a/t/t5701-git-serve.sh
+++ b/t/t5701-git-serve.sh
@@ -14,7 +14,7 @@ test_expect_success 'test capability advertisement' '
 	0000
 	EOF
 
-	git serve --advertise-capabilities >out &&
+	GIT_TEST_SIDEBAND_ALL=0 git serve --advertise-capabilities >out &&
 	test-tool pkt-line unpack <out >actual &&
 	test_cmp expect actual
 '
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 0f2b09ebb8..b491c62e3e 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -583,8 +583,8 @@ test_expect_success 'when server does not send "ready", expect FLUSH' '
 	test_must_fail env GIT_TRACE_PACKET="$(pwd)/log" git -C http_child \
 		-c protocol.version=2 \
 		fetch "$HTTPD_URL/one_time_sed/http_parent" 2> err &&
-	grep "fetch< acknowledgments" log &&
-	! grep "fetch< ready" log &&
+	grep "fetch< .*acknowledgments" log &&
+	! grep "fetch< .*ready" log &&
 	test_i18ngrep "expected no other sections to be sent after no .ready." err
 '
 
diff --git a/upload-pack.c b/upload-pack.c
index 765b7695d2..0c1feccaab 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1288,7 +1288,9 @@ static void process_args(struct packet_reader *request,
 			continue;
 		}
 
-		if (allow_sideband_all && !strcmp(arg, "sideband-all")) {
+		if ((git_env_bool("GIT_TEST_SIDEBAND_ALL", 0) ||
+		     allow_sideband_all) &&
+		    !strcmp(arg, "sideband-all")) {
 			data->writer.use_sideband = 1;
 			continue;
 		}
@@ -1521,10 +1523,11 @@ int upload_pack_advertise(struct repository *r,
 		    allow_ref_in_want)
 			strbuf_addstr(value, " ref-in-want");
 
-		if (!repo_config_get_bool(the_repository,
-					 "uploadpack.allowsidebandall",
-					 &allow_sideband_all_value) &&
-		    allow_sideband_all_value)
+		if (git_env_bool("GIT_TEST_SIDEBAND_ALL", 0) ||
+		    (!repo_config_get_bool(the_repository,
+					   "uploadpack.allowsidebandall",
+					   &allow_sideband_all_value) &&
+		     allow_sideband_all_value))
 			strbuf_addstr(value, " sideband-all");
 	}
 

diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index 1b0633f59f..c76f2311c2 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -319,11 +319,11 @@ included in the client's request:
 
 The response of `fetch` is broken into a number of sections separated by
 delimiter packets (0001), with each section beginning with its section
-header.
+header. Most sections are sent only when the packfile is sent.
 
-    output = *section
-    section = (acknowledgments | shallow-info | wanted-refs | packfile)
-	      (flush-pkt | delim-pkt)
+    output = acknowledgements flush-pkt |
+	     [acknowledgments delim-pkt] [shallow-info delim-pkt]
+	     [wanted-refs delim-pkt] packfile flush-pkt
 
     acknowledgments = PKT-LINE("acknowledgments" LF)
 		      (nak | *ack)
@@ -345,9 +345,10 @@ header.
 	       *PKT-LINE(%x01-03 *%x00-ff)
 
     acknowledgments section
-	* If the client determines that it is finished with negotiations
-	  by sending a "done" line, the acknowledgments sections MUST be
-	  omitted from the server's response.
+	* If the client determines that it is finished with negotiations by
+	  sending a "done" line (thus requiring the server to send a packfile),
+	  the acknowledgments sections MUST be omitted from the server's
+	  response.
 
 	* Always begins with the section header "acknowledgments"
 
@@ -398,9 +399,6 @@ header.
 	  which the client has not indicated was shallow as a part of
 	  its request.
 
-	* This section is only included if a packfile section is also
-	  included in the response.
-
     wanted-refs section
 	* This section is only included if the client has requested a
 	  ref using a 'want-ref' line and if a packfile section is also

Documentation/technical/packfile-uri.txt, Documentation/technical/protocol-v2.txt

diff --git a/Documentation/technical/packfile-uri.txt b/Documentation/technical/packfile-uri.txt
new file mode 100644
index 0000000000..6535801486
--- /dev/null
+++ b/Documentation/technical/packfile-uri.txt
@@ -0,0 +1,83 @@
+Packfile URIs
+=============
+
+This feature allows servers to serve part of their packfile response as URIs.
+This allows server designs that improve scalability in bandwidth and CPU usage
+(for example, by serving some data through a CDN), and (in the future) provides
+some measure of resumability to clients.
+
+This feature is available only in protocol version 2.
+
+Protocol
+--------
+
+The server advertises `packfile-uris`.
+
+If the client replies with the following arguments:
+
+ * packfile-uris
+ * thin-pack
+ * ofs-delta
+
+when the server sends the packfile, it MAY send a `packfile-uris` section
+directly before the `packfile` section (right after `wanted-refs` if it is
+sent) containing HTTP(S) URIs. See protocol-v2.txt for the documentation of
+this section.
+
+Clients then should understand that the returned packfile could be incomplete,
+and that it needs to download all the given URIs before the fetch or clone is
+complete. Each URI should point to a Git packfile (which may be a thin pack and
+which may contain offset deltas).
+
+Server design
+-------------
+
+The server can be trivially made compatible with the proposed protocol by
+having it advertise `packfile-uris`, tolerating the client sending
+`packfile-uris`, and never sending any `packfile-uris` section. But we should
+include some sort of non-trivial implementation in the Minimum Viable Product,
+at least so that we can test the client.
+
+This is the implementation: a feature, marked experimental, that allows the
+server to be configured by one or more `uploadpack.blobPackfileUri=<sha1>
+<uri>` entries. Whenever the list of objects to be sent is assembled, a blob
+with the given sha1 can be replaced by the given URI. This allows, for example,
+servers to delegate serving of large blobs to CDNs.
+
+Client design
+-------------
+
+While fetching, the client needs to remember the list of URIs and cannot
+declare that the fetch is complete until all URIs have been downloaded as
+packfiles.
+
+The division of work (initial fetch + additional URIs) introduces convenient
+points for resumption of an interrupted clone - such resumption can be done
+after the Minimum Viable Product (see "Future work").
+
+The client can inhibit this feature (i.e. refrain from sending the
+`packfile-urls` parameter) by passing --no-packfile-urls to `git fetch`.
+
+Future work
+-----------
+
+The protocol design allows some evolution of the server and client without any
+need for protocol changes, so only a small-scoped design is included here to
+form the MVP. For example, the following can be done:
+
+ * On the server, a long-running process that takes in entire requests and
+   outputs a list of URIs and the corresponding inclusion and exclusion sets of
+   objects. This allows, e.g., signed URIs to be used and packfiles for common
+   requests to be cached.
+ * On the client, resumption of clone. If a clone is interrupted, information
+   could be recorded in the repository's config and a "clone-resume" command
+   can resume the clone in progress. (Resumption of subsequent fetches is more
+   difficult because that must deal with the user wanting to use the repository
+   even after the fetch was interrupted.)
+
+There are some possible features that will require a change in protocol:
+
+ * Additional HTTP headers (e.g. authentication)
+ * Byte range support
+ * Different file formats referenced by URIs (e.g. raw object)
+
diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index c76f2311c2..de5c63bc9b 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -323,7 +323,8 @@ header. Most sections are sent only when the packfile is sent.
 
     output = acknowledgements flush-pkt |
 	     [acknowledgments delim-pkt] [shallow-info delim-pkt]
-	     [wanted-refs delim-pkt] packfile flush-pkt
+	     [wanted-refs delim-pkt] [packfile-uris delim-pkt]
+	     packfile flush-pkt
 
     acknowledgments = PKT-LINE("acknowledgments" LF)
 		      (nak | *ack)
@@ -341,6 +342,9 @@ header. Most sections are sent only when the packfile is sent.
 		  *PKT-LINE(wanted-ref LF)
     wanted-ref = obj-id SP refname
 
+    packfile-uris = PKT-LINE("packfile-uris" LF) *packfile-uri
+    packfile-uri = PKT-LINE("uri" SP *%x20-ff LF)
+
     packfile = PKT-LINE("packfile" LF)
 	       *PKT-LINE(%x01-03 *%x00-ff)
 

diff --git a/upload-pack.c b/upload-pack.c
index 0c1feccaab..c87b752550 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -103,14 +103,51 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 	return 0;
 }
 
+struct output_state {
+	char buffer[8193];
+	int used;
+};
+
+static int read_pack_objects_stdout(int outfd, struct output_state *os)
+{
+	/* Data ready; we keep the last byte to ourselves
+	 * in case we detect broken rev-list, so that we
+	 * can leave the stream corrupted.  This is
+	 * unfortunate -- unpack-objects would happily
+	 * accept a valid packdata with trailing garbage,
+	 * so appending garbage after we pass all the
+	 * pack data is not good enough to signal
+	 * breakage to downstream.
+	 */
+	ssize_t readsz;
+
+	readsz = xread(outfd, os->buffer + os->used,
+		       sizeof(os->buffer) - os->used);
+	if (readsz < 0) {
+		return readsz;
+	}
+	os->used += readsz;
+
+	if (os->used > 1) {
+		send_client_data(1, os->buffer, os->used - 1);
+		os->buffer[0] = os->buffer[os->used - 1];
+		os->used = 1;
+	} else {
+		send_client_data(1, os->buffer, os->used);
+		os->used = 0;
+	}
+
+	return readsz;
+}
+
 static void create_pack_file(const struct object_array *have_obj,
 			     const struct object_array *want_obj)
 {
 	struct child_process pack_objects = CHILD_PROCESS_INIT;
-	char data[8193], progress[128];
+	struct output_state output_state = {0};
+	char progress[128];
 	char abort_msg[] = "aborting due to possible repository "
 		"corruption on the remote side.";
-	int buffered = -1;
 	ssize_t sz;
 	int i;
 	FILE *pipe_fd;
@@ -237,39 +274,15 @@ static void create_pack_file(const struct object_array *have_obj,
 			continue;
 		}
 		if (0 <= pu && (pfd[pu].revents & (POLLIN|POLLHUP))) {
-			/* Data ready; we keep the last byte to ourselves
-			 * in case we detect broken rev-list, so that we
-			 * can leave the stream corrupted.  This is
-			 * unfortunate -- unpack-objects would happily
-			 * accept a valid packdata with trailing garbage,
-			 * so appending garbage after we pass all the
-			 * pack data is not good enough to signal
-			 * breakage to downstream.
-			 */
-			char *cp = data;
-			ssize_t outsz = 0;
-			if (0 <= buffered) {
-				*cp++ = buffered;
-				outsz++;
-			}
-			sz = xread(pack_objects.out, cp,
-				  sizeof(data) - outsz);
-			if (0 < sz)
-				;
-			else if (sz == 0) {
+			int result = read_pack_objects_stdout(pack_objects.out,
+							      &output_state);
+
+			if (result == 0) {
 				close(pack_objects.out);
 				pack_objects.out = -1;
-			}
-			else
+			} else if (result < 0) {
 				goto fail;
-			sz += outsz;
-			if (1 < sz) {
-				buffered = data[sz-1] & 0xFF;
-				sz--;
 			}
-			else
-				buffered = -1;
-			send_client_data(1, data, sz);
 		}
 
 		/*
@@ -294,9 +307,8 @@ static void create_pack_file(const struct object_array *have_obj,
 	}
 
 	/* flush the data */
-	if (0 <= buffered) {
-		data[0] = buffered;
-		send_client_data(1, data, 1);
+	if (output_state.used > 0) {
+		send_client_data(1, output_state.buffer, output_state.used);
 		fprintf(stderr, "flushed.\n");
 	}
 	if (use_sideband)

builtin/pack-objects.c, fetch-pack.c, t/t5702-protocol-v2.sh, upload-pack.c

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 411aefd687..448c42a666 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -117,6 +117,15 @@ enum missing_action {
 static enum missing_action arg_missing_action;
 static show_object_fn fn_show_object;
 
+struct configured_exclusion {
+	struct oidmap_entry e;
+	char *uri;
+};
+static struct oidmap configured_exclusions;
+
+static int exclude_configured_blobs;
+static struct oidset excluded_by_config;
+
 /*
  * stats
  */
@@ -831,6 +840,23 @@ static off_t write_reused_pack(struct hashfile *f)
 	return reuse_packfile_offset - sizeof(struct pack_header);
 }
 
+static void write_excluded_by_configs(void)
+{
+	struct oidset_iter iter;
+	const struct object_id *oid;
+
+	oidset_iter_init(&excluded_by_config, &iter);
+	while ((oid = oidset_iter_next(&iter))) {
+		struct configured_exclusion *ex =
+			oidmap_get(&configured_exclusions, oid);
+
+		if (!ex)
+			BUG("configured exclusion wasn't configured");
+		write_in_full(1, ex->uri, strlen(ex->uri));
+		write_in_full(1, "\n", 1);
+	}
+}
+
 static const char no_split_warning[] = N_(
 "disabling bitmap writing, packs are split due to pack.packSizeLimit"
 );
@@ -1124,6 +1150,12 @@ static int want_object_in_pack(const struct object_id *oid,
 		}
 	}
 
+	if (exclude_configured_blobs &&
+	    oidmap_get(&configured_exclusions, oid)) {
+		oidset_insert(&excluded_by_config, oid);
+		return 0;
+	}
+
 	return 1;
 }
 
@@ -2728,6 +2760,19 @@ static int git_pack_config(const char *k, const char *v, void *cb)
 			    pack_idx_opts.version);
 		return 0;
 	}
+	if (!strcmp(k, "uploadpack.blobpackfileuri")) {
+		struct configured_exclusion *ex = xmalloc(sizeof(*ex));
+		const char *end;
+
+		if (parse_oid_hex(v, &ex->e.oid, &end) || *end != ' ')
+			die(_("value of uploadpack.blobpackfileuri must be "
+			      "of the form '<sha-1> <uri>' (got '%s')"), v);
+		if (oidmap_get(&configured_exclusions, &ex->e.oid))
+			die(_("object already configured in another "
+			      "uploadpack.blobpackfileuri (got '%s')"), v);
+		ex->uri = xstrdup(end + 1);
+		oidmap_put(&configured_exclusions, ex);
+	}
 	return git_default_config(k, v, cb);
 }
 
@@ -3316,6 +3361,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 			 N_("do not pack objects in promisor packfiles")),
 		OPT_BOOL(0, "delta-islands", &use_delta_islands,
 			 N_("respect islands during delta compression")),
+		OPT_BOOL(0, "exclude-configured-blobs", &exclude_configured_blobs,
+			 N_("respect uploadpack.blobpackfileuri")),
 		OPT_END(),
 	};
 
@@ -3489,6 +3536,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		return 0;
 	if (nr_result)
 		prepare_pack(window, depth);
+	write_excluded_by_configs();
 	write_pack_file();
 	if (progress)
 		fprintf_ln(stderr,
diff --git a/fetch-pack.c b/fetch-pack.c
index 4618568fee..79af87b2cf 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -1429,6 +1429,15 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 				receive_wanted_refs(&reader, sought, nr_sought);
 
 			/* get the pack */
+			if (process_section_header(&reader, "packfile-uris", 1)) {
+				/* skip the whole section */
+				process_section_header(&reader, "packfile-uris", 0);
+				while (packet_reader_read(&reader) == PACKET_READ_NORMAL) {
+					/* do nothing */
+				}
+				if (reader.status != PACKET_READ_DELIM)
+					die("expected DELIM");
+			}
 			process_section_header(&reader, "packfile", 0);
 			if (get_pack(args, fd, pack_lockfile))
 				die(_("git fetch-pack: fetch failed."));
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index b491c62e3e..ba85ee4dd9 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -588,6 +588,33 @@ test_expect_success 'when server does not send "ready", expect FLUSH' '
 	test_i18ngrep "expected no other sections to be sent after no .ready." err
 '
 
