v2object-info: support for retrieving object info

> Sometimes it is useful to get information of an object without having to
> download it completely.
>
> Add the "object-info" capability that lets the client ask for
> object-related information with their full hexadecimal object names.
>
> Only sizes are returned for now.
>
> Signed-off-by: Bruno Albuquerque <bga@google.com>
> ---
>
> This version is a small change with just style fixes.

> Sometimes it is useful to get information of an object without having to
> download it completely.
>
> Add the "object-info" capability that lets the client ask for
> object-related information with their full hexadecimal object names.
>
> Only sizes are returned for now.
>
> Signed-off-by: Bruno Albuquerque <bga@google.com>
> ---

> This version is a small change with just style fixes.
>
>  Documentation/technical/protocol-v2.txt |  31 +++++++
>  Makefile                                |   1 +
>  protocol-caps.c                         | 113 ++++++++++++++++++++++++
>  protocol-caps.h                         |  10 +++
>  serve.c                                 |   2 +
>  t/t5701-git-serve.sh                    |  26 ++++++
>  6 files changed, 183 insertions(+)
>  create mode 100644 protocol-caps.c
>  create mode 100644 protocol-caps.h
>
> diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
> index a7c806a73e..f4ed141774 100644
> --- a/Documentation/technical/protocol-v2.txt
> +++ b/Documentation/technical/protocol-v2.txt
> @@ -514,3 +514,34 @@ packet-line, and must not contain non-printable or whitespace characters. The
>  current implementation uses trace2 session IDs (see
>  link:api-trace2.html[api-trace2] for details), but this may change and users of
>  the session ID should not rely on this fact.
> +
> +object-info
> +~~~~~~~~~~~
> +
> +`object-info` is the command to retrieve information about one or more objects.
> +Its main purpose is to allow a client to make decisions based on this
> +information without having to fully fetch objects. Object size is the only
> +information that is currently supported.
> +
> +An `object-info` request takes the following arguments:
> +
> +	size
> +	Requests size information to be returned for each listed object id.
> +
> +	oid <oid>
> +	Indicates to the server an object which the client wants to obtain
> +	information for.
> +
> +The response of `object-info` is a list of the the requested object ids
> +and associated requested information, each separated by a single space.
> +
> +	output = info flush-pkt
> +
> +	info = PKT-LINE(attrs) LF)
> +		*PKT-LINE(obj-info LF)
> +
> +	attrs = attr | attrs SP attrs
> +
> +	attr = "size"
> +
> +	obj-info = obj-id SP obj-size
> diff --git a/Makefile b/Makefile
> index 21c0bf1667..3225e37b63 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -961,6 +961,7 @@ LIB_OBJS += progress.o
>  LIB_OBJS += promisor-remote.o
>  LIB_OBJS += prompt.o
>  LIB_OBJS += protocol.o
> +LIB_OBJS += protocol-caps.o
>  LIB_OBJS += prune-packed.o
>  LIB_OBJS += quote.o
>  LIB_OBJS += range-diff.o
> diff --git a/protocol-caps.c b/protocol-caps.c
> new file mode 100644
> index 0000000000..13a9e63a04
> --- /dev/null
> +++ b/protocol-caps.c
> @@ -0,0 +1,113 @@
> +#include "git-compat-util.h"
> +#include "protocol-caps.h"
> +#include "gettext.h"
> +#include "pkt-line.h"
> +#include "strvec.h"
> +#include "hash.h"
> +#include "object.h"
> +#include "object-store.h"
> +#include "string-list.h"
> +#include "strbuf.h"
> +
> +struct requested_info {
> +	unsigned size : 1;
> +};
> +
> +/*
> + * Parses oids from the given line and collects them in the given
> + * oid_str_list. Returns 1 if parsing was successful and 0 otherwise.
> + */
> +static int parse_oid(const char *line, struct string_list *oid_str_list)
> +{
> +	const char *arg;
> +
> +	if (!skip_prefix(line, "oid ", &arg))
> +		return 0;
> +
> +	string_list_append(oid_str_list, arg);
> +
> +	return 1;
> +}
> +
> +/*
> + * Validates and send requested info back to the client. Any errors detected
> + * are returned as they are detected.
> + */
> +static void send_info(struct repository *r, struct packet_writer *writer,
> +		      struct string_list *oid_str_list,
> +		      struct requested_info *info)
> +{
> +	struct string_list_item *item;
