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[PATCH v2 1/2] http: avoid concurrent appends to partial packs

From
Ted Nyman <tnyman@openai.com>
Date
Jul 20, 2026, 22:33 UTC
Message-ID
<160a9b9fd0982dadfbf6f8fbb378d1a3e9173698.1784582665.git.tnyman@openai.com>
In-Reply-To
<cover.1784582665.git.tnyman@openai.com>

Pack requests stage downloads in a predictable partial-pack file so an interrupted transfer can be resumed. Both packfile URI and ordinary dumb HTTP requests use this staging path. Opening it in append mode lets concurrent fetches interleave their writes, corrupting the pack or causing a later fetch to request a range at EOF.

Open the partial pack read-write, seek to its current end, and retain a per-descriptor offset for incoming data. Reopen newly created partial packs without O_CREAT so Windows permits concurrent unlink, and keep the descriptor for index-pack when another downloader removes the staging path. Accept HTTP 416 when a partial pack is already complete.

Exercise resumed transfers, EOF ranges, and overlapping 200 and 206 responses. Clarify the staging-key documentation and correct the stale --index-pack-args spelling in the documentation and error messages; the repeatable --index-pack-arg option is already accepted.

Signed-off-by: Ted Nyman <tnyman@openai.com>
---
 Documentation/git-http-fetch.adoc |  13 +-
 http-fetch.c                      |   7 +-
 http-push.c                       |   3 +-
 http-walker.c                     |   3 +-
 http.c                            |  53 ++++---
 t/t5550-http-fetch-dumb.sh        | 223 ++++++++++++++++++++++++++++++
 6 files changed, 271 insertions(+), 31 deletions(-)
diff --git a/Documentation/git-http-fetch.adoc b/Documentation/git-http-fetch.adoc
index 2200f073c4..60ca91cf3a 100644
--- a/Documentation/git-http-fetch.adoc
+++ b/Documentation/git-http-fetch.adoc
@@ -48,13 +48,14 @@ commit-id::
 	line (which is not expected in
 	this case), 'git http-fetch' fetches the packfile directly at the given
 	URL and uses index-pack to generate corresponding .idx and .keep files.
-	The hash is used to determine the name of the temporary file and is
-	arbitrary. The output of index-pack is printed to stdout. Requires
-	--index-pack-args.
+	The hash is used to determine the name of the temporary file. It need
+	not be the pack hash, but it must uniquely identify the pack contents
+	for resumption. The output of index-pack is printed to stdout. Requires
+	one or more --index-pack-arg options.
 
---index-pack-args=<args>::
-	For internal use only. The command to run on the contents of the
-	downloaded pack. Arguments are URL-encoded separated by spaces.
+--index-pack-arg=<arg>::
+	For internal use only. An argument to the command run on the contents
+	of the downloaded pack. This option can be specified multiple times.
 
 --recover::
 	Verify that everything reachable from target is fetched.  Used after
diff --git a/http-fetch.c b/http-fetch.c
index f9b6ecb061..05f68f306a 100644
--- a/http-fetch.c
+++ b/http-fetch.c
@@ -70,7 +70,8 @@ static void fetch_single_packfile(struct object_id *packfile_hash,
 
 	if (start_active_slot(preq->slot)) {
 		run_active_slot(preq->slot);
-		if (results.curl_result != CURLE_OK) {
+		if (results.curl_result != CURLE_OK &&
+		    results.http_code != 416) {
 			struct url_info url;
 			char *nurl = url_normalize(preq->url, &url);
 			if (!nurl || !git_env_bool("GIT_TRACE_REDACT", 1)) {
@@ -155,7 +156,7 @@ int cmd_main(int argc, const char **argv)
 
 	if (packfile) {
 		if (!index_pack_args.nr)
-			die(_("the option '%s' requires '%s'"), "--packfile", "--index-pack-args");
+			die(_("the option '%s' requires '%s'"), "--packfile", "--index-pack-arg");
 
 		fetch_single_packfile(&packfile_hash, argv[arg],
 				      index_pack_args.v);
@@ -164,7 +165,7 @@ int cmd_main(int argc, const char **argv)
 	}
 
 	if (index_pack_args.nr)
-		die(_("the option '%s' requires '%s'"), "--index-pack-args", "--packfile");
+		die(_("the option '%s' requires '%s'"), "--index-pack-arg", "--packfile");
 
 	if (commits_on_stdin) {
 		commits = walker_targets_stdin(&commit_id, &write_ref);
diff --git a/http-push.c b/http-push.c
index 3c23cbba27..03dc8102a1 100644
--- a/http-push.c
+++ b/http-push.c
@@ -595,7 +595,8 @@ static void finish_request(struct transfer_request *request)
 