+test_expect_success 'part of packfile response provided as URI' '
+	rm -rf "$HTTPD_DOCUMENT_ROOT_PATH/http_parent" http_child log &&
+
+	git init "$HTTPD_DOCUMENT_ROOT_PATH/http_parent" &&
+	echo my-blob >"$HTTPD_DOCUMENT_ROOT_PATH/http_parent/my-blob" &&
+	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent/" add my-blob &&
+	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent/" commit -m x &&
+
+	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent/" hash-object my-blob >h &&
+	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent/" config \
+		"uploadpack.blobpackfileuri" \
+		"$(cat h) https://example.com/a-uri" &&
+	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent/" config \
+		"uploadpack.allowsidebandall" "true" &&
+
+	# NEEDSWORK: "git clone" fails here because it ignores the URI provided
+	# instead of fetching it.
+	test_must_fail env GIT_TRACE_PACKET="$(pwd)/log" \
+		git -c protocol.version=2 clone \
+		"$HTTPD_URL/smart/http_parent" http_child 2>err &&
+	# Although "git clone" fails, we can still check that the server
+	# provided the URI we requested and that the error message pinpoints
+	# the object that is missing.
+	grep "clone< .*uri https://example.com/a-uri" log &&
+	test_i18ngrep "did not receive expected object $(cat h)" err
+'
+
 stop_httpd
 
 test_done
diff --git a/upload-pack.c b/upload-pack.c
index c87b752550..fe6e2300e3 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -106,9 +106,12 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 struct output_state {
 	char buffer[8193];
 	int used;
+	unsigned packfile_uris_started : 1;
+	unsigned packfile_started : 1;
 };
 
-static int read_pack_objects_stdout(int outfd, struct output_state *os)
+static int read_pack_objects_stdout(int outfd, struct output_state *os,
+				    int write_packfile_line)
 {
 	/* Data ready; we keep the last byte to ourselves
 	 * in case we detect broken rev-list, so that we
@@ -128,6 +131,37 @@ static int read_pack_objects_stdout(int outfd, struct output_state *os)
 	}
 	os->used += readsz;
 
+	while (!os->packfile_started) {
+		char *p;
+		if (os->used >= 4 && !memcmp(os->buffer, "PACK", 4)) {
+			os->packfile_started = 1;
+			if (write_packfile_line) {
+				if (os->packfile_uris_started)
+					packet_delim(1);
+				packet_write_fmt(1, "\1packfile\n");
+			}
+			break;
+		}
+		if ((p = memchr(os->buffer, '\n', os->used))) {
+			if (!os->packfile_uris_started) {
+				os->packfile_uris_started = 1;
+				if (!write_packfile_line)
+					BUG("packfile_uris requires sideband-all");
+				packet_write_fmt(1, "\1packfile-uris\n");
+			}
+			*p = '\0';
+			packet_write_fmt(1, "\1uri %s\n", os->buffer);
+
+			os->used -= p - os->buffer + 1;
+			memmove(os->buffer, p, os->used);
+		} else {
+			/*
+			 * Incomplete line.
+			 */
+			return readsz;
+		}
+	}
+
 	if (os->used > 1) {
 		send_client_data(1, os->buffer, os->used - 1);
 		os->buffer[0] = os->buffer[os->used - 1];
@@ -141,7 +175,8 @@ static int read_pack_objects_stdout(int outfd, struct output_state *os)
 }
 
 static void create_pack_file(const struct object_array *have_obj,
-			     const struct object_array *want_obj)
+			     const struct object_array *want_obj,
+			     int write_packfile_line)
 {
 	struct child_process pack_objects = CHILD_PROCESS_INIT;
 	struct output_state output_state = {0};
@@ -189,6 +224,9 @@ static void create_pack_file(const struct object_array *have_obj,
 					 filter_options.filter_spec);
 		}
 	}
+	if (write_packfile_line)
+		argv_array_push(&pack_objects.args,
+				"--exclude-configured-blobs");
 
 	pack_objects.in = -1;
 	pack_objects.out = -1;
@@ -275,8 +313,8 @@ static void create_pack_file(const struct object_array *have_obj,
 		}
 		if (0 <= pu && (pfd[pu].revents & (POLLIN|POLLHUP))) {
 			int result = read_pack_objects_stdout(pack_objects.out,
-							      &output_state);
-
+							      &output_state,
+							      write_packfile_line);
 			if (result == 0) {
 				close(pack_objects.out);
 				pack_objects.out = -1;
@@ -1108,7 +1146,7 @@ void upload_pack(struct upload_pack_options *options)
 	if (want_obj.nr) {
 		struct object_array have_obj = OBJECT_ARRAY_INIT;
 		get_common_commits(&reader, &have_obj, &want_obj);
-		create_pack_file(&have_obj, &want_obj);
+		create_pack_file(&have_obj, &want_obj, 0);
 	}
 }
 
@@ -1498,8 +1536,12 @@ int upload_pack_v2(struct repository *r, struct argv_array *keys,
 			send_wanted_ref_info(&data);
 			send_shallow_info(&data, &want_obj);
 
-			packet_writer_write(&data.writer, "packfile\n");
-			create_pack_file(&have_obj, &want_obj);
+			if (data.writer.use_sideband) {
+				create_pack_file(&have_obj, &want_obj, 1);
+			} else {
+				packet_write_fmt(1, "packfile\n");
+				create_pack_file(&have_obj, &want_obj, 0);
+			}
 			state = FETCH_DONE;
 			break;
 		case FETCH_DONE:

pkt-line.c, sideband.c, sideband.h

diff --git a/pkt-line.c b/pkt-line.c
index 71b17e6b93..69162f3990 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -447,16 +447,16 @@ int recv_sideband(const char *me, int in_stream, int out)
 
 	while (1) {
 		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX, 0);
-		retval = diagnose_sideband(me, buf, len, 0);
+		retval = demultiplex_sideband(me, buf, len, 0);
 		switch (retval) {
-			case SIDEBAND_PRIMARY:
-				write_or_die(out, buf + 1, len - 1);
-				break;
-			case SIDEBAND_PROGRESS:
-				/* already written by diagnose_sideband() */
-				break;
-			default: /* flush or error */
-				return retval;
+		case SIDEBAND_PRIMARY:
+			write_or_die(out, buf + 1, len - 1);
+			break;
+		case SIDEBAND_PROGRESS:
+			/* already written by demultiplex_sideband() */
+			break;
+		default: /* errors: message already written */
+			return retval;
 		}
 	}
 }
@@ -474,6 +474,7 @@ void packet_reader_init(struct packet_reader *reader, int fd,
 	reader->buffer = packet_buffer;
 	reader->buffer_size = sizeof(packet_buffer);
 	reader->options = options;
+	reader->me = "git";
 }
 
 enum packet_read_status packet_reader_read(struct packet_reader *reader)
@@ -483,6 +484,10 @@ enum packet_read_status packet_reader_read(struct packet_reader *reader)
 		return reader->status;
 	}
 
+	/*
+	 * Consume all progress and keepalive packets until a primary payload
+	 * packet is received
+	 */
 	while (1) {
 		int retval;
 		reader->status = packet_read_with_status(reader->fd,
@@ -494,29 +499,31 @@ enum packet_read_status packet_reader_read(struct packet_reader *reader)
 							 reader->options);
 		if (!reader->use_sideband)
 			break;
-		retval = diagnose_sideband(reader->me, reader->buffer,
-					   reader->pktlen, 1);
+		retval = demultiplex_sideband(reader->me, reader->buffer,
+					      reader->pktlen, 1);
 		switch (retval) {
-			case SIDEBAND_PROTOCOL_ERROR:
-			case SIDEBAND_REMOTE_ERROR:
-				BUG("should have died in diagnose_sideband");
-			case SIDEBAND_FLUSH:
-				goto nonprogress;
-			case SIDEBAND_PRIMARY:
-				if (reader->pktlen != 1)
-					goto nonprogress;
-				/*
-				 * Since pktlen is 1, this is a keepalive
-				 * packet. Wait for the next one.
-				 */
-				break;
-			default: /* SIDEBAND_PROGRESS */
-				;
+		case SIDEBAND_PROTOCOL_ERROR:
+		case SIDEBAND_REMOTE_ERROR:
+			BUG("should have died in diagnose_sideband");
+		case SIDEBAND_FLUSH:
+			goto nonprogress_received;
+		case SIDEBAND_PRIMARY:
+			if (reader->pktlen != 1)
+				goto nonprogress_received;
+			/*
+			 * Since the packet contains nothing but the sideband
+			 * designator, this is a keepalive packet. Wait for the
+			 * next one.
+			 */
+			break;
+		default: /* SIDEBAND_PROGRESS */
+			;
 		}
 	}
 
-nonprogress:
+nonprogress_received:
 	if (reader->status == PACKET_READ_NORMAL)
+		/* Skip the sideband designator if sideband is used */
 		reader->line = reader->use_sideband ?
 			reader->buffer + 1 : reader->buffer;
 	else
@@ -549,7 +556,7 @@ void packet_writer_write(struct packet_writer *writer, const char *fmt, ...)
 
 	va_start(args, fmt);
 	packet_write_fmt_1(writer->dest_fd, 0,
-			   writer->use_sideband ? "\1" : "", fmt, args);
+			   writer->use_sideband ? "\001" : "", fmt, args);
 	va_end(args);
 }
 
@@ -559,7 +566,7 @@ void packet_writer_error(struct packet_writer *writer, const char *fmt, ...)
 
 	va_start(args, fmt);
 	packet_write_fmt_1(writer->dest_fd, 0,
-			   writer->use_sideband ? "\3" : "ERR ", fmt, args);
+			   writer->use_sideband ? "\003" : "ERR ", fmt, args);
 	va_end(args);
 }
 
diff --git a/sideband.c b/sideband.c
index cbeab380ae..9d3051e3fe 100644
--- a/sideband.c
+++ b/sideband.c
@@ -113,7 +113,7 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
 #define ANSI_SUFFIX "\033[K"
 #define DUMB_SUFFIX "        "
 
-int diagnose_sideband(const char *me, char *buf, int len, int die_on_error)
+int demultiplex_sideband(const char *me, char *buf, int len, int die_on_error)
 {
 	static const char *suffix;
 	struct strbuf outbuf = STRBUF_INIT;
diff --git a/sideband.h b/sideband.h
index 3786670694..f75c4fde2a 100644
--- a/sideband.h
+++ b/sideband.h
@@ -8,15 +8,14 @@
 #define SIDEBAND_PROGRESS 2
 
 /*
- * buf and len should be the result of reading a line from a remote sending
- * multiplexed data.
+ * Inspects a multiplexed packet read from the remote and returns which
+ * sideband it is for.
  *
- * Determines the nature of the result and returns it. If
- * SIDEBAND_PROTOCOL_ERROR, SIDEBAND_REMOTE_ERROR, or SIDEBAND_PROGRESS, also
- * prints a message (or the formatted contents of the notice in the case of
- * SIDEBAND_PROGRESS) to stderr.
+ * If SIDEBAND_PROTOCOL_ERROR, SIDEBAND_REMOTE_ERROR, or SIDEBAND_PROGRESS,
+ * also prints a message (or the formatted contents of the notice in the case
+ * of SIDEBAND_PROGRESS) to stderr.
  */
-int diagnose_sideband(const char *me, char *buf, int len, int die_on_error);
+int demultiplex_sideband(const char *me, char *buf, int len, int die_on_error);
 
 void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_max);
 

pkt-line.c, pkt-line.h, upload-pack.c

diff --git a/pkt-line.c b/pkt-line.c
index e70ea6d88f..9d3e402d41 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -129,12 +129,14 @@ static void set_packet_header(char *buf, const int size)
 	#undef hex
 }
 
-static void format_packet(struct strbuf *out, const char *fmt, va_list args)
+static void format_packet(struct strbuf *out, const char *prefix,
+			  const char *fmt, va_list args)
 {
 	size_t orig_len, n;
 
 	orig_len = out->len;
 	strbuf_addstr(out, "0000");
+	strbuf_addstr(out, prefix);
 	strbuf_vaddf(out, fmt, args);
 	n = out->len - orig_len;
 
@@ -145,13 +147,13 @@ static void format_packet(struct strbuf *out, const char *fmt, va_list args)
 	packet_trace(out->buf + orig_len + 4, n - 4, 1);
 }
 
-static int packet_write_fmt_1(int fd, int gently,
+static int packet_write_fmt_1(int fd, int gently, const char *prefix,
 			      const char *fmt, va_list args)
 {
 	static struct strbuf buf = STRBUF_INIT;
 
 	strbuf_reset(&buf);
-	format_packet(&buf, fmt, args);
+	format_packet(&buf, prefix, fmt, args);
 	if (write_in_full(fd, buf.buf, buf.len) < 0) {
 		if (!gently) {
 			check_pipe(errno);
@@ -168,7 +170,7 @@ void packet_write_fmt(int fd, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	packet_write_fmt_1(fd, 0, fmt, args);
+	packet_write_fmt_1(fd, 0, "", fmt, args);
 	va_end(args);
 }
 
@@ -178,7 +180,7 @@ int packet_write_fmt_gently(int fd, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	status = packet_write_fmt_1(fd, 1, fmt, args);
+	status = packet_write_fmt_1(fd, 1, "", fmt, args);
 	va_end(args);
 	return status;
 }
@@ -211,7 +213,7 @@ void packet_buf_write(struct strbuf *buf, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	format_packet(buf, fmt, args);
+	format_packet(buf, "", fmt, args);
 	va_end(args);
 }
 
@@ -486,3 +488,36 @@ enum packet_read_status packet_reader_peek(struct packet_reader *reader)
 	reader->line_peeked = 1;
 	return reader->status;
 }
+
+void packet_writer_init(struct packet_writer *writer, int dest_fd)
+{
+	writer->dest_fd = dest_fd;
+}
+
+void packet_writer_write(struct packet_writer *writer, const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	packet_write_fmt_1(writer->dest_fd, 0, "", fmt, args);
+	va_end(args);
+}
+
+void packet_writer_error(struct packet_writer *writer, const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	packet_write_fmt_1(writer->dest_fd, 0, "ERR ", fmt, args);
+	va_end(args);
+}
+
+void packet_writer_delim(struct packet_writer *writer)
+{
+	packet_delim(writer->dest_fd);
+}
+
+void packet_writer_flush(struct packet_writer *writer)
+{
+	packet_flush(writer->dest_fd);
+}
diff --git a/pkt-line.h b/pkt-line.h
index d7e1dbc047..023ad2951d 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -183,4 +183,18 @@ extern enum packet_read_status packet_reader_peek(struct packet_reader *reader);
 #define LARGE_PACKET_DATA_MAX (LARGE_PACKET_MAX - 4)
 extern char packet_buffer[LARGE_PACKET_MAX];
 
+struct packet_writer {
+	int dest_fd;
+};
+
+void packet_writer_init(struct packet_writer *writer, int dest_fd);
+
+/* These functions die upon failure. */
+__attribute__((format (printf, 2, 3)))
+void packet_writer_write(struct packet_writer *writer, const char *fmt, ...);
+__attribute__((format (printf, 2, 3)))
+void packet_writer_error(struct packet_writer *writer, const char *fmt, ...);
+void packet_writer_delim(struct packet_writer *writer);
+void packet_writer_flush(struct packet_writer *writer);
+
 #endif
diff --git a/upload-pack.c b/upload-pack.c
index 08b547cf01..60a26bbbfd 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -613,13 +613,14 @@ static void check_non_tip(struct object_array *want_obj)
 	}
 }
 