> +	struct strbuf send_buffer = STRBUF_INIT;
> +
> +	if (!oid_str_list->nr)
> +		return;
> +
> +	if (info->size)
> +		packet_writer_write(writer, "size");
> +
> +	for_each_string_list_item (item, oid_str_list) {
> +		const char *oid_str = item->string;
> +		struct object_id oid;
> +		unsigned long object_size;
> +
> +		if (get_oid_hex(oid_str, &oid) < 0) {
> +			packet_writer_error(
> +				writer,
> +				"object-info: protocol error, expected to get oid, not '%s'",
> +				oid_str);
> +			continue;
> +		}
> +
> +		strbuf_addstr(&send_buffer, oid_str);
> +
> +		if (info->size) {
> +			if (oid_object_info(r, &oid, &object_size) < 0) {
> +				strbuf_addstr(&send_buffer, " ");
> +			} else {
> +				strbuf_addf(&send_buffer, " %lu", object_size);
> +			}
> +		}
> +
> +		packet_writer_write(writer, "%s",
> +				    strbuf_detach(&send_buffer, NULL));
> +	}
> +}
> +
> +int cap_object_info(struct repository *r, struct strvec *keys,
> +		    struct packet_reader *request)
> +{
> +	struct requested_info info;
> +	struct packet_writer writer;
> +	struct string_list oid_str_list = STRING_LIST_INIT_DUP;
> +
> +	packet_writer_init(&writer, 1);
> +
> +	while (packet_reader_read(request) == PACKET_READ_NORMAL) {
> +		if (!strcmp("size", request->line)) {
> +			info.size = 1;
> +			continue;
> +		}
> +
> +		if (parse_oid(request->line, &oid_str_list))
> +			continue;
> +
> +		packet_writer_error(&writer,
> +				    "object-info: unexpected line: '%s'",
> +				    request->line);
> +	}
> +
> +	if (request->status != PACKET_READ_FLUSH) {
> +		packet_writer_error(
> +			&writer, "object-info: expected flush after arguments");
> +		die(_("object-info: expected flush after arguments"));
> +	}
> +
> +	send_info(r, &writer, &oid_str_list, &info);
> +
> +	string_list_clear(&oid_str_list, 1);
> +
> +	packet_flush(1);
> +
> +	return 0;
> +}
> diff --git a/protocol-caps.h b/protocol-caps.h
> new file mode 100644
> index 0000000000..6351648e37
> --- /dev/null
> +++ b/protocol-caps.h
> @@ -0,0 +1,10 @@
> +#ifndef PROTOCOL_CAPS_H
> +#define PROTOCOL_CAPS_H
> +
> +struct repository;
> +struct strvec;
> +struct packet_reader;
> +int cap_object_info(struct repository *r, struct strvec *keys,
> +		    struct packet_reader *request);
> +
> +#endif /* PROTOCOL_CAPS_H */
> \ No newline at end of file
> diff --git a/serve.c b/serve.c
> index ac20c72763..aa8209f147 100644
> --- a/serve.c
> +++ b/serve.c
> @@ -5,6 +5,7 @@
>  #include "version.h"
>  #include "strvec.h"
>  #include "ls-refs.h"
> +#include "protocol-caps.h"
>  #include "serve.h"
>  #include "upload-pack.h"
>  
> @@ -78,6 +79,7 @@ static struct protocol_capability capabilities[] = {
>  	{ "server-option", always_advertise, NULL },
>  	{ "object-format", object_format_advertise, NULL },
>  	{ "session-id", session_id_advertise, NULL },
> +	{ "object-info", always_advertise, cap_object_info },
>  };
>  
>  static void advertise_capabilities(void)
> diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
> index 509f379d49..73e74a9c54 100755
> --- a/t/t5701-git-serve.sh
> +++ b/t/t5701-git-serve.sh
> @@ -19,6 +19,7 @@ test_expect_success 'test capability advertisement' '
>  	fetch=shallow
>  	server-option
>  	object-format=$(test_oid algo)
> +	object-info
>  	0000
>  	EOF
>  
> @@ -240,4 +241,29 @@ test_expect_success 'unexpected lines are not allowed in fetch request' '
>  	grep "unexpected line: .this-is-not-a-command." err
>  '
>  
> +# Test the basics of object-info
> +#
> +test_expect_success 'basics of object-info' '
> +	test-tool pkt-line pack >in <<-EOF &&
> +	command=object-info
> +	object-format=$(test_oid algo)
> +	0001
> +	size
> +	oid $(git rev-parse two:two.t)
> +	oid $(git rev-parse two:two.t)
> +	0000
> +	EOF
> +
> +	cat >expect <<-EOF &&
> +	size
> +	$(git rev-parse two:two.t) $(wc -c <two.t | xargs)
> +	$(git rev-parse two:two.t) $(wc -c <two.t | xargs)
> +	0000
> +	EOF
> +
> +	test-tool serve-v2 --stateless-rpc <in >out &&
> +	test-tool pkt-line unpack <out >actual &&
> +	test_cmp expect actual
> +'
> +
>  test_done

>
> Add a trailing newline to the protocol-caps.h file added in the recent
> a2ba162cda (object-info: support for retrieving object info,
> 2021-04-20). Various editors add this implicitly, and some compilers
> warn about the lack of a \n here.