 	} else if (request->state == RUN_FETCH_PACKED) {
 		int fail = 1;
-		if (request->curl_result != CURLE_OK) {
+		if (request->curl_result != CURLE_OK &&
+		    request->http_code != 416) {
 			fprintf(stderr, "Unable to get pack file %s\n%s",
 				request->url, curl_errorstr);
 		} else {
diff --git a/http-walker.c b/http-walker.c
index b58a3b2a92..abafca84d6 100644
--- a/http-walker.c
+++ b/http-walker.c
@@ -451,7 +451,8 @@ static int http_fetch_pack(struct walker *walker, struct alt_base *repo,
 
 	if (start_active_slot(preq->slot)) {
 		run_active_slot(preq->slot);
-		if (results.curl_result != CURLE_OK) {
+		if (results.curl_result != CURLE_OK &&
+		    results.http_code != 416) {
 			error("Unable to get pack file %s\n%s", preq->url,
 			      curl_errorstr);
 			goto abort;
diff --git a/http.c b/http.c
index b4e7b8d00b..9b9f4efe28 100644
--- a/http.c
+++ b/http.c
@@ -2688,10 +2688,13 @@ int finish_http_pack_request(struct http_pack_request *preq)
 	int tmpfile_fd;
 	int ret = 0;
 
+	/* Another downloader may unlink the staging path while we index it. */
+	tmpfile_fd = xdup(fileno(preq->packfile));
 	fclose(preq->packfile);
 	preq->packfile = NULL;
-
-	tmpfile_fd = xopen(preq->tmpfile.buf, O_RDONLY);
+	if (lseek(tmpfile_fd, 0, SEEK_SET) < 0)
+		die_errno("unable to seek local file %s for pack",
+			  preq->tmpfile.buf);
 
 	ip.git_cmd = 1;
 	ip.in = tmpfile_fd;
@@ -2704,13 +2707,8 @@ int finish_http_pack_request(struct http_pack_request *preq)
 	else
 		ip.no_stdout = 1;
 
-	if (run_command(&ip)) {
+	if (run_command(&ip))
 		ret = -1;
-		goto cleanup;
-	}
-
-cleanup:
-	close(tmpfile_fd);
 	unlink(preq->tmpfile.buf);
 	return ret;
 }
@@ -2738,22 +2736,42 @@ struct http_pack_request *new_http_pack_request(
 struct http_pack_request *new_direct_http_pack_request(
 	const unsigned char *packed_git_hash, char *url)
 {
-	off_t prev_posn = 0;
+	off_t prev_posn;
 	struct http_pack_request *preq;
+	int fd;
 
 	CALLOC_ARRAY(preq, 1);
 	strbuf_init(&preq->tmpfile, 0);
-
 	preq->url = url;
 
 	odb_pack_name(the_repository, &preq->tmpfile, packed_git_hash, "pack");
 	strbuf_addstr(&preq->tmpfile, ".temp");
-	preq->packfile = fopen(preq->tmpfile.buf, "a");
-	if (!preq->packfile) {
-		error("Unable to open local file %s for pack",
-		      preq->tmpfile.buf);
+	/* Reopen without O_CREAT so MinGW permits another writer to unlink it. */
+	for (;;) {
+		fd = open(preq->tmpfile.buf, O_RDWR);
+		if (fd >= 0 || errno != ENOENT)
+			break;
+		fd = open(preq->tmpfile.buf, O_RDWR | O_CREAT | O_EXCL, 0666);
+		if (fd >= 0) {
+			close(fd);
+			continue;
+		}
+		if (errno != EEXIST)
+			break;
+	}
+	if (fd < 0) {
+		error_errno("unable to open local file %s for pack",
+			    preq->tmpfile.buf);
+		goto abort;
+	}
+	prev_posn = lseek(fd, 0, SEEK_END);
+	if (prev_posn < 0) {
+		error_errno("unable to seek local file %s for pack",
+			    preq->tmpfile.buf);
+		close(fd);
 		goto abort;
 	}
+	preq->packfile = xfdopen(fd, "w");
 