-static void send_shallow(struct commit_list *result)
+static void send_shallow(struct packet_writer *writer,
+			 struct commit_list *result)
 {
 	while (result) {
 		struct object *object = &result->item->object;
 		if (!(object->flags & (CLIENT_SHALLOW|NOT_SHALLOW))) {
-			packet_write_fmt(1, "shallow %s",
-					 oid_to_hex(&object->oid));
+			packet_writer_write(writer, "shallow %s",
+					    oid_to_hex(&object->oid));
 			register_shallow(the_repository, &object->oid);
 			shallow_nr++;
 		}
@@ -627,7 +628,8 @@ static void send_shallow(struct commit_list *result)
 	}
 }
 
-static void send_unshallow(const struct object_array *shallows,
+static void send_unshallow(struct packet_writer *writer,
+			   const struct object_array *shallows,
 			   struct object_array *want_obj)
 {
 	int i;
@@ -636,8 +638,8 @@ static void send_unshallow(const struct object_array *shallows,
 		struct object *object = shallows->objects[i].item;
 		if (object->flags & NOT_SHALLOW) {
 			struct commit_list *parents;
-			packet_write_fmt(1, "unshallow %s",
-					 oid_to_hex(&object->oid));
+			packet_writer_write(writer, "unshallow %s",
+					    oid_to_hex(&object->oid));
 			object->flags &= ~CLIENT_SHALLOW;
 			/*
 			 * We want to _register_ "object" as shallow, but we
@@ -662,7 +664,7 @@ static void send_unshallow(const struct object_array *shallows,
 	}
 }
 
-static void deepen(int depth, int deepen_relative,
+static void deepen(struct packet_writer *writer, int depth, int deepen_relative,
 		   struct object_array *shallows, struct object_array *want_obj)
 {
 	if (depth == INFINITE_DEPTH && !is_repository_shallow(the_repository)) {
@@ -680,7 +682,7 @@ static void deepen(int depth, int deepen_relative,
 		result = get_shallow_commits(&reachable_shallows,
 					     depth + 1,
 					     SHALLOW, NOT_SHALLOW);
-		send_shallow(result);
+		send_shallow(writer, result);
 		free_commit_list(result);
 		object_array_clear(&reachable_shallows);
 	} else {
@@ -688,14 +690,15 @@ static void deepen(int depth, int deepen_relative,
 
 		result = get_shallow_commits(want_obj, depth,
 					     SHALLOW, NOT_SHALLOW);
-		send_shallow(result);
+		send_shallow(writer, result);
 		free_commit_list(result);
 	}
 
-	send_unshallow(shallows, want_obj);
+	send_unshallow(writer, shallows, want_obj);
 }
 
-static void deepen_by_rev_list(int ac, const char **av,
+static void deepen_by_rev_list(struct packet_writer *writer, int ac,
+			       const char **av,
 			       struct object_array *shallows,
 			       struct object_array *want_obj)
 {
@@ -703,13 +706,14 @@ static void deepen_by_rev_list(int ac, const char **av,
 
 	close_commit_graph(the_repository);
 	result = get_shallow_commits_by_rev_list(ac, av, SHALLOW, NOT_SHALLOW);
-	send_shallow(result);
+	send_shallow(writer, result);
 	free_commit_list(result);
-	send_unshallow(shallows, want_obj);
+	send_unshallow(writer, shallows, want_obj);
 }
 
 /* Returns 1 if a shallow list is sent or 0 otherwise */
-static int send_shallow_list(int depth, int deepen_rev_list,
+static int send_shallow_list(struct packet_writer *writer,
+			     int depth, int deepen_rev_list,
 			     timestamp_t deepen_since,
 			     struct string_list *deepen_not,
 			     struct object_array *shallows,
@@ -720,7 +724,7 @@ static int send_shallow_list(int depth, int deepen_rev_list,
 	if (depth > 0 && deepen_rev_list)
 		die("git upload-pack: deepen and deepen-since (or deepen-not) cannot be used together");
 	if (depth > 0) {
-		deepen(depth, deepen_relative, shallows, want_obj);
+		deepen(writer, depth, deepen_relative, shallows, want_obj);
 		ret = 1;
 	} else if (deepen_rev_list) {
 		struct argv_array av = ARGV_ARRAY_INIT;
@@ -741,7 +745,7 @@ static int send_shallow_list(int depth, int deepen_rev_list,
 			struct object *o = want_obj->objects[i].item;
 			argv_array_push(&av, oid_to_hex(&o->oid));
 		}
-		deepen_by_rev_list(av.argc, av.argv, shallows, want_obj);
+		deepen_by_rev_list(writer, av.argc, av.argv, shallows, want_obj);
 		argv_array_clear(&av);
 		ret = 1;
 	} else {
@@ -834,8 +838,10 @@ static void receive_needs(struct packet_reader *reader, struct object_array *wan
 	int has_non_tip = 0;
 	timestamp_t deepen_since = 0;
 	int deepen_rev_list = 0;
+	struct packet_writer writer;
 
 	shallow_nr = 0;
+	packet_writer_init(&writer, 1);
 	for (;;) {
 		struct object *o;
 		const char *features;
@@ -892,9 +898,9 @@ static void receive_needs(struct packet_reader *reader, struct object_array *wan
 
 		o = parse_object(the_repository, &oid_buf);
 		if (!o) {
-			packet_write_fmt(1,
-					 "ERR upload-pack: not our ref %s",
-					 oid_to_hex(&oid_buf));
+			packet_writer_error(&writer,
+					    "upload-pack: not our ref %s",
+					    oid_to_hex(&oid_buf));
 			die("git upload-pack: not our ref %s",
 			    oid_to_hex(&oid_buf));
 		}
@@ -923,7 +929,7 @@ static void receive_needs(struct packet_reader *reader, struct object_array *wan
 	if (depth == 0 && !deepen_rev_list && shallows.nr == 0)
 		return;
 
-	if (send_shallow_list(depth, deepen_rev_list, deepen_since,
+	if (send_shallow_list(&writer, depth, deepen_rev_list, deepen_since,
 			      &deepen_not, &shallows, want_obj))
 		packet_flush(1);
 	object_array_clear(&shallows);
@@ -1102,6 +1108,8 @@ struct upload_pack_data {
 	int deepen_rev_list;
 	int deepen_relative;
 
+	struct packet_writer writer;
+
 	unsigned stateless_rpc : 1;
 
 	unsigned use_thin_pack : 1;
@@ -1125,6 +1133,7 @@ static void upload_pack_data_init(struct upload_pack_data *data)
 	data->haves = haves;
 	data->shallows = shallows;
 	data->deepen_not = deepen_not;
+	packet_writer_init(&data->writer, 1);
 }
 
 static void upload_pack_data_clear(struct upload_pack_data *data)
@@ -1136,7 +1145,8 @@ static void upload_pack_data_clear(struct upload_pack_data *data)
 	string_list_clear(&data->deepen_not, 0);
 }
 
-static int parse_want(const char *line, struct object_array *want_obj)
+static int parse_want(struct packet_writer *writer, const char *line,
+		      struct object_array *want_obj)
 {
 	const char *arg;
 	if (skip_prefix(line, "want ", &arg)) {
@@ -1149,9 +1159,9 @@ static int parse_want(const char *line, struct object_array *want_obj)
 
 		o = parse_object(the_repository, &oid);
 		if (!o) {
-			packet_write_fmt(1,
-					 "ERR upload-pack: not our ref %s",
-					 oid_to_hex(&oid));
+			packet_writer_error(writer,
+					    "upload-pack: not our ref %s",
+					    oid_to_hex(&oid));
 			die("git upload-pack: not our ref %s",
 			    oid_to_hex(&oid));
 		}
@@ -1167,7 +1177,8 @@ static int parse_want(const char *line, struct object_array *want_obj)
 	return 0;
 }
 
-static int parse_want_ref(const char *line, struct string_list *wanted_refs,
+static int parse_want_ref(struct packet_writer *writer, const char *line,
+			  struct string_list *wanted_refs,
 			  struct object_array *want_obj)
 {
 	const char *arg;
@@ -1177,7 +1188,7 @@ static int parse_want_ref(const char *line, struct string_list *wanted_refs,
 		struct object *o;
 
 		if (read_ref(arg, &oid)) {
-			packet_write_fmt(1, "ERR unknown ref %s", arg);
+			packet_writer_error(writer, "unknown ref %s", arg);
 			die("unknown ref %s", arg);
 		}
 
@@ -1220,10 +1231,11 @@ static void process_args(struct packet_reader *request,
 		const char *p;
 
 		/* process want */
-		if (parse_want(arg, want_obj))
+		if (parse_want(&data->writer, arg, want_obj))
 			continue;
 		if (allow_ref_in_want &&
-		    parse_want_ref(arg, &data->wanted_refs, want_obj))
+		    parse_want_ref(&data->writer, arg, &data->wanted_refs,
+				   want_obj))
 			continue;
 		/* process have line */
 		if (parse_have(arg, &data->haves))
@@ -1317,26 +1329,26 @@ static int process_haves(struct oid_array *haves, struct oid_array *common,
 	return 0;
 }
 
-static int send_acks(struct oid_array *acks, struct strbuf *response,
+static int send_acks(struct packet_writer *writer, struct oid_array *acks,
 		     const struct object_array *have_obj,
 		     struct object_array *want_obj)
 {
 	int i;
 
-	packet_buf_write(response, "acknowledgments\n");
+	packet_writer_write(writer, "acknowledgments\n");
 
 	/* Send Acks */
 	if (!acks->nr)
-		packet_buf_write(response, "NAK\n");
+		packet_writer_write(writer, "NAK\n");
 
 	for (i = 0; i < acks->nr; i++) {
-		packet_buf_write(response, "ACK %s\n",
-				 oid_to_hex(&acks->oid[i]));
+		packet_writer_write(writer, "ACK %s\n",
+				    oid_to_hex(&acks->oid[i]));
 	}
 
 	if (ok_to_give_up(have_obj, want_obj)) {
 		/* Send Ready */
-		packet_buf_write(response, "ready\n");
+		packet_writer_write(writer, "ready\n");
 		return 1;
 	}
 
@@ -1348,25 +1360,20 @@ static int process_haves_and_send_acks(struct upload_pack_data *data,
 				       struct object_array *want_obj)
 {
 	struct oid_array common = OID_ARRAY_INIT;
-	struct strbuf response = STRBUF_INIT;
 	int ret = 0;
 
 	process_haves(&data->haves, &common, have_obj);
 	if (data->done) {
 		ret = 1;
-	} else if (send_acks(&common, &response, have_obj, want_obj)) {
-		packet_buf_delim(&response);
+	} else if (send_acks(&data->writer, &common, have_obj, want_obj)) {
+		packet_writer_delim(&data->writer);
 		ret = 1;
 	} else {
 		/* Add Flush */
-		packet_buf_flush(&response);
+		packet_writer_flush(&data->writer);
 		ret = 0;
 	}
 
-	/* Send response */
-	write_or_die(1, response.buf, response.len);
-	strbuf_release(&response);
-
 	oid_array_clear(&data->haves);
 	oid_array_clear(&common);
 	return ret;
@@ -1379,15 +1386,15 @@ static void send_wanted_ref_info(struct upload_pack_data *data)
 	if (!data->wanted_refs.nr)
 		return;
 
-	packet_write_fmt(1, "wanted-refs\n");
+	packet_writer_write(&data->writer, "wanted-refs\n");
 
 	for_each_string_list_item(item, &data->wanted_refs) {
-		packet_write_fmt(1, "%s %s\n",
-				 oid_to_hex(item->util),
-				 item->string);
+		packet_writer_write(&data->writer, "%s %s\n",
+				    oid_to_hex(item->util),
+				    item->string);
 	}
 
-	packet_delim(1);
+	packet_writer_delim(&data->writer);
 }
 
 static void send_shallow_info(struct upload_pack_data *data,
@@ -1398,14 +1405,15 @@ static void send_shallow_info(struct upload_pack_data *data,
 	    !is_repository_shallow(the_repository))
 		return;
 
-	packet_write_fmt(1, "shallow-info\n");
+	packet_writer_write(&data->writer, "shallow-info\n");
 
-	if (!send_shallow_list(data->depth, data->deepen_rev_list,
+	if (!send_shallow_list(&data->writer, data->depth,
+			       data->deepen_rev_list,
 			       data->deepen_since, &data->deepen_not,
 			       &data->shallows, want_obj) &&
 	    is_repository_shallow(the_repository))
-		deepen(INFINITE_DEPTH, data->deepen_relative, &data->shallows,
-		       want_obj);
+		deepen(&data->writer, INFINITE_DEPTH, data->deepen_relative,
+		       &data->shallows, want_obj);
 
 	packet_delim(1);
 }
@@ -1467,7 +1475,7 @@ int upload_pack_v2(struct repository *r, struct argv_array *keys,
 			send_wanted_ref_info(&data);
 			send_shallow_info(&data, &want_obj);
 
-			packet_write_fmt(1, "packfile\n");
+			packet_writer_write(&data.writer, "packfile\n");
 			create_pack_file(&have_obj, &want_obj);
 			state = FETCH_DONE;
 			break;

pkt-line.c, pkt-line.h, sideband.c, sideband.h

diff --git a/pkt-line.c b/pkt-line.c
index 9d3e402d41..5feb73ebec 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -439,6 +439,28 @@ ssize_t read_packetized_to_strbuf(int fd_in, struct strbuf *sb_out)
 	return sb_out->len - orig_len;
 }
 
+int recv_sideband(const char *me, int in_stream, int out)
+{
+	char buf[LARGE_PACKET_MAX + 1];
+	int retval = 0;
+	int len;
+
+	while (1) {
+		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX, 0);
+		retval = demultiplex_sideband(me, buf, len);
+		switch (retval) {
+		case SIDEBAND_PRIMARY:
+			write_or_die(out, buf + 1, len - 1);
+			break;
+		case SIDEBAND_PROGRESS:
+			/* already written by demultiplex_sideband() */
+			break;
+		default: /* errors: message already written */
+			return retval;
+		}
+	}
+}
+
 /* Packet Reader Functions */
 void packet_reader_init(struct packet_reader *reader, int fd,
 			char *src_buffer, size_t src_len,
diff --git a/pkt-line.h b/pkt-line.h
index 023ad2951d..a8e92a4b63 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -3,6 +3,7 @@
 
 #include "git-compat-util.h"
 #include "strbuf.h"
+#include "sideband.h"
 
 /*
  * Write a packetized stream, where each line is preceded by
@@ -120,6 +121,21 @@ char *packet_read_line_buf(char **src_buf, size_t *src_len, int *size);
  */
 ssize_t read_packetized_to_strbuf(int fd_in, struct strbuf *sb_out);
 
+/*
+ * Receive multiplexed output stream over git native protocol.
+ * in_stream is the input stream from the remote, which carries data
+ * in pkt_line format with band designator.  Demultiplex it into out
+ * and err and return error appropriately.  Band #1 carries the
+ * primary payload.  Things coming over band #2 is not necessarily
+ * error; they are usually informative message on the standard error
+ * stream, aka "verbose").  A message over band #3 is a signal that
+ * the remote died unexpectedly.  A flush() concludes the stream.
+ *
+ * Returns SIDEBAND_FLUSH upon a normal conclusion, and SIDEBAND_PROTOCOL_ERROR
+ * or SIDEBAND_REMOTE_ERROR if an error occurred.
+ */
+int recv_sideband(const char *me, int in_stream, int out);
+
 struct packet_reader {
 	/* source file descriptor */
 	int fd;
diff --git a/sideband.c b/sideband.c
index 368647acf8..dce22d20b1 100644
--- a/sideband.c
+++ b/sideband.c
@@ -1,7 +1,6 @@
 #include "cache.h"
 #include "color.h"
 #include "config.h"
-#include "pkt-line.h"
 #include "sideband.h"
 #include "help.h"
 
@@ -109,103 +108,94 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
 }
 
 
-/*
- * Receive multiplexed output stream over git native protocol.
- * in_stream is the input stream from the remote, which carries data
- * in pkt_line format with band designator.  Demultiplex it into out
- * and err and return error appropriately.  Band #1 carries the
- * primary payload.  Things coming over band #2 is not necessarily
- * error; they are usually informative message on the standard error
- * stream, aka "verbose").  A message over band #3 is a signal that
- * the remote died unexpectedly.  A flush() concludes the stream.
- */
-
 #define DISPLAY_PREFIX "remote: "
 
 #define ANSI_SUFFIX "\033[K"
 #define DUMB_SUFFIX "        "
 
-int recv_sideband(const char *me, int in_stream, int out)
+int demultiplex_sideband(const char *me, char *buf, int len)
 {
-	const char *suffix;
-	char buf[LARGE_PACKET_MAX + 1];
+	static const char *suffix;
 	struct strbuf outbuf = STRBUF_INIT;
 	int retval = 0;
+	const char *b, *brk;
+	int band;
+
+	if (!suffix) {
+		if (isatty(2) && !is_terminal_dumb())
+			suffix = ANSI_SUFFIX;
+		else
+			suffix = DUMB_SUFFIX;
+	}
 
-	if (isatty(2) && !is_terminal_dumb())
-		suffix = ANSI_SUFFIX;
-	else
-		suffix = DUMB_SUFFIX;
+	if (len == 0) {
+		retval = SIDEBAND_FLUSH;
+		goto cleanup;
+	}
+	if (len < 1) {
+		strbuf_addf(&outbuf,
+			    "%s%s: protocol error: no band designator",
+			    outbuf.len ? "\n" : "", me);
+		retval = SIDEBAND_PROTOCOL_ERROR;
+		goto cleanup;
+	}
+	band = buf[0] & 0xff;
+	buf[len] = '\0';
+	len--;
+	switch (band) {
+	case 3:
+		strbuf_addf(&outbuf, "%s%s", outbuf.len ? "\n" : "",
+			    DISPLAY_PREFIX);
+		maybe_colorize_sideband(&outbuf, buf + 1, len);
+
+		retval = SIDEBAND_REMOTE_ERROR;
+		break;
+	case 2:
+		b = buf + 1;
 
-	while (!retval) {
-		const char *b, *brk;
-		int band, len;
-		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX, 0);
-		if (len == 0)
-			break;
-		if (len < 1) {
-			strbuf_addf(&outbuf,
-				    "%s%s: protocol error: no band designator",
-				    outbuf.len ? "\n" : "", me);
-			retval = SIDEBAND_PROTOCOL_ERROR;
-			break;
-		}
-		band = buf[0] & 0xff;
-		buf[len] = '\0';
-		len--;
-		switch (band) {
-		case 3:
-			strbuf_addf(&outbuf, "%s%s", outbuf.len ? "\n" : "",
-				    DISPLAY_PREFIX);
-			maybe_colorize_sideband(&outbuf, buf + 1, len);
-
-			retval = SIDEBAND_REMOTE_ERROR;
-			break;
-		case 2:
-			b = buf + 1;
-
-			/*
-			 * Append a suffix to each nonempty line to clear the
-			 * end of the screen line.
-			 *
-			 * The output is accumulated in a buffer and
-			 * each line is printed to stderr using
-			 * write(2) to ensure inter-process atomicity.
-			 */
-			while ((brk = strpbrk(b, "\n\r"))) {
-				int linelen = brk - b;
-
-				if (!outbuf.len)
-					strbuf_addstr(&outbuf, DISPLAY_PREFIX);
-				if (linelen > 0) {
-					maybe_colorize_sideband(&outbuf, b, linelen);
-					strbuf_addstr(&outbuf, suffix);
-				}
-
-				strbuf_addch(&outbuf, *brk);
-				xwrite(2, outbuf.buf, outbuf.len);
-				strbuf_reset(&outbuf);
-
-				b = brk + 1;
+		/*
+		 * Append a suffix to each nonempty line to clear the
+		 * end of the screen line.
+		 *
+		 * The output is accumulated in a buffer and
+		 * each line is printed to stderr using
+		 * write(2) to ensure inter-process atomicity.
+		 */
+		while ((brk = strpbrk(b, "\n\r"))) {
+			int linelen = brk - b;
+
+			if (!outbuf.len)
+				strbuf_addstr(&outbuf, DISPLAY_PREFIX);
+			if (linelen > 0) {
+				maybe_colorize_sideband(&outbuf, b, linelen);
+				strbuf_addstr(&outbuf, suffix);
 			}
 
-			if (*b) {
-				strbuf_addstr(&outbuf, outbuf.len ?
-					    "" : DISPLAY_PREFIX);
-				maybe_colorize_sideband(&outbuf, b, strlen(b));
-			}
-			break;
-		case 1:
-			write_or_die(out, buf + 1, len);
-			break;
-		default:
-			strbuf_addf(&outbuf, "%s%s: protocol error: bad band #%d",
-				    outbuf.len ? "\n" : "", me, band);
-			retval = SIDEBAND_PROTOCOL_ERROR;
-			break;
+			strbuf_addch(&outbuf, *brk);
+			xwrite(2, outbuf.buf, outbuf.len);
+			strbuf_reset(&outbuf);
+
+			b = brk + 1;
+		}
+
+		if (*b) {
+			strbuf_addstr(&outbuf, outbuf.len ?
+				    "" : DISPLAY_PREFIX);
+			maybe_colorize_sideband(&outbuf, b, strlen(b));
 		}
+		retval = SIDEBAND_PROGRESS;
+		break;
+	case 1:
+		retval = SIDEBAND_PRIMARY;
+		break;
+	default:
+		strbuf_addf(&outbuf, "%s%s: protocol error: bad band #%d",
+			    outbuf.len ? "\n" : "", me, band);
+		retval = SIDEBAND_PROTOCOL_ERROR;
+		break;
 	}
 
+cleanup:
 	if (outbuf.len) {
 		strbuf_addch(&outbuf, '\n');
 		xwrite(2, outbuf.buf, outbuf.len);
diff --git a/sideband.h b/sideband.h
index 7a8146f161..7244971231 100644
--- a/sideband.h
+++ b/sideband.h
@@ -3,8 +3,20 @@
 
 #define SIDEBAND_PROTOCOL_ERROR -2
 #define SIDEBAND_REMOTE_ERROR -1
+#define SIDEBAND_FLUSH 0
+#define SIDEBAND_PRIMARY 1
+#define SIDEBAND_PROGRESS 2
+
+/*
+ * Inspects a multiplexed packet read from the remote and returns which
+ * sideband it is for.
+ *
+ * If SIDEBAND_PROTOCOL_ERROR, SIDEBAND_REMOTE_ERROR, or SIDEBAND_PROGRESS,
+ * also prints a message (or the formatted contents of the notice in the case
+ * of SIDEBAND_PROGRESS) to stderr.
+ */
+int demultiplex_sideband(const char *me, char *buf, int len);
 
-int recv_sideband(const char *me, int in_stream, int out);
 void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_max);
 
 #endif

Documentation/technical/protocol-v2.txt, fetch-pack.c, pkt-line.c, pkt-line.h, sideband.c, sideband.h, upload-pack.c

diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index 09e4e0273f..1b0633f59f 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -307,6 +307,16 @@ the 'wanted-refs' section in the server's response as explained below.
 	particular ref, where <ref> is the full name of a ref on the
 	server.
 
+If the 'sideband-all' feature is advertised, the following argument can be
+included in the client's request:
+
+    sideband-all
+	Instruct the server to send the whole response multiplexed, not just
+	the packfile section. All non-flush and non-delim PKT-LINE in the
+	response (not only in the packfile section) will then start with a byte
+	indicating its sideband (1, 2, or 3), and the server may send "0005\1"
+	(a PKT-LINE of sideband 1 with no payload) as a keepalive packet.
+
 The response of `fetch` is broken into a number of sections separated by
 delimiter packets (0001), with each section beginning with its section
 header.
diff --git a/fetch-pack.c b/fetch-pack.c
index 3f9626dbf7..b89199891d 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -1125,7 +1125,8 @@ static int add_haves(struct fetch_negotiator *negotiator,
 static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 			      const struct fetch_pack_args *args,
 			      const struct ref *wants, struct oidset *common,
-			      int *haves_to_send, int *in_vain)
+			      int *haves_to_send, int *in_vain,
+			      int sideband_all)
 {
 	int ret = 0;
 	struct strbuf req_buf = STRBUF_INIT;
@@ -1151,6 +1152,8 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 		packet_buf_write(&req_buf, "include-tag");
 	if (prefer_ofs_delta)
 		packet_buf_write(&req_buf, "ofs-delta");
+	if (sideband_all)
+		packet_buf_write(&req_buf, "sideband-all");
 
 	/* Add shallow-info and deepen request */
 	if (server_supports_feature("fetch", "shallow", 0))
@@ -1359,6 +1362,10 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 	packet_reader_init(&reader, fd[0], NULL, 0,
 			   PACKET_READ_CHOMP_NEWLINE |
 			   PACKET_READ_DIE_ON_ERR_PACKET);
+	if (server_supports_feature("fetch", "sideband-all", 0)) {
+		reader.use_sideband = 1;
+		reader.me = "fetch-pack";
+	}
 
 	while (state != FETCH_DONE) {
 		switch (state) {
@@ -1392,7 +1399,8 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 		case FETCH_SEND_REQUEST:
 			if (send_fetch_request(&negotiator, fd[1], args, ref,
 					       &common,
-					       &haves_to_send, &in_vain))
+					       &haves_to_send, &in_vain,
+					       reader.use_sideband))
 				state = FETCH_GET_PACK;
 			else
 				state = FETCH_PROCESS_ACKS;
diff --git a/pkt-line.c b/pkt-line.c
index 5feb73ebec..69162f3990 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -447,7 +447,7 @@ int recv_sideband(const char *me, int in_stream, int out)
 
 	while (1) {
 		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX, 0);
-		retval = demultiplex_sideband(me, buf, len);
+		retval = demultiplex_sideband(me, buf, len, 0);
 		switch (retval) {
 		case SIDEBAND_PRIMARY:
 			write_or_die(out, buf + 1, len - 1);
@@ -474,6 +474,7 @@ void packet_reader_init(struct packet_reader *reader, int fd,
 	reader->buffer = packet_buffer;
 	reader->buffer_size = sizeof(packet_buffer);
 	reader->options = options;
+	reader->me = "git";
 }
 
 enum packet_read_status packet_reader_read(struct packet_reader *reader)
@@ -483,16 +484,48 @@ enum packet_read_status packet_reader_read(struct packet_reader *reader)
 		return reader->status;
 	}
 
-	reader->status = packet_read_with_status(reader->fd,
-						 &reader->src_buffer,
-						 &reader->src_len,
-						 reader->buffer,
-						 reader->buffer_size,
-						 &reader->pktlen,
-						 reader->options);
+	/*
+	 * Consume all progress and keepalive packets until a primary payload
+	 * packet is received
+	 */
+	while (1) {
+		int retval;
+		reader->status = packet_read_with_status(reader->fd,
+							 &reader->src_buffer,
+							 &reader->src_len,
+							 reader->buffer,
+							 reader->buffer_size,
+							 &reader->pktlen,
+							 reader->options);
+		if (!reader->use_sideband)
+			break;
+		retval = demultiplex_sideband(reader->me, reader->buffer,
+					      reader->pktlen, 1);
+		switch (retval) {
+		case SIDEBAND_PROTOCOL_ERROR:
+		case SIDEBAND_REMOTE_ERROR:
+			BUG("should have died in diagnose_sideband");
+		case SIDEBAND_FLUSH:
+			goto nonprogress_received;
+		case SIDEBAND_PRIMARY:
+			if (reader->pktlen != 1)
+				goto nonprogress_received;
+			/*
+			 * Since the packet contains nothing but the sideband
+			 * designator, this is a keepalive packet. Wait for the
+			 * next one.
+			 */
+			break;
+		default: /* SIDEBAND_PROGRESS */
+			;
+		}
+	}
 
+nonprogress_received:
 	if (reader->status == PACKET_READ_NORMAL)
-		reader->line = reader->buffer;
+		/* Skip the sideband designator if sideband is used */
+		reader->line = reader->use_sideband ?
+			reader->buffer + 1 : reader->buffer;
 	else
 		reader->line = NULL;
 
@@ -514,6 +547,7 @@ enum packet_read_status packet_reader_peek(struct packet_reader *reader)
 void packet_writer_init(struct packet_writer *writer, int dest_fd)
 {
 	writer->dest_fd = dest_fd;
+	writer->use_sideband = 0;
 }
 
 void packet_writer_write(struct packet_writer *writer, const char *fmt, ...)
@@ -521,7 +555,8 @@ void packet_writer_write(struct packet_writer *writer, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	packet_write_fmt_1(writer->dest_fd, 0, "", fmt, args);
+	packet_write_fmt_1(writer->dest_fd, 0,
+			   writer->use_sideband ? "\001" : "", fmt, args);
 	va_end(args);
 }
 
@@ -530,7 +565,8 @@ void packet_writer_error(struct packet_writer *writer, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	packet_write_fmt_1(writer->dest_fd, 0, "ERR ", fmt, args);
+	packet_write_fmt_1(writer->dest_fd, 0,
+			   writer->use_sideband ? "\003" : "ERR ", fmt, args);
 	va_end(args);
 }
 
diff --git a/pkt-line.h b/pkt-line.h
index a8e92a4b63..ad9a4a2cd7 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -162,6 +162,9 @@ struct packet_reader {
 
 	/* indicates if a line has been peeked */
 	int line_peeked;
+
+	unsigned use_sideband : 1;
+	const char *me;
 };
 
 /*
@@ -201,6 +204,7 @@ extern char packet_buffer[LARGE_PACKET_MAX];
 
 struct packet_writer {
 	int dest_fd;
+	unsigned use_sideband : 1;
 };
 
 void packet_writer_init(struct packet_writer *writer, int dest_fd);
diff --git a/sideband.c b/sideband.c
index dce22d20b1..9d3051e3fe 100644
--- a/sideband.c
+++ b/sideband.c
@@ -113,7 +113,7 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
 #define ANSI_SUFFIX "\033[K"
 #define DUMB_SUFFIX "        "
 