 	preq->slot = get_active_slot();
 	preq->headers = object_request_headers();
@@ -2762,12 +2780,7 @@ struct http_pack_request *new_direct_http_pack_request(
 	curl_easy_setopt(preq->slot->curl, CURLOPT_URL, preq->url);
 	curl_easy_setopt(preq->slot->curl, CURLOPT_HTTPHEADER, preq->headers);
 
-	/*
-	 * If there is data present from a previous transfer attempt,
-	 * resume where it left off
-	 */
-	prev_posn = ftello(preq->packfile);
-	if (prev_posn>0) {
+	if (prev_posn > 0) {
 		if (http_is_verbose)
 			fprintf(stderr,
 				"Resuming fetch of pack %s at byte %"PRIuMAX"\n",
diff --git a/t/t5550-http-fetch-dumb.sh b/t/t5550-http-fetch-dumb.sh
index b0080bf204..7acae96a96 100755
--- a/t/t5550-http-fetch-dumb.sh
+++ b/t/t5550-http-fetch-dumb.sh
@@ -293,6 +293,229 @@ test_expect_success 'http-fetch --packfile' '
 	git -C packfileclient cat-file -e "$HASH"
 '
 
+test_expect_success 'http-fetch --packfile resumes a partial download' '
+	git init packfileclient-resume &&
+	p=$(cd "$HTTPD_DOCUMENT_ROOT_PATH"/repo_pack.git &&
+		ls objects/pack/pack-*.pack) &&
+	tmpfile="packfileclient-resume/.git/objects/pack/pack-$ARBITRARY.pack.temp" &&
+	test_copy_bytes 64 <"$HTTPD_DOCUMENT_ROOT_PATH/repo_pack.git/$p" >"$tmpfile" &&
+	GIT_TRACE_CURL="$TRASH_DIRECTORY/resume.trace" \
+	git -C packfileclient-resume http-fetch --packfile="$ARBITRARY" \
+		--index-pack-arg=index-pack --index-pack-arg=--stdin \
+		--index-pack-arg=--keep \
+		"$HTTPD_URL/dumb/repo_pack.git/$p" >out &&
+	test_grep "Range: bytes=64-" resume.trace &&
+	test_path_is_missing "$tmpfile" &&
+	git -C packfileclient-resume cat-file -e "$HASH"
+'
+
+test_expect_success PIPE 'concurrent http-fetch --packfile accepts a complete partial' '
+	git init packfileclient-concurrent &&
+	p=$(cd "$HTTPD_DOCUMENT_ROOT_PATH"/repo_pack.git &&
+		ls objects/pack/pack-*.pack) &&
+	packhash=$(basename "$p" .pack) &&
+	packhash=${packhash#pack-} &&
+	tmpfile="packfileclient-concurrent/.git/objects/pack/pack-$packhash.pack.temp" &&
+	test_copy_bytes 64 <"$HTTPD_DOCUMENT_ROOT_PATH/repo_pack.git/$p" >"$tmpfile" &&
+	mkfifo first-ready first-continue &&
+	exec 8<>first-ready &&
+	exec 9<>first-continue &&
+	write_script git-wait-index-pack <<-\EOF &&
+	echo ready >"$GIT_TEST_WAIT_READY" &&
+	read continue <"$GIT_TEST_WAIT_CONTINUE" &&
+	exec git index-pack "$@"
+	EOF
+	{
+		(
+			if ! PATH="$TRASH_DIRECTORY:$PATH" \
+			GIT_TEST_WAIT_READY="$TRASH_DIRECTORY/first-ready" \
+			GIT_TEST_WAIT_CONTINUE="$TRASH_DIRECTORY/first-continue" \
+			GIT_TRACE_CURL="$TRASH_DIRECTORY/first.trace" \
+			git -C packfileclient-concurrent http-fetch --packfile="$packhash" \
+				--index-pack-arg=wait-index-pack \
+				--index-pack-arg=--stdin --index-pack-arg=--keep \
+				"$HTTPD_URL/dumb/repo_pack.git/$p" >first.out
+			then
+				echo failed >"$TRASH_DIRECTORY/first-ready" &&
+				exit 1
+			fi
+		) &
+		first_pid=$!
+	} &&
+	test_when_finished "
+		echo continue >&9