-int demultiplex_sideband(const char *me, char *buf, int len)
+int demultiplex_sideband(const char *me, char *buf, int len, int die_on_error)
 {
 	static const char *suffix;
 	struct strbuf outbuf = STRBUF_INIT;
@@ -144,6 +144,9 @@ int demultiplex_sideband(const char *me, char *buf, int len)
 	len--;
 	switch (band) {
 	case 3:
+		if (die_on_error)
+			die("remote error: %s", buf + 1);
+
 		strbuf_addf(&outbuf, "%s%s", outbuf.len ? "\n" : "",
 			    DISPLAY_PREFIX);
 		maybe_colorize_sideband(&outbuf, buf + 1, len);
@@ -196,6 +199,8 @@ int demultiplex_sideband(const char *me, char *buf, int len)
 	}
 
 cleanup:
+	if (die_on_error && retval == SIDEBAND_PROTOCOL_ERROR)
+		die("%s", outbuf.buf);
 	if (outbuf.len) {
 		strbuf_addch(&outbuf, '\n');
 		xwrite(2, outbuf.buf, outbuf.len);
diff --git a/sideband.h b/sideband.h
index 7244971231..f75c4fde2a 100644
--- a/sideband.h
+++ b/sideband.h
@@ -15,7 +15,7 @@
  * also prints a message (or the formatted contents of the notice in the case
  * of SIDEBAND_PROGRESS) to stderr.
  */
-int demultiplex_sideband(const char *me, char *buf, int len);
+int demultiplex_sideband(const char *me, char *buf, int len, int die_on_error);
 
 void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_max);
 
diff --git a/upload-pack.c b/upload-pack.c
index 60a26bbbfd..765b7695d2 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -71,6 +71,8 @@ static int allow_filter;
 static int allow_ref_in_want;
 static struct list_objects_filter_options filter_options;
 
+static int allow_sideband_all;
+
 static void reset_timeout(void)
 {
 	alarm(timeout);
@@ -1046,6 +1048,8 @@ static int upload_pack_config(const char *var, const char *value, void *unused)
 		allow_filter = git_config_bool(var, value);
 	} else if (!strcmp("uploadpack.allowrefinwant", var)) {
 		allow_ref_in_want = git_config_bool(var, value);
+	} else if (!strcmp("uploadpack.allowsidebandall", var)) {
+		allow_sideband_all = git_config_bool(var, value);
 	}
 
 	if (current_config_scope() != CONFIG_SCOPE_REPO) {
@@ -1284,6 +1288,11 @@ static void process_args(struct packet_reader *request,
 			continue;
 		}
 
+		if (allow_sideband_all && !strcmp(arg, "sideband-all")) {
+			data->writer.use_sideband = 1;
+			continue;
+		}
+
 		/* ignore unknown lines maybe? */
 		die("unexpected line: '%s'", arg);
 	}
@@ -1496,6 +1505,7 @@ int upload_pack_advertise(struct repository *r,
 	if (value) {
 		int allow_filter_value;
 		int allow_ref_in_want;
+		int allow_sideband_all_value;
 
 		strbuf_addstr(value, "shallow");
 
@@ -1510,6 +1520,12 @@ int upload_pack_advertise(struct repository *r,
 					 &allow_ref_in_want) &&
 		    allow_ref_in_want)
 			strbuf_addstr(value, " ref-in-want");
+
+		if (!repo_config_get_bool(the_repository,
+					 "uploadpack.allowsidebandall",
+					 &allow_sideband_all_value) &&
+		    allow_sideband_all_value)
+			strbuf_addstr(value, " sideband-all");
 	}
 
 	return 1;

fetch-pack.c, t/README, t/lib-httpd/apache.conf, t/t5537-fetch-shallow.sh, t/t5701-git-serve.sh, t/t5702-protocol-v2.sh, upload-pack.c

diff --git a/fetch-pack.c b/fetch-pack.c
index b89199891d..4618568fee 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -1362,7 +1362,8 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 	packet_reader_init(&reader, fd[0], NULL, 0,
 			   PACKET_READ_CHOMP_NEWLINE |
 			   PACKET_READ_DIE_ON_ERR_PACKET);
-	if (server_supports_feature("fetch", "sideband-all", 0)) {
+	if (git_env_bool("GIT_TEST_SIDEBAND_ALL", 1) &&
+	    server_supports_feature("fetch", "sideband-all", 0)) {
 		reader.use_sideband = 1;
 		reader.me = "fetch-pack";
 	}
diff --git a/t/README b/t/README
index 28711cc508..b275c883b8 100644
--- a/t/README
+++ b/t/README
@@ -358,6 +358,11 @@ GIT_TEST_MULTI_PACK_INDEX=<boolean>, when true, forces the multi-pack-
 index to be written after every 'git repack' command, and overrides the
 'core.multiPackIndex' setting to true.
 
+GIT_TEST_SIDEBAND_ALL=<boolean>, when true, overrides the
+'uploadpack.allowSidebandAll' setting to true, and when false, forces
+fetch-pack to not request sideband-all (even if the server advertises
+sideband-all).
+
 Naming Tests
 ------------
 
diff --git a/t/lib-httpd/apache.conf b/t/lib-httpd/apache.conf
index 581c010d8f..9a6d368247 100644
--- a/t/lib-httpd/apache.conf
+++ b/t/lib-httpd/apache.conf
@@ -78,6 +78,7 @@ PassEnv GNUPGHOME
 PassEnv ASAN_OPTIONS
 PassEnv GIT_TRACE
 PassEnv GIT_CONFIG_NOSYSTEM
+PassEnv GIT_TEST_SIDEBAND_ALL
 
 SetEnvIf Git-Protocol ".*" GIT_PROTOCOL=$0
 
diff --git a/t/t5537-fetch-shallow.sh b/t/t5537-fetch-shallow.sh
index 6faf17e17a..6caf628efa 100755
--- a/t/t5537-fetch-shallow.sh
+++ b/t/t5537-fetch-shallow.sh
@@ -243,7 +243,8 @@ test_expect_success 'shallow fetches check connectivity before writing shallow f
 	       "$(git -C "$REPO" rev-parse HEAD)" \
 	       "$(git -C "$REPO" rev-parse HEAD^)" \
 	       >"$HTTPD_ROOT_PATH/one-time-sed" &&
-	test_must_fail git -C client fetch --depth=1 "$HTTPD_URL/one_time_sed/repo" \
+	test_must_fail env GIT_TEST_SIDEBAND_ALL=0 git -C client \
+		fetch --depth=1 "$HTTPD_URL/one_time_sed/repo" \
 		master:a_branch &&
 
 	# Ensure that the one-time-sed script was used.
diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
index ae79c6bbc0..fe45bf828d 100755
--- a/t/t5701-git-serve.sh
+++ b/t/t5701-git-serve.sh
@@ -14,7 +14,7 @@ test_expect_success 'test capability advertisement' '
 	0000
 	EOF
 
-	git serve --advertise-capabilities >out &&
+	GIT_TEST_SIDEBAND_ALL=0 git serve --advertise-capabilities >out &&
 	test-tool pkt-line unpack <out >actual &&
 	test_cmp expect actual
 '
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 0f2b09ebb8..b491c62e3e 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -583,8 +583,8 @@ test_expect_success 'when server does not send "ready", expect FLUSH' '
 	test_must_fail env GIT_TRACE_PACKET="$(pwd)/log" git -C http_child \
 		-c protocol.version=2 \
 		fetch "$HTTPD_URL/one_time_sed/http_parent" 2> err &&
-	grep "fetch< acknowledgments" log &&
-	! grep "fetch< ready" log &&
+	grep "fetch< .*acknowledgments" log &&
+	! grep "fetch< .*ready" log &&
 	test_i18ngrep "expected no other sections to be sent after no .ready." err
 '
 
diff --git a/upload-pack.c b/upload-pack.c
index 765b7695d2..0c1feccaab 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1288,7 +1288,9 @@ static void process_args(struct packet_reader *request,
 			continue;
 		}
 
-		if (allow_sideband_all && !strcmp(arg, "sideband-all")) {
+		if ((git_env_bool("GIT_TEST_SIDEBAND_ALL", 0) ||
+		     allow_sideband_all) &&
+		    !strcmp(arg, "sideband-all")) {
 			data->writer.use_sideband = 1;
 			continue;
 		}
@@ -1521,10 +1523,11 @@ int upload_pack_advertise(struct repository *r,
 		    allow_ref_in_want)
 			strbuf_addstr(value, " ref-in-want");
 
-		if (!repo_config_get_bool(the_repository,
-					 "uploadpack.allowsidebandall",
-					 &allow_sideband_all_value) &&
-		    allow_sideband_all_value)
+		if (git_env_bool("GIT_TEST_SIDEBAND_ALL", 0) ||
+		    (!repo_config_get_bool(the_repository,
+					   "uploadpack.allowsidebandall",
+					   &allow_sideband_all_value) &&
+		     allow_sideband_all_value))
 			strbuf_addstr(value, " sideband-all");
 	}
 

Documentation/technical/protocol-v2.txt, pkt-line.c, sideband.c, sideband.h

diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index 1b0633f59f..39b40c0dc1 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -314,8 +314,8 @@ included in the client's request:
 	Instruct the server to send the whole response multiplexed, not just
 	the packfile section. All non-flush and non-delim PKT-LINE in the
 	response (not only in the packfile section) will then start with a byte
-	indicating its sideband (1, 2, or 3), and the server may send "0005\1"
-	(a PKT-LINE of sideband 1 with no payload) as a keepalive packet.
+	indicating its sideband (1, 2, or 3), and the server may send "0005\2"
+	(a PKT-LINE of sideband 2 with no payload) as a keepalive packet.
 
 The response of `fetch` is broken into a number of sections separated by
 delimiter packets (0001), with each section beginning with its section
diff --git a/pkt-line.c b/pkt-line.c
index 69162f3990..d4b71d3e82 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -442,21 +442,22 @@ ssize_t read_packetized_to_strbuf(int fd_in, struct strbuf *sb_out)
 int recv_sideband(const char *me, int in_stream, int out)
 {
 	char buf[LARGE_PACKET_MAX + 1];
-	int retval = 0;
 	int len;
+	struct strbuf scratch = STRBUF_INIT;
+	enum sideband_type sideband_type;
 
 	while (1) {
-		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX, 0);
-		retval = demultiplex_sideband(me, buf, len, 0);
-		switch (retval) {
+		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX,
+				  0);
+		if (!demultiplex_sideband(me, buf, len, 0, &scratch,
+					  &sideband_type))
+			continue;
+		switch (sideband_type) {
 		case SIDEBAND_PRIMARY:
 			write_or_die(out, buf + 1, len - 1);
 			break;
-		case SIDEBAND_PROGRESS:
-			/* already written by demultiplex_sideband() */
-			break;
 		default: /* errors: message already written */
-			return retval;
+			return sideband_type;
 		}
 	}
 }
@@ -479,17 +480,19 @@ void packet_reader_init(struct packet_reader *reader, int fd,
 
 enum packet_read_status packet_reader_read(struct packet_reader *reader)
 {
+	struct strbuf scratch = STRBUF_INIT;
+
 	if (reader->line_peeked) {
 		reader->line_peeked = 0;
 		return reader->status;
 	}
 
 	/*
-	 * Consume all progress and keepalive packets until a primary payload
-	 * packet is received
+	 * Consume all progress packets until a primary payload packet is
+	 * received
 	 */
 	while (1) {
-		int retval;
+		enum sideband_type sideband_type;
 		reader->status = packet_read_with_status(reader->fd,
 							 &reader->src_buffer,
 							 &reader->src_len,
@@ -499,29 +502,12 @@ enum packet_read_status packet_reader_read(struct packet_reader *reader)
 							 reader->options);
 		if (!reader->use_sideband)
 			break;
-		retval = demultiplex_sideband(reader->me, reader->buffer,
-					      reader->pktlen, 1);
-		switch (retval) {
-		case SIDEBAND_PROTOCOL_ERROR:
-		case SIDEBAND_REMOTE_ERROR:
-			BUG("should have died in diagnose_sideband");
-		case SIDEBAND_FLUSH:
-			goto nonprogress_received;
-		case SIDEBAND_PRIMARY:
-			if (reader->pktlen != 1)
-				goto nonprogress_received;
-			/*
-			 * Since the packet contains nothing but the sideband
-			 * designator, this is a keepalive packet. Wait for the
-			 * next one.
-			 */
+		if (demultiplex_sideband(reader->me, reader->buffer,
+					 reader->pktlen, 1, &scratch,
+					 &sideband_type))
 			break;
-		default: /* SIDEBAND_PROGRESS */
-			;
-		}
 	}
 
-nonprogress_received:
 	if (reader->status == PACKET_READ_NORMAL)
 		/* Skip the sideband designator if sideband is used */
 		reader->line = reader->use_sideband ?
diff --git a/sideband.c b/sideband.c
index 9d3051e3fe..6a16feb262 100644
--- a/sideband.c
+++ b/sideband.c
@@ -113,11 +113,12 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
 #define ANSI_SUFFIX "\033[K"
 #define DUMB_SUFFIX "        "
 
-int demultiplex_sideband(const char *me, char *buf, int len, int die_on_error)
+int demultiplex_sideband(const char *me, char *buf, int len,
+			 int die_on_error,
+			 struct strbuf *scratch,
+			 enum sideband_type *sideband_type)
 {
 	static const char *suffix;
-	struct strbuf outbuf = STRBUF_INIT;
-	int retval = 0;
 	const char *b, *brk;
 	int band;
 
@@ -129,14 +130,14 @@ int demultiplex_sideband(const char *me, char *buf, int len, int die_on_error)
 	}
 
 	if (len == 0) {
-		retval = SIDEBAND_FLUSH;
+		*sideband_type = SIDEBAND_FLUSH;
 		goto cleanup;
 	}
 	if (len < 1) {
-		strbuf_addf(&outbuf,
+		strbuf_addf(scratch,
 			    "%s%s: protocol error: no band designator",
-			    outbuf.len ? "\n" : "", me);
-		retval = SIDEBAND_PROTOCOL_ERROR;
+			    scratch->len ? "\n" : "", me);
+		*sideband_type = SIDEBAND_PROTOCOL_ERROR;
 		goto cleanup;
 	}
 	band = buf[0] & 0xff;
@@ -146,12 +147,11 @@ int demultiplex_sideband(const char *me, char *buf, int len, int die_on_error)
 	case 3:
 		if (die_on_error)
 			die("remote error: %s", buf + 1);
-
-		strbuf_addf(&outbuf, "%s%s", outbuf.len ? "\n" : "",
+		strbuf_addf(scratch, "%s%s", scratch->len ? "\n" : "",
 			    DISPLAY_PREFIX);
-		maybe_colorize_sideband(&outbuf, buf + 1, len);
+		maybe_colorize_sideband(scratch, buf + 1, len);
 
-		retval = SIDEBAND_REMOTE_ERROR;
+		*sideband_type = SIDEBAND_REMOTE_ERROR;
 		break;
 	case 2:
 		b = buf + 1;
@@ -167,46 +167,45 @@ int demultiplex_sideband(const char *me, char *buf, int len, int die_on_error)
 		while ((brk = strpbrk(b, "\n\r"))) {
 			int linelen = brk - b;
 
-			if (!outbuf.len)
-				strbuf_addstr(&outbuf, DISPLAY_PREFIX);
+			if (!scratch->len)
+				strbuf_addstr(scratch, DISPLAY_PREFIX);
 			if (linelen > 0) {
-				maybe_colorize_sideband(&outbuf, b, linelen);
-				strbuf_addstr(&outbuf, suffix);
+				maybe_colorize_sideband(scratch, b, linelen);
+				strbuf_addstr(scratch, suffix);
 			}
 
-			strbuf_addch(&outbuf, *brk);
-			xwrite(2, outbuf.buf, outbuf.len);
-			strbuf_reset(&outbuf);
+			strbuf_addch(scratch, *brk);
+			xwrite(2, scratch->buf, scratch->len);
+			strbuf_reset(scratch);
 
 			b = brk + 1;
 		}
 
 		if (*b) {
-			strbuf_addstr(&outbuf, outbuf.len ?
+			strbuf_addstr(scratch, scratch->len ?
 				    "" : DISPLAY_PREFIX);
-			maybe_colorize_sideband(&outbuf, b, strlen(b));
+			maybe_colorize_sideband(scratch, b, strlen(b));
 		}
-		retval = SIDEBAND_PROGRESS;
-		break;
+		return 0;
 	case 1:
-		retval = SIDEBAND_PRIMARY;
+		*sideband_type = SIDEBAND_PRIMARY;
 		break;
 	default:
-		strbuf_addf(&outbuf, "%s%s: protocol error: bad band #%d",
-			    outbuf.len ? "\n" : "", me, band);
-		retval = SIDEBAND_PROTOCOL_ERROR;
+		strbuf_addf(scratch, "%s%s: protocol error: bad band #%d",
+			    scratch->len ? "\n" : "", me, band);
+		*sideband_type = SIDEBAND_PROTOCOL_ERROR;
 		break;
 	}
 
 cleanup:
-	if (die_on_error && retval == SIDEBAND_PROTOCOL_ERROR)
-		die("%s", outbuf.buf);
-	if (outbuf.len) {
-		strbuf_addch(&outbuf, '\n');
-		xwrite(2, outbuf.buf, outbuf.len);
+	if (die_on_error && *sideband_type == SIDEBAND_PROTOCOL_ERROR)
+		die("%s", scratch->buf);
+	if (scratch->len) {
+		strbuf_addch(scratch, '\n');
+		xwrite(2, scratch->buf, scratch->len);
 	}
-	strbuf_release(&outbuf);
-	return retval;
+	strbuf_release(scratch);
+	return 1;
 }
 
 /*
diff --git a/sideband.h b/sideband.h
index f75c4fde2a..227740a58e 100644
--- a/sideband.h
+++ b/sideband.h
@@ -1,21 +1,28 @@
 #ifndef SIDEBAND_H
 #define SIDEBAND_H
 
-#define SIDEBAND_PROTOCOL_ERROR -2
-#define SIDEBAND_REMOTE_ERROR -1
-#define SIDEBAND_FLUSH 0
-#define SIDEBAND_PRIMARY 1
-#define SIDEBAND_PROGRESS 2
+enum sideband_type {
+	SIDEBAND_PROTOCOL_ERROR = -2,
+	SIDEBAND_REMOTE_ERROR = -1,
+	SIDEBAND_FLUSH = 0,
+	SIDEBAND_PRIMARY = 1
+};
 
 /*
- * Inspects a multiplexed packet read from the remote and returns which
- * sideband it is for.
+ * Inspects a multiplexed packet read from the remote. If this packet is a
+ * progress packet and thus should not be processed by the caller, returns 0.
+ * Otherwise, returns 1, releases scratch, and sets sideband_type.
  *
- * If SIDEBAND_PROTOCOL_ERROR, SIDEBAND_REMOTE_ERROR, or SIDEBAND_PROGRESS,
- * also prints a message (or the formatted contents of the notice in the case
- * of SIDEBAND_PROGRESS) to stderr.
+ * If this packet is SIDEBAND_PROTOCOL_ERROR, SIDEBAND_REMOTE_ERROR, or a
+ * progress packet, also prints a message to stderr.
+ *
+ * scratch must be a struct strbuf allocated by the caller. It is used to store
+ * progress messages split across multiple packets.
  */
-int demultiplex_sideband(const char *me, char *buf, int len, int die_on_error);
+int demultiplex_sideband(const char *me, char *buf, int len,
+			 int die_on_error,
+			 struct strbuf *scratch,
+			 enum sideband_type *sideband_type);
 
 void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_max);
 

pkt-line.c, pkt-line.h, upload-pack.c

diff --git a/pkt-line.c b/pkt-line.c
index e70ea6d88f..9d3e402d41 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -129,12 +129,14 @@ static void set_packet_header(char *buf, const int size)
 	#undef hex
 }
 
-static void format_packet(struct strbuf *out, const char *fmt, va_list args)
+static void format_packet(struct strbuf *out, const char *prefix,
+			  const char *fmt, va_list args)
 {
 	size_t orig_len, n;
 
 	orig_len = out->len;
 	strbuf_addstr(out, "0000");
+	strbuf_addstr(out, prefix);
 	strbuf_vaddf(out, fmt, args);
 	n = out->len - orig_len;
 
@@ -145,13 +147,13 @@ static void format_packet(struct strbuf *out, const char *fmt, va_list args)
 	packet_trace(out->buf + orig_len + 4, n - 4, 1);
 }
 
-static int packet_write_fmt_1(int fd, int gently,
+static int packet_write_fmt_1(int fd, int gently, const char *prefix,
 			      const char *fmt, va_list args)
 {
 	static struct strbuf buf = STRBUF_INIT;
 
 	strbuf_reset(&buf);
-	format_packet(&buf, fmt, args);
+	format_packet(&buf, prefix, fmt, args);
 	if (write_in_full(fd, buf.buf, buf.len) < 0) {
 		if (!gently) {
 			check_pipe(errno);
@@ -168,7 +170,7 @@ void packet_write_fmt(int fd, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	packet_write_fmt_1(fd, 0, fmt, args);
+	packet_write_fmt_1(fd, 0, "", fmt, args);
 	va_end(args);
 }
 
@@ -178,7 +180,7 @@ int packet_write_fmt_gently(int fd, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	status = packet_write_fmt_1(fd, 1, fmt, args);
+	status = packet_write_fmt_1(fd, 1, "", fmt, args);
 	va_end(args);
 	return status;
 }
@@ -211,7 +213,7 @@ void packet_buf_write(struct strbuf *buf, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	format_packet(buf, fmt, args);
+	format_packet(buf, "", fmt, args);
 	va_end(args);
 }
 
@@ -486,3 +488,36 @@ enum packet_read_status packet_reader_peek(struct packet_reader *reader)
 	reader->line_peeked = 1;
 	return reader->status;
 }
+
+void packet_writer_init(struct packet_writer *writer, int dest_fd)
+{
+	writer->dest_fd = dest_fd;
+}
+
+void packet_writer_write(struct packet_writer *writer, const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	packet_write_fmt_1(writer->dest_fd, 0, "", fmt, args);
+	va_end(args);
+}
+
+void packet_writer_error(struct packet_writer *writer, const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	packet_write_fmt_1(writer->dest_fd, 0, "ERR ", fmt, args);
+	va_end(args);
+}
+
+void packet_writer_delim(struct packet_writer *writer)
+{
+	packet_delim(writer->dest_fd);
+}
+
+void packet_writer_flush(struct packet_writer *writer)
+{
+	packet_flush(writer->dest_fd);
+}
diff --git a/pkt-line.h b/pkt-line.h
index d7e1dbc047..023ad2951d 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -183,4 +183,18 @@ extern enum packet_read_status packet_reader_peek(struct packet_reader *reader);
 #define LARGE_PACKET_DATA_MAX (LARGE_PACKET_MAX - 4)
 extern char packet_buffer[LARGE_PACKET_MAX];
 
+struct packet_writer {
+	int dest_fd;
+};
+
+void packet_writer_init(struct packet_writer *writer, int dest_fd);
+
+/* These functions die upon failure. */
+__attribute__((format (printf, 2, 3)))
+void packet_writer_write(struct packet_writer *writer, const char *fmt, ...);
+__attribute__((format (printf, 2, 3)))
+void packet_writer_error(struct packet_writer *writer, const char *fmt, ...);
+void packet_writer_delim(struct packet_writer *writer);
+void packet_writer_flush(struct packet_writer *writer);
+
 #endif
diff --git a/upload-pack.c b/upload-pack.c
index 08b547cf01..60a26bbbfd 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -613,13 +613,14 @@ static void check_non_tip(struct object_array *want_obj)
 	}
 }
 
-static void send_shallow(struct commit_list *result)
+static void send_shallow(struct packet_writer *writer,
+			 struct commit_list *result)
 {
 	while (result) {
 		struct object *object = &result->item->object;
 		if (!(object->flags & (CLIENT_SHALLOW|NOT_SHALLOW))) {
-			packet_write_fmt(1, "shallow %s",
-					 oid_to_hex(&object->oid));
+			packet_writer_write(writer, "shallow %s",
+					    oid_to_hex(&object->oid));
 			register_shallow(the_repository, &object->oid);
 			shallow_nr++;
 		}
@@ -627,7 +628,8 @@ static void send_shallow(struct commit_list *result)
 	}
 }
 
-static void send_unshallow(const struct object_array *shallows,
+static void send_unshallow(struct packet_writer *writer,
+			   const struct object_array *shallows,
 			   struct object_array *want_obj)
 {
 	int i;
@@ -636,8 +638,8 @@ static void send_unshallow(const struct object_array *shallows,
 		struct object *object = shallows->objects[i].item;
 		if (object->flags & NOT_SHALLOW) {
 			struct commit_list *parents;
-			packet_write_fmt(1, "unshallow %s",
-					 oid_to_hex(&object->oid));
+			packet_writer_write(writer, "unshallow %s",
+					    oid_to_hex(&object->oid));
 			object->flags &= ~CLIENT_SHALLOW;
 			/*
 			 * We want to _register_ "object" as shallow, but we
@@ -662,7 +664,7 @@ static void send_unshallow(const struct object_array *shallows,
 	}
 }
 
-static void deepen(int depth, int deepen_relative,
+static void deepen(struct packet_writer *writer, int depth, int deepen_relative,
 		   struct object_array *shallows, struct object_array *want_obj)
 {
 	if (depth == INFINITE_DEPTH && !is_repository_shallow(the_repository)) {
@@ -680,7 +682,7 @@ static void deepen(int depth, int deepen_relative,
 		result = get_shallow_commits(&reachable_shallows,
 					     depth + 1,
 					     SHALLOW, NOT_SHALLOW);
-		send_shallow(result);
+		send_shallow(writer, result);
 		free_commit_list(result);
 		object_array_clear(&reachable_shallows);
 	} else {
@@ -688,14 +690,15 @@ static void deepen(int depth, int deepen_relative,
 
 		result = get_shallow_commits(want_obj, depth,
 					     SHALLOW, NOT_SHALLOW);
-		send_shallow(result);
+		send_shallow(writer, result);
 		free_commit_list(result);
 	}
 
-	send_unshallow(shallows, want_obj);
+	send_unshallow(writer, shallows, want_obj);
 }
 
-static void deepen_by_rev_list(int ac, const char **av,
+static void deepen_by_rev_list(struct packet_writer *writer, int ac,
+			       const char **av,
 			       struct object_array *shallows,
 			       struct object_array *want_obj)
 {
@@ -703,13 +706,14 @@ static void deepen_by_rev_list(int ac, const char **av,
 
 	close_commit_graph(the_repository);
 	result = get_shallow_commits_by_rev_list(ac, av, SHALLOW, NOT_SHALLOW);
-	send_shallow(result);
+	send_shallow(writer, result);
 	free_commit_list(result);
-	send_unshallow(shallows, want_obj);
+	send_unshallow(writer, shallows, want_obj);
 }
 
 /* Returns 1 if a shallow list is sent or 0 otherwise */
-static int send_shallow_list(int depth, int deepen_rev_list,
+static int send_shallow_list(struct packet_writer *writer,
+			     int depth, int deepen_rev_list,
 			     timestamp_t deepen_since,
 			     struct string_list *deepen_not,
 			     struct object_array *shallows,
@@ -720,7 +724,7 @@ static int send_shallow_list(int depth, int deepen_rev_list,
 	if (depth > 0 && deepen_rev_list)
 		die("git upload-pack: deepen and deepen-since (or deepen-not) cannot be used together");
 	if (depth > 0) {
-		deepen(depth, deepen_relative, shallows, want_obj);
+		deepen(writer, depth, deepen_relative, shallows, want_obj);
 		ret = 1;
 	} else if (deepen_rev_list) {
 		struct argv_array av = ARGV_ARRAY_INIT;
@@ -741,7 +745,7 @@ static int send_shallow_list(int depth, int deepen_rev_list,
 			struct object *o = want_obj->objects[i].item;
 			argv_array_push(&av, oid_to_hex(&o->oid));
 		}
-		deepen_by_rev_list(av.argc, av.argv, shallows, want_obj);
+		deepen_by_rev_list(writer, av.argc, av.argv, shallows, want_obj);
 		argv_array_clear(&av);
 		ret = 1;
 	} else {
@@ -834,8 +838,10 @@ static void receive_needs(struct packet_reader *reader, struct object_array *wan
 	int has_non_tip = 0;
 	timestamp_t deepen_since = 0;
 	int deepen_rev_list = 0;
+	struct packet_writer writer;
 
 	shallow_nr = 0;
+	packet_writer_init(&writer, 1);
 	for (;;) {
 		struct object *o;
 		const char *features;
@@ -892,9 +898,9 @@ static void receive_needs(struct packet_reader *reader, struct object_array *wan
 
 		o = parse_object(the_repository, &oid_buf);
 		if (!o) {
-			packet_write_fmt(1,
-					 "ERR upload-pack: not our ref %s",
-					 oid_to_hex(&oid_buf));
+			packet_writer_error(&writer,
+					    "upload-pack: not our ref %s",
+					    oid_to_hex(&oid_buf));
 			die("git upload-pack: not our ref %s",
 			    oid_to_hex(&oid_buf));
 		}
@@ -923,7 +929,7 @@ static void receive_needs(struct packet_reader *reader, struct object_array *wan
 	if (depth == 0 && !deepen_rev_list && shallows.nr == 0)
 		return;
 
-	if (send_shallow_list(depth, deepen_rev_list, deepen_since,
+	if (send_shallow_list(&writer, depth, deepen_rev_list, deepen_since,
 			      &deepen_not, &shallows, want_obj))
 		packet_flush(1);
 	object_array_clear(&shallows);
@@ -1102,6 +1108,8 @@ struct upload_pack_data {
 	int deepen_rev_list;
 	int deepen_relative;
 
+	struct packet_writer writer;
+
 	unsigned stateless_rpc : 1;
 
 	unsigned use_thin_pack : 1;
@@ -1125,6 +1133,7 @@ static void upload_pack_data_init(struct upload_pack_data *data)
 	data->haves = haves;
 	data->shallows = shallows;
 	data->deepen_not = deepen_not;
+	packet_writer_init(&data->writer, 1);
 }
 
 static void upload_pack_data_clear(struct upload_pack_data *data)
@@ -1136,7 +1145,8 @@ static void upload_pack_data_clear(struct upload_pack_data *data)
 	string_list_clear(&data->deepen_not, 0);
 }
 
-static int parse_want(const char *line, struct object_array *want_obj)
+static int parse_want(struct packet_writer *writer, const char *line,
+		      struct object_array *want_obj)
 {
 	const char *arg;
 	if (skip_prefix(line, "want ", &arg)) {
@@ -1149,9 +1159,9 @@ static int parse_want(const char *line, struct object_array *want_obj)
 
 		o = parse_object(the_repository, &oid);
 		if (!o) {
-			packet_write_fmt(1,
-					 "ERR upload-pack: not our ref %s",
-					 oid_to_hex(&oid));
+			packet_writer_error(writer,
+					    "upload-pack: not our ref %s",
+					    oid_to_hex(&oid));
 			die("git upload-pack: not our ref %s",
 			    oid_to_hex(&oid));
 		}
@@ -1167,7 +1177,8 @@ static int parse_want(const char *line, struct object_array *want_obj)
 	return 0;
 }
 