+		kill $first_pid 2>/dev/null || :
+		wait $first_pid 2>/dev/null || :
+		exec 8>&-
+		exec 9>&-
+		rm -f first-ready first-continue git-wait-index-pack
+	" &&
+	read ready <&8 &&
+	test "$ready" = ready &&
+	GIT_TRACE_CURL="$TRASH_DIRECTORY/second.trace" \
+	git -C packfileclient-concurrent http-fetch --packfile="$packhash" \
+		--index-pack-arg=index-pack \
+		--index-pack-arg=--stdin --index-pack-arg=--keep \
+		"$HTTPD_URL/dumb/repo_pack.git/$p" >second.out &&
+	echo continue >&9 &&
+	wait "$first_pid" &&
+	printf "pack\t%s\n" "$packhash" >expect &&
+	test_cmp expect first.out &&
+	printf "keep\t%s\n" "$packhash" >expect &&
+	test_cmp expect second.out &&
+	test_grep "Range: bytes=64-" first.trace &&
+	test_grep "Range: bytes=[0-9]*-" second.trace &&
+	test_grep "HTTP/[0-9.]* 416" second.trace &&
+	test_path_is_missing "$tmpfile" &&
+	git -C packfileclient-concurrent cat-file -e "$HASH"
+'
+
+test_expect_success PERL,PIPE 'concurrent http-fetch --packfile cannot corrupt an overlapping download' '
+	git init packfileclient-overlap &&
+	blob=$(test-tool genrandom pack-overlap 2m |
+		git -C "$HTTPD_DOCUMENT_ROOT_PATH"/repo_pack.git \
+			hash-object -w --stdin) &&
+	packhash=$(printf "%s\n" "$blob" |
+		git -C "$HTTPD_DOCUMENT_ROOT_PATH"/repo_pack.git \
+			pack-objects "$TRASH_DIRECTORY/overlap-pack") &&
+	pack="$TRASH_DIRECTORY/overlap-pack-$packhash.pack" &&
+	tmpfile="packfileclient-overlap/.git/objects/pack/pack-$packhash.pack.temp" &&
+	mkfifo server-ready first-ready &&
+	exec 7<>server-ready &&
+	exec 8<>first-ready &&
+	write_script slow-pack-server "$PERL_PATH" <<-\EOF &&
+	use strict;
+	use warnings;
+	use IO::Socket::INET;
+
+	my ($packfile, $server_ready, $first_ready) = @ARGV;
+	open(my $in, "<:raw", $packfile) or die "open $packfile: $!";
+	my $pack = do { local $/; <$in> };
+	close($in) or die "close $packfile: $!";
+	my $server = IO::Socket::INET->new(LocalAddr => "127.0.0.1",
+		LocalPort => 0, Proto => "tcp", Listen => 2, ReuseAddr => 1)
+		or die "listen: $!";
+
+	sub signal_ready {
+		my ($file, $value) = @_;
+		open(my $out, ">", $file) or die "open $file: $!";
+		print $out "$value\n" or die "write $file: $!";
+		close($out) or die "close $file: $!";
+	}
+
+	sub write_all {
+		my ($out, $data) = @_;
+		my $offset = 0;
+		while ($offset < length($data)) {
+			my $written = syswrite($out, $data,
+				length($data) - $offset, $offset);
+			defined($written) && $written or die "write response: $!";
+			$offset += $written;
+		}
+	}
+
+	sub start_response {
+		my $out = $server->accept() or die "accept: $!";
+		<$out> or die "read request: $!";
+		my $start = 0;
+		while (<$out>) {
+			last if /^\r?\n$/;
+			$start = $1 if /^Range: bytes=(\d+)-/i;
+		}
+		$start < length($pack) or die "invalid range $start";
+		my $length = length($pack) - $start;
+		my $middle = int($length / 2);
+		my $status = $start ? "206 Partial Content" : "200 OK";
+		my $headers = "HTTP/1.1 $status\r\n" .