-static int parse_want_ref(const char *line, struct string_list *wanted_refs,
+static int parse_want_ref(struct packet_writer *writer, const char *line,
+			  struct string_list *wanted_refs,
 			  struct object_array *want_obj)
 {
 	const char *arg;
@@ -1177,7 +1188,7 @@ static int parse_want_ref(const char *line, struct string_list *wanted_refs,
 		struct object *o;
 
 		if (read_ref(arg, &oid)) {
-			packet_write_fmt(1, "ERR unknown ref %s", arg);
+			packet_writer_error(writer, "unknown ref %s", arg);
 			die("unknown ref %s", arg);
 		}
 
@@ -1220,10 +1231,11 @@ static void process_args(struct packet_reader *request,
 		const char *p;
 
 		/* process want */
-		if (parse_want(arg, want_obj))
+		if (parse_want(&data->writer, arg, want_obj))
 			continue;
 		if (allow_ref_in_want &&
-		    parse_want_ref(arg, &data->wanted_refs, want_obj))
+		    parse_want_ref(&data->writer, arg, &data->wanted_refs,
+				   want_obj))
 			continue;
 		/* process have line */
 		if (parse_have(arg, &data->haves))
@@ -1317,26 +1329,26 @@ static int process_haves(struct oid_array *haves, struct oid_array *common,
 	return 0;
 }
 
-static int send_acks(struct oid_array *acks, struct strbuf *response,
+static int send_acks(struct packet_writer *writer, struct oid_array *acks,
 		     const struct object_array *have_obj,
 		     struct object_array *want_obj)
 {
 	int i;
 
-	packet_buf_write(response, "acknowledgments\n");
+	packet_writer_write(writer, "acknowledgments\n");
 
 	/* Send Acks */
 	if (!acks->nr)
-		packet_buf_write(response, "NAK\n");
+		packet_writer_write(writer, "NAK\n");
 
 	for (i = 0; i < acks->nr; i++) {
-		packet_buf_write(response, "ACK %s\n",
-				 oid_to_hex(&acks->oid[i]));
+		packet_writer_write(writer, "ACK %s\n",
+				    oid_to_hex(&acks->oid[i]));
 	}
 
 	if (ok_to_give_up(have_obj, want_obj)) {
 		/* Send Ready */
-		packet_buf_write(response, "ready\n");
+		packet_writer_write(writer, "ready\n");
 		return 1;
 	}
 
@@ -1348,25 +1360,20 @@ static int process_haves_and_send_acks(struct upload_pack_data *data,
 				       struct object_array *want_obj)
 {
 	struct oid_array common = OID_ARRAY_INIT;
-	struct strbuf response = STRBUF_INIT;
 	int ret = 0;
 
 	process_haves(&data->haves, &common, have_obj);
 	if (data->done) {
 		ret = 1;
-	} else if (send_acks(&common, &response, have_obj, want_obj)) {
-		packet_buf_delim(&response);
+	} else if (send_acks(&data->writer, &common, have_obj, want_obj)) {
+		packet_writer_delim(&data->writer);
 		ret = 1;
 	} else {
 		/* Add Flush */
-		packet_buf_flush(&response);
+		packet_writer_flush(&data->writer);
 		ret = 0;
 	}
 
-	/* Send response */
-	write_or_die(1, response.buf, response.len);
-	strbuf_release(&response);
-
 	oid_array_clear(&data->haves);
 	oid_array_clear(&common);
 	return ret;
@@ -1379,15 +1386,15 @@ static void send_wanted_ref_info(struct upload_pack_data *data)
 	if (!data->wanted_refs.nr)
 		return;
 
-	packet_write_fmt(1, "wanted-refs\n");
+	packet_writer_write(&data->writer, "wanted-refs\n");
 
 	for_each_string_list_item(item, &data->wanted_refs) {
-		packet_write_fmt(1, "%s %s\n",
-				 oid_to_hex(item->util),
-				 item->string);
+		packet_writer_write(&data->writer, "%s %s\n",
+				    oid_to_hex(item->util),
+				    item->string);
 	}
 
-	packet_delim(1);
+	packet_writer_delim(&data->writer);
 }
 
 static void send_shallow_info(struct upload_pack_data *data,
@@ -1398,14 +1405,15 @@ static void send_shallow_info(struct upload_pack_data *data,
 	    !is_repository_shallow(the_repository))
 		return;
 
-	packet_write_fmt(1, "shallow-info\n");
+	packet_writer_write(&data->writer, "shallow-info\n");
 
-	if (!send_shallow_list(data->depth, data->deepen_rev_list,
+	if (!send_shallow_list(&data->writer, data->depth,
+			       data->deepen_rev_list,
 			       data->deepen_since, &data->deepen_not,
 			       &data->shallows, want_obj) &&
 	    is_repository_shallow(the_repository))
-		deepen(INFINITE_DEPTH, data->deepen_relative, &data->shallows,
-		       want_obj);
+		deepen(&data->writer, INFINITE_DEPTH, data->deepen_relative,
+		       &data->shallows, want_obj);
 
 	packet_delim(1);
 }
@@ -1467,7 +1475,7 @@ int upload_pack_v2(struct repository *r, struct argv_array *keys,
 			send_wanted_ref_info(&data);
 			send_shallow_info(&data, &want_obj);
 
-			packet_write_fmt(1, "packfile\n");
+			packet_writer_write(&data.writer, "packfile\n");
 			create_pack_file(&have_obj, &want_obj);
 			state = FETCH_DONE;
 			break;

pkt-line.c, pkt-line.h, sideband.c, sideband.h

diff --git a/pkt-line.c b/pkt-line.c
index 9d3e402d41..321ff632a5 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -439,6 +439,29 @@ ssize_t read_packetized_to_strbuf(int fd_in, struct strbuf *sb_out)
 	return sb_out->len - orig_len;
 }
 
+int recv_sideband(const char *me, int in_stream, int out)
+{
+	char buf[LARGE_PACKET_MAX + 1];
+	int len;
+	struct strbuf scratch = STRBUF_INIT;
+	enum sideband_type sideband_type;
+
+	while (1) {
+		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX,
+				  0);
+		if (!demultiplex_sideband(me, buf, len, &scratch,
+					  &sideband_type))
+			continue;
+		switch (sideband_type) {
+		case SIDEBAND_PRIMARY:
+			write_or_die(out, buf + 1, len - 1);
+			break;
+		default: /* errors: message already written */
+			return sideband_type;
+		}
+	}
+}
+
 /* Packet Reader Functions */
 void packet_reader_init(struct packet_reader *reader, int fd,
 			char *src_buffer, size_t src_len,
diff --git a/pkt-line.h b/pkt-line.h
index 023ad2951d..a8e92a4b63 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -3,6 +3,7 @@
 
 #include "git-compat-util.h"
 #include "strbuf.h"
+#include "sideband.h"
 
 /*
  * Write a packetized stream, where each line is preceded by
@@ -120,6 +121,21 @@ char *packet_read_line_buf(char **src_buf, size_t *src_len, int *size);
  */
 ssize_t read_packetized_to_strbuf(int fd_in, struct strbuf *sb_out);
 
+/*
+ * Receive multiplexed output stream over git native protocol.
+ * in_stream is the input stream from the remote, which carries data
+ * in pkt_line format with band designator.  Demultiplex it into out
+ * and err and return error appropriately.  Band #1 carries the
+ * primary payload.  Things coming over band #2 is not necessarily
+ * error; they are usually informative message on the standard error
+ * stream, aka "verbose").  A message over band #3 is a signal that
+ * the remote died unexpectedly.  A flush() concludes the stream.
+ *
+ * Returns SIDEBAND_FLUSH upon a normal conclusion, and SIDEBAND_PROTOCOL_ERROR
+ * or SIDEBAND_REMOTE_ERROR if an error occurred.
+ */
+int recv_sideband(const char *me, int in_stream, int out);
+
 struct packet_reader {
 	/* source file descriptor */
 	int fd;
diff --git a/sideband.c b/sideband.c
index 368647acf8..e89d677626 100644
--- a/sideband.c
+++ b/sideband.c
@@ -1,7 +1,6 @@
 #include "cache.h"
 #include "color.h"
 #include "config.h"
-#include "pkt-line.h"
 #include "sideband.h"
 #include "help.h"
 
@@ -109,109 +108,99 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
 }
 
 
-/*
- * Receive multiplexed output stream over git native protocol.
- * in_stream is the input stream from the remote, which carries data
- * in pkt_line format with band designator.  Demultiplex it into out
- * and err and return error appropriately.  Band #1 carries the
- * primary payload.  Things coming over band #2 is not necessarily
- * error; they are usually informative message on the standard error
- * stream, aka "verbose").  A message over band #3 is a signal that
- * the remote died unexpectedly.  A flush() concludes the stream.
- */
-
 #define DISPLAY_PREFIX "remote: "
 
 #define ANSI_SUFFIX "\033[K"
 #define DUMB_SUFFIX "        "
 
-int recv_sideband(const char *me, int in_stream, int out)
+int demultiplex_sideband(const char *me, char *buf, int len,
+			 struct strbuf *scratch,
+			 enum sideband_type *sideband_type)
 {
-	const char *suffix;
-	char buf[LARGE_PACKET_MAX + 1];
-	struct strbuf outbuf = STRBUF_INIT;
-	int retval = 0;
-
-	if (isatty(2) && !is_terminal_dumb())
-		suffix = ANSI_SUFFIX;
-	else
-		suffix = DUMB_SUFFIX;
-
-	while (!retval) {
-		const char *b, *brk;
-		int band, len;
-		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX, 0);
-		if (len == 0)
-			break;
-		if (len < 1) {
-			strbuf_addf(&outbuf,
-				    "%s%s: protocol error: no band designator",
-				    outbuf.len ? "\n" : "", me);
-			retval = SIDEBAND_PROTOCOL_ERROR;
-			break;
-		}
-		band = buf[0] & 0xff;
-		buf[len] = '\0';
-		len--;
-		switch (band) {
-		case 3:
-			strbuf_addf(&outbuf, "%s%s", outbuf.len ? "\n" : "",
-				    DISPLAY_PREFIX);
-			maybe_colorize_sideband(&outbuf, buf + 1, len);
-
-			retval = SIDEBAND_REMOTE_ERROR;
-			break;
-		case 2:
-			b = buf + 1;
-
-			/*
-			 * Append a suffix to each nonempty line to clear the
-			 * end of the screen line.
-			 *
-			 * The output is accumulated in a buffer and
-			 * each line is printed to stderr using
-			 * write(2) to ensure inter-process atomicity.
-			 */
-			while ((brk = strpbrk(b, "\n\r"))) {
-				int linelen = brk - b;
-
-				if (!outbuf.len)
-					strbuf_addstr(&outbuf, DISPLAY_PREFIX);
-				if (linelen > 0) {
-					maybe_colorize_sideband(&outbuf, b, linelen);
-					strbuf_addstr(&outbuf, suffix);
-				}
-
-				strbuf_addch(&outbuf, *brk);
-				xwrite(2, outbuf.buf, outbuf.len);
-				strbuf_reset(&outbuf);
-
-				b = brk + 1;
-			}
+	static const char *suffix;
+	const char *b, *brk;
+	int band;
+
+	if (!suffix) {
+		if (isatty(2) && !is_terminal_dumb())
+			suffix = ANSI_SUFFIX;
+		else
+			suffix = DUMB_SUFFIX;
+	}
 
-			if (*b) {
-				strbuf_addstr(&outbuf, outbuf.len ?
-					    "" : DISPLAY_PREFIX);
-				maybe_colorize_sideband(&outbuf, b, strlen(b));
+	if (len == 0) {
+		*sideband_type = SIDEBAND_FLUSH;
+		goto cleanup;
+	}
+	if (len < 1) {
+		strbuf_addf(scratch,
+			    "%s%s: protocol error: no band designator",
+			    scratch->len ? "\n" : "", me);
+		*sideband_type = SIDEBAND_PROTOCOL_ERROR;
+		goto cleanup;
+	}
+	band = buf[0] & 0xff;
+	buf[len] = '\0';
+	len--;
+	switch (band) {
+	case 3:
+		strbuf_addf(scratch, "%s%s", scratch->len ? "\n" : "",
+			    DISPLAY_PREFIX);
+		maybe_colorize_sideband(scratch, buf + 1, len);
+
+		*sideband_type = SIDEBAND_REMOTE_ERROR;
+		break;
+	case 2:
+		b = buf + 1;
+
+		/*
+		 * Append a suffix to each nonempty line to clear the
+		 * end of the screen line.
+		 *
+		 * The output is accumulated in a buffer and
+		 * each line is printed to stderr using
+		 * write(2) to ensure inter-process atomicity.
+		 */
+		while ((brk = strpbrk(b, "\n\r"))) {
+			int linelen = brk - b;
+
+			if (!scratch->len)
+				strbuf_addstr(scratch, DISPLAY_PREFIX);
+			if (linelen > 0) {
+				maybe_colorize_sideband(scratch, b, linelen);
+				strbuf_addstr(scratch, suffix);
 			}
-			break;
-		case 1:
-			write_or_die(out, buf + 1, len);
-			break;
-		default:
-			strbuf_addf(&outbuf, "%s%s: protocol error: bad band #%d",
-				    outbuf.len ? "\n" : "", me, band);
-			retval = SIDEBAND_PROTOCOL_ERROR;
-			break;
+
+			strbuf_addch(scratch, *brk);
+			xwrite(2, scratch->buf, scratch->len);
+			strbuf_reset(scratch);
+
+			b = brk + 1;
+		}
+
+		if (*b) {
+			strbuf_addstr(scratch, scratch->len ?
+				    "" : DISPLAY_PREFIX);
+			maybe_colorize_sideband(scratch, b, strlen(b));
 		}
+		return 0;
+	case 1:
+		*sideband_type = SIDEBAND_PRIMARY;
+		break;
+	default:
+		strbuf_addf(scratch, "%s%s: protocol error: bad band #%d",
+			    scratch->len ? "\n" : "", me, band);
+		*sideband_type = SIDEBAND_PROTOCOL_ERROR;
+		break;
 	}
 
-	if (outbuf.len) {
-		strbuf_addch(&outbuf, '\n');
-		xwrite(2, outbuf.buf, outbuf.len);
+cleanup:
+	if (scratch->len) {
+		strbuf_addch(scratch, '\n');
+		xwrite(2, scratch->buf, scratch->len);
 	}
-	strbuf_release(&outbuf);
-	return retval;
+	strbuf_release(scratch);
+	return 1;
 }
 
 /*
diff --git a/sideband.h b/sideband.h
index 7a8146f161..2c4f021645 100644
--- a/sideband.h
+++ b/sideband.h
@@ -1,10 +1,28 @@
 #ifndef SIDEBAND_H
 #define SIDEBAND_H
 
-#define SIDEBAND_PROTOCOL_ERROR -2
-#define SIDEBAND_REMOTE_ERROR -1
+enum sideband_type {
+	SIDEBAND_PROTOCOL_ERROR = -2,
+	SIDEBAND_REMOTE_ERROR = -1,
+	SIDEBAND_FLUSH = 0,
+	SIDEBAND_PRIMARY = 1
+};
+
+/*
+ * Inspects a multiplexed packet read from the remote. If this packet is a
+ * progress packet and thus should not be processed by the caller, returns 0.
+ * Otherwise, returns 1, releases scratch, and sets sideband_type.
+ *
+ * If this packet is SIDEBAND_PROTOCOL_ERROR, SIDEBAND_REMOTE_ERROR, or a
+ * progress packet, also prints a message to stderr.
+ *
+ * scratch must be a struct strbuf allocated by the caller. It is used to store
+ * progress messages split across multiple packets.
+ */
+int demultiplex_sideband(const char *me, char *buf, int len,
+			 struct strbuf *scratch,
+			 enum sideband_type *sideband_type);
 
-int recv_sideband(const char *me, int in_stream, int out);
 void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_max);
 
 #endif

Documentation/technical/protocol-v2.txt, fetch-pack.c, pkt-line.c, pkt-line.h, sideband.c, sideband.h, upload-pack.c

diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
index 09e4e0273f..39b40c0dc1 100644
--- a/Documentation/technical/protocol-v2.txt
+++ b/Documentation/technical/protocol-v2.txt
@@ -307,6 +307,16 @@ the 'wanted-refs' section in the server's response as explained below.
 	particular ref, where <ref> is the full name of a ref on the
 	server.
 