+			"Content-Length: $length\r\n" .
+			($start ? "Content-Range: bytes $start-" .
+				(length($pack) - 1) . "/" . length($pack) . "\r\n" : "") .
+			"Connection: close\r\n\r\n";
+		write_all($out, $headers);
+		write_all($out, substr($pack, $start, $middle));
+		return ($out, $start + $middle);
+	}
+
+	signal_ready($server_ready, $server->sockport());
+	my ($first, $first_pos) = start_response();
+	signal_ready($first_ready, "ready");
+	my ($second, $second_pos) = start_response();
+	write_all($first, substr($pack, $first_pos));
+	write_all($second, substr($pack, $second_pos));
+	close($first) or die "close first response: $!";
+	close($second) or die "close second response: $!";
+	EOF
+	{
+		(
+			if ! "$TRASH_DIRECTORY/slow-pack-server" "$pack" \
+				"$TRASH_DIRECTORY/server-ready" \
+				"$TRASH_DIRECTORY/first-ready"
+			then
+				echo failed >"$TRASH_DIRECTORY/server-ready" &&
+				echo failed >"$TRASH_DIRECTORY/first-ready" &&
+				exit 1
+			fi
+		) >server.log 2>&1 &
+		server_pid=$!
+	} &&
+	test_when_finished "
+		kill $server_pid 2>/dev/null || :
+		wait $server_pid 2>/dev/null || :
+		exec 7>&-
+		exec 8>&-
+		rm -f server-ready first-ready slow-pack-server
+	" &&
+	read port <&7 &&
+	url="http://127.0.0.1:$port/pack" &&
+	{
+		(
+			if ! GIT_TRACE_CURL="$TRASH_DIRECTORY/overlap-first.trace" \
+			GIT_TRACE_CURL_NO_DATA=1 \
+			git -C packfileclient-overlap http-fetch --packfile="$packhash" \
+				--index-pack-arg=index-pack \
+				--index-pack-arg=--stdin --index-pack-arg=--keep \
+				"$url" >first.out
+			then
+				echo failed >"$TRASH_DIRECTORY/first-ready" &&
+				exit 1
+			fi
+		) &
+		first_pid=$!
+	} &&
+	test_when_finished "
+		kill $first_pid 2>/dev/null || :
+		wait $first_pid 2>/dev/null || :
+	" &&
+	read ready <&8 &&
+	test "$ready" = ready &&
+	test_path_is_file "$tmpfile" &&
+	test -s "$tmpfile" &&
+	{
+		GIT_TRACE_CURL="$TRASH_DIRECTORY/overlap-second.trace" \
+		GIT_TRACE_CURL_NO_DATA=1 \
+		git -C packfileclient-overlap http-fetch --packfile="$packhash" \
+			--index-pack-arg=index-pack \
+			--index-pack-arg=--stdin --index-pack-arg=--keep \
+			"$url" >second.out &
+		second_pid=$!
+	} &&
+	test_when_finished "
+		kill $second_pid 2>/dev/null || :
+		wait $second_pid 2>/dev/null || :
+	" &&
+	wait "$server_pid" &&
+	wait "$first_pid" &&
+	wait "$second_pid" &&
+	test_grep "HTTP/[0-9.]* 200" overlap-first.trace &&
+	test_grep "Range: bytes=[1-9][0-9]*-" overlap-second.trace &&
+	test_grep "HTTP/[0-9.]* 206" overlap-second.trace &&
+	printf "keep\t%s\npack\t%s\n" "$packhash" "$packhash" | sort >expect &&
+	sort first.out second.out >actual &&
+	test_cmp expect actual &&
+	test_path_is_missing "$tmpfile" &&
+	git -C packfileclient-overlap cat-file -e "$blob"
+'
+
 test_expect_success 'fetch notices corrupt pack' '
 	cp -R "$HTTPD_DOCUMENT_ROOT_PATH"/repo_pack.git "$HTTPD_DOCUMENT_ROOT_PATH"/repo_bad1.git &&
 	(cd "$HTTPD_DOCUMENT_ROOT_PATH"/repo_bad1.git &&

-- 
2.55.0.125.g9b41d4ddb3
Previous: Ted NymanNext: Junio C Hamano
Message 19 of 57 in “packfile URIs: support concurrent downloads”
  1. 0/2 packfile URIs: support concurrent downloadsTed Nyman, Jul 13, 2026
  2. 1/2 http: use unique tempfiles for packfile URI downloadsTed Nyman, Jul 13, 2026
  3. Junio C HamanoJul 14, 2026
  4. Ted NymanJul 14, 2026
  5. Taylor BlauJul 14, 2026
  6. Jeff KingJul 14, 2026
  7. Junio C HamanoJul 14, 2026
  8. Ted NymanJul 14, 2026
  9. Taylor BlauJul 14, 2026
  10. Jeff KingJul 14, 2026
  11. Jeff KingJul 14, 2026
  12. 2/2 fetch-pack: accept "pack" output for packfile URIsTed Nyman, Jul 13, 2026
  13. Jeff KingJul 14, 2026
  14. Jeff KingJul 14, 2026
  15. Ted NymanJul 14, 2026
  16. Jeff KingJul 14, 2026
  17. Taylor BlauJul 14, 2026
  18. 0/2 packfile URIs: support concurrent downloadsTed Nyman, Jul 20, 2026
  19. 1/2 http: avoid concurrent appends to partial packsTed Nyman, Jul 20, 2026
  20. Junio C HamanoJul 21, 2026
  21. 2/2 fetch-pack: accept "pack" output for packfile URIsTed Nyman, Jul 20, 2026
  22. 0/3 packfile URIs: support concurrent downloadsTed Nyman, Jul 21, 2026
  23. 1/3 http-fetch: correct --index-pack-arg documentationTed Nyman, Jul 21, 2026
  24. 2/3 http: avoid concurrent appends to partial packsTed Nyman, Jul 21, 2026
  25. 3/3 fetch-pack: accept "pack" output for packfile URIsTed Nyman, Jul 21, 2026
  26. Junio C HamanoJul 24, 2026
  27. Jeff KingJul 25, 2026
  28. Jeff KingJul 25, 2026
  29. Jeff KingJul 25, 2026
  30. Jeff KingJul 25, 2026
  31. Junio C HamanoJul 25, 2026
  32. 0/3 packfile URIs: support concurrent downloadsTed Nyman, Jul 24, 2026
  33. 1/3 http-fetch: correct --index-pack-arg documentationTed Nyman, Jul 24, 2026
  34. Taylor BlauJul 24, 2026
  35. 2/3 http: avoid concurrent appends to partial packsTed Nyman, Jul 24, 2026
  36. 3/3 fetch-pack: accept "pack" output for packfile URIsTed Nyman, Jul 24, 2026
  37. Taylor BlauJul 24, 2026
  38. 0/3 packfile URIs: support concurrent downloadsTed Nyman, Jul 26, 2026
  39. 1/3 http-fetch: correct --index-pack-arg documentationTed Nyman, Jul 26, 2026
  40. 2/3 http: avoid concurrent appends to partial packsTed Nyman, Jul 26, 2026
  41. Jeff KingJul 26, 2026
  42. Ted NymanJul 26, 2026
  43. Jeff KingJul 26, 2026
  44. 3/3 fetch-pack: accept "pack" output for packfile URIsTed Nyman, Jul 26, 2026
  45. Jeff KingJul 26, 2026
  46. 0/6 packfile URIs: support concurrent downloadsTed Nyman, Jul 27, 2026
  47. 1/6 http-fetch: correct --index-pack-arg documentationTed Nyman, Jul 27, 2026
  48. 2/6 http: avoid closing index-pack input twiceTed Nyman, Jul 27, 2026
  49. Jeff KingAug 1, 2026
  50. 3/6 http: accept HTTP 416 for complete partial packsTed Nyman, Jul 27, 2026
  51. Jeff KingAug 1, 2026
  52. 4/6 http: avoid concurrent appends to partial packsTed Nyman, Jul 27, 2026
  53. 5/6 http: permit unlinking partial packs on WindowsTed Nyman, Jul 27, 2026
  54. 6/6 fetch-pack: accept "pack" output for packfile URIsTed Nyman, Jul 27, 2026
  55. Junio C HamanoJul 29, 2026
  56. Jeff KingAug 1, 2026
  57. Junio C HamanoAug 8, 2026

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