+If the 'sideband-all' feature is advertised, the following argument can be
+included in the client's request:
+
+    sideband-all
+	Instruct the server to send the whole response multiplexed, not just
+	the packfile section. All non-flush and non-delim PKT-LINE in the
+	response (not only in the packfile section) will then start with a byte
+	indicating its sideband (1, 2, or 3), and the server may send "0005\2"
+	(a PKT-LINE of sideband 2 with no payload) as a keepalive packet.
+
 The response of `fetch` is broken into a number of sections separated by
 delimiter packets (0001), with each section beginning with its section
 header.
diff --git a/fetch-pack.c b/fetch-pack.c
index ee0847e6ae..86e9e18901 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -1090,7 +1090,8 @@ static int add_haves(struct fetch_negotiator *negotiator,
 static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 			      const struct fetch_pack_args *args,
 			      const struct ref *wants, struct oidset *common,
-			      int *haves_to_send, int *in_vain)
+			      int *haves_to_send, int *in_vain,
+			      int sideband_all)
 {
 	int ret = 0;
 	struct strbuf req_buf = STRBUF_INIT;
@@ -1116,6 +1117,8 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 		packet_buf_write(&req_buf, "include-tag");
 	if (prefer_ofs_delta)
 		packet_buf_write(&req_buf, "ofs-delta");
+	if (sideband_all)
+		packet_buf_write(&req_buf, "sideband-all");
 
 	/* Add shallow-info and deepen request */
 	if (server_supports_feature("fetch", "shallow", 0))
@@ -1324,6 +1327,10 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 	packet_reader_init(&reader, fd[0], NULL, 0,
 			   PACKET_READ_CHOMP_NEWLINE |
 			   PACKET_READ_DIE_ON_ERR_PACKET);
+	if (server_supports_feature("fetch", "sideband-all", 0)) {
+		reader.use_sideband = 1;
+		reader.me = "fetch-pack";
+	}
 
 	while (state != FETCH_DONE) {
 		switch (state) {
@@ -1357,7 +1364,8 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 		case FETCH_SEND_REQUEST:
 			if (send_fetch_request(&negotiator, fd[1], args, ref,
 					       &common,
-					       &haves_to_send, &in_vain))
+					       &haves_to_send, &in_vain,
+					       reader.use_sideband))
 				state = FETCH_GET_PACK;
 			else
 				state = FETCH_PROCESS_ACKS;
diff --git a/pkt-line.c b/pkt-line.c
index 321ff632a5..d4b71d3e82 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -449,7 +449,7 @@ int recv_sideband(const char *me, int in_stream, int out)
 	while (1) {
 		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX,
 				  0);
-		if (!demultiplex_sideband(me, buf, len, &scratch,
+		if (!demultiplex_sideband(me, buf, len, 0, &scratch,
 					  &sideband_type))
 			continue;
 		switch (sideband_type) {
@@ -475,25 +475,43 @@ void packet_reader_init(struct packet_reader *reader, int fd,
 	reader->buffer = packet_buffer;
 	reader->buffer_size = sizeof(packet_buffer);
 	reader->options = options;
+	reader->me = "git";
 }
 
 enum packet_read_status packet_reader_read(struct packet_reader *reader)
 {
+	struct strbuf scratch = STRBUF_INIT;
+
 	if (reader->line_peeked) {
 		reader->line_peeked = 0;
 		return reader->status;
 	}
 
-	reader->status = packet_read_with_status(reader->fd,
-						 &reader->src_buffer,
-						 &reader->src_len,
-						 reader->buffer,
-						 reader->buffer_size,
-						 &reader->pktlen,
-						 reader->options);
+	/*
+	 * Consume all progress packets until a primary payload packet is
+	 * received
+	 */
+	while (1) {
+		enum sideband_type sideband_type;
+		reader->status = packet_read_with_status(reader->fd,
+							 &reader->src_buffer,
+							 &reader->src_len,
+							 reader->buffer,
+							 reader->buffer_size,
+							 &reader->pktlen,
+							 reader->options);
+		if (!reader->use_sideband)
+			break;
+		if (demultiplex_sideband(reader->me, reader->buffer,
+					 reader->pktlen, 1, &scratch,
+					 &sideband_type))
+			break;
+	}
 
 	if (reader->status == PACKET_READ_NORMAL)
-		reader->line = reader->buffer;
+		/* Skip the sideband designator if sideband is used */
+		reader->line = reader->use_sideband ?
+			reader->buffer + 1 : reader->buffer;
 	else
 		reader->line = NULL;
 
@@ -515,6 +533,7 @@ enum packet_read_status packet_reader_peek(struct packet_reader *reader)
 void packet_writer_init(struct packet_writer *writer, int dest_fd)
 {
 	writer->dest_fd = dest_fd;
+	writer->use_sideband = 0;
 }
 
 void packet_writer_write(struct packet_writer *writer, const char *fmt, ...)
@@ -522,7 +541,8 @@ void packet_writer_write(struct packet_writer *writer, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	packet_write_fmt_1(writer->dest_fd, 0, "", fmt, args);
+	packet_write_fmt_1(writer->dest_fd, 0,
+			   writer->use_sideband ? "\001" : "", fmt, args);
 	va_end(args);
 }
 
@@ -531,7 +551,8 @@ void packet_writer_error(struct packet_writer *writer, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	packet_write_fmt_1(writer->dest_fd, 0, "ERR ", fmt, args);
+	packet_write_fmt_1(writer->dest_fd, 0,
+			   writer->use_sideband ? "\003" : "ERR ", fmt, args);
 	va_end(args);
 }
 
diff --git a/pkt-line.h b/pkt-line.h
index a8e92a4b63..ad9a4a2cd7 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -162,6 +162,9 @@ struct packet_reader {
 
 	/* indicates if a line has been peeked */
 	int line_peeked;
+
+	unsigned use_sideband : 1;
+	const char *me;
 };
 
 /*
@@ -201,6 +204,7 @@ extern char packet_buffer[LARGE_PACKET_MAX];
 
 struct packet_writer {
 	int dest_fd;
+	unsigned use_sideband : 1;
 };
 
 void packet_writer_init(struct packet_writer *writer, int dest_fd);
diff --git a/sideband.c b/sideband.c
index e89d677626..6a16feb262 100644
--- a/sideband.c
+++ b/sideband.c
@@ -114,6 +114,7 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
 #define DUMB_SUFFIX "        "
 
 int demultiplex_sideband(const char *me, char *buf, int len,
+			 int die_on_error,
 			 struct strbuf *scratch,
 			 enum sideband_type *sideband_type)
 {
@@ -144,6 +145,8 @@ int demultiplex_sideband(const char *me, char *buf, int len,
 	len--;
 	switch (band) {
 	case 3:
+		if (die_on_error)
+			die("remote error: %s", buf + 1);
 		strbuf_addf(scratch, "%s%s", scratch->len ? "\n" : "",
 			    DISPLAY_PREFIX);
 		maybe_colorize_sideband(scratch, buf + 1, len);
@@ -195,6 +198,8 @@ int demultiplex_sideband(const char *me, char *buf, int len,
 	}
 
 cleanup:
+	if (die_on_error && *sideband_type == SIDEBAND_PROTOCOL_ERROR)
+		die("%s", scratch->buf);
 	if (scratch->len) {
 		strbuf_addch(scratch, '\n');
 		xwrite(2, scratch->buf, scratch->len);
diff --git a/sideband.h b/sideband.h
index 2c4f021645..227740a58e 100644
--- a/sideband.h
+++ b/sideband.h
@@ -20,6 +20,7 @@ enum sideband_type {
  * progress messages split across multiple packets.
  */
 int demultiplex_sideband(const char *me, char *buf, int len,
+			 int die_on_error,
 			 struct strbuf *scratch,
 			 enum sideband_type *sideband_type);
 
diff --git a/upload-pack.c b/upload-pack.c
index 60a26bbbfd..765b7695d2 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -71,6 +71,8 @@ static int allow_filter;
 static int allow_ref_in_want;
 static struct list_objects_filter_options filter_options;
 
+static int allow_sideband_all;
+
 static void reset_timeout(void)
 {
 	alarm(timeout);
@@ -1046,6 +1048,8 @@ static int upload_pack_config(const char *var, const char *value, void *unused)
 		allow_filter = git_config_bool(var, value);
 	} else if (!strcmp("uploadpack.allowrefinwant", var)) {
 		allow_ref_in_want = git_config_bool(var, value);
+	} else if (!strcmp("uploadpack.allowsidebandall", var)) {
+		allow_sideband_all = git_config_bool(var, value);
 	}
 
 	if (current_config_scope() != CONFIG_SCOPE_REPO) {
@@ -1284,6 +1288,11 @@ static void process_args(struct packet_reader *request,
 			continue;
 		}
 
+		if (allow_sideband_all && !strcmp(arg, "sideband-all")) {
+			data->writer.use_sideband = 1;
+			continue;
+		}
+
 		/* ignore unknown lines maybe? */
 		die("unexpected line: '%s'", arg);
 	}
@@ -1496,6 +1505,7 @@ int upload_pack_advertise(struct repository *r,
 	if (value) {
 		int allow_filter_value;
 		int allow_ref_in_want;
+		int allow_sideband_all_value;
 
 		strbuf_addstr(value, "shallow");
 
@@ -1510,6 +1520,12 @@ int upload_pack_advertise(struct repository *r,
 					 &allow_ref_in_want) &&
 		    allow_ref_in_want)
 			strbuf_addstr(value, " ref-in-want");
+
+		if (!repo_config_get_bool(the_repository,
+					 "uploadpack.allowsidebandall",
+					 &allow_sideband_all_value) &&
+		    allow_sideband_all_value)
+			strbuf_addstr(value, " sideband-all");
 	}
 
 	return 1;

fetch-pack.c, t/README, t/lib-httpd/apache.conf, t/t5537-fetch-shallow.sh, t/t5701-git-serve.sh, t/t5702-protocol-v2.sh, upload-pack.c

diff --git a/fetch-pack.c b/fetch-pack.c
index 86e9e18901..e98294d918 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -1327,7 +1327,8 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 	packet_reader_init(&reader, fd[0], NULL, 0,
 			   PACKET_READ_CHOMP_NEWLINE |
 			   PACKET_READ_DIE_ON_ERR_PACKET);
-	if (server_supports_feature("fetch", "sideband-all", 0)) {
+	if (git_env_bool("GIT_TEST_SIDEBAND_ALL", 1) &&
+	    server_supports_feature("fetch", "sideband-all", 0)) {
 		reader.use_sideband = 1;
 		reader.me = "fetch-pack";
 	}
diff --git a/t/README b/t/README
index 28711cc508..b275c883b8 100644
--- a/t/README
+++ b/t/README
@@ -358,6 +358,11 @@ GIT_TEST_MULTI_PACK_INDEX=<boolean>, when true, forces the multi-pack-
 index to be written after every 'git repack' command, and overrides the
 'core.multiPackIndex' setting to true.
 
+GIT_TEST_SIDEBAND_ALL=<boolean>, when true, overrides the
+'uploadpack.allowSidebandAll' setting to true, and when false, forces
+fetch-pack to not request sideband-all (even if the server advertises
+sideband-all).
+
 Naming Tests
 ------------
 
diff --git a/t/lib-httpd/apache.conf b/t/lib-httpd/apache.conf
index 581c010d8f..9a6d368247 100644
--- a/t/lib-httpd/apache.conf
+++ b/t/lib-httpd/apache.conf
@@ -78,6 +78,7 @@ PassEnv GNUPGHOME
 PassEnv ASAN_OPTIONS
 PassEnv GIT_TRACE
 PassEnv GIT_CONFIG_NOSYSTEM
+PassEnv GIT_TEST_SIDEBAND_ALL
 
 SetEnvIf Git-Protocol ".*" GIT_PROTOCOL=$0
 
diff --git a/t/t5537-fetch-shallow.sh b/t/t5537-fetch-shallow.sh
index 6faf17e17a..6caf628efa 100755
--- a/t/t5537-fetch-shallow.sh
+++ b/t/t5537-fetch-shallow.sh
@@ -243,7 +243,8 @@ test_expect_success 'shallow fetches check connectivity before writing shallow f
 	       "$(git -C "$REPO" rev-parse HEAD)" \
 	       "$(git -C "$REPO" rev-parse HEAD^)" \
 	       >"$HTTPD_ROOT_PATH/one-time-sed" &&
-	test_must_fail git -C client fetch --depth=1 "$HTTPD_URL/one_time_sed/repo" \
+	test_must_fail env GIT_TEST_SIDEBAND_ALL=0 git -C client \
+		fetch --depth=1 "$HTTPD_URL/one_time_sed/repo" \
 		master:a_branch &&
 
 	# Ensure that the one-time-sed script was used.
diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
index ae79c6bbc0..fe45bf828d 100755
--- a/t/t5701-git-serve.sh
+++ b/t/t5701-git-serve.sh
@@ -14,7 +14,7 @@ test_expect_success 'test capability advertisement' '
 	0000
 	EOF
 
-	git serve --advertise-capabilities >out &&
+	GIT_TEST_SIDEBAND_ALL=0 git serve --advertise-capabilities >out &&
 	test-tool pkt-line unpack <out >actual &&
 	test_cmp expect actual
 '
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 0f2b09ebb8..b491c62e3e 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -583,8 +583,8 @@ test_expect_success 'when server does not send "ready", expect FLUSH' '
 	test_must_fail env GIT_TRACE_PACKET="$(pwd)/log" git -C http_child \
 		-c protocol.version=2 \
 		fetch "$HTTPD_URL/one_time_sed/http_parent" 2> err &&
-	grep "fetch< acknowledgments" log &&
-	! grep "fetch< ready" log &&
+	grep "fetch< .*acknowledgments" log &&
+	! grep "fetch< .*ready" log &&
 	test_i18ngrep "expected no other sections to be sent after no .ready." err
 '
 
diff --git a/upload-pack.c b/upload-pack.c
index 765b7695d2..0c1feccaab 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1288,7 +1288,9 @@ static void process_args(struct packet_reader *request,
 			continue;
 		}
 
-		if (allow_sideband_all && !strcmp(arg, "sideband-all")) {
+		if ((git_env_bool("GIT_TEST_SIDEBAND_ALL", 0) ||
+		     allow_sideband_all) &&
+		    !strcmp(arg, "sideband-all")) {
 			data->writer.use_sideband = 1;
 			continue;
 		}
@@ -1521,10 +1523,11 @@ int upload_pack_advertise(struct repository *r,
 		    allow_ref_in_want)
 			strbuf_addstr(value, " ref-in-want");
 
-		if (!repo_config_get_bool(the_repository,
-					 "uploadpack.allowsidebandall",
-					 &allow_sideband_all_value) &&
-		    allow_sideband_all_value)
+		if (git_env_bool("GIT_TEST_SIDEBAND_ALL", 0) ||
+		    (!repo_config_get_bool(the_repository,
+					   "uploadpack.allowsidebandall",
+					   &allow_sideband_all_value) &&
+		     allow_sideband_all_value))
 			strbuf_addstr(value, " sideband-all");
 	}