[PATCH v2] fetch-pack: write fetched refs to .promisor

[Jonathan Tan <jonathantanmy@google.com>]

The specification of promisor packfiles (in partial-clone.txt) states
that the .promisor files that accompany packfiles do not matter (just
like .keep files), so whenever a packfile is fetched from the promisor
remote, Git has been writing empty .promisor files. But these files
could contain more useful information.

So instead of writing empty files, write the refs fetched to these
files. This makes it easier to debug issues with partial clones, as we
can identify what refs (and their associated hashes) were fetched at the
time the packfile was downloaded, and if necessary, compare those hashes
against what the promisor remote reports now.

This is implemented by teaching fetch-pack to write its own non-empty
.promisor file whenever it knows the name of the pack's lockfile. This
covers the case wherein the user runs "git fetch" with an internal
protocol or HTTP protocol v2 (fetch_refs_via_pack() in transport.c sets
lock_pack) and with HTTP protocol v0/v1 (fetch_git() in remote-curl.c
passes "--lock-pack" to "fetch-pack").

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
Changes from v1:
 - commit message explains scope of change ("it knows the name of the
   pack's lockfile)
 - explained what .promisor contains in comment in builtin/repack.c
 - moved .promisor writing until after we know that index-pack succeeded
---
 builtin/repack.c         |  7 ++++++
 fetch-pack.c             | 47 ++++++++++++++++++++++++++++++++++++----
 t/t5616-partial-clone.sh |  8 +++++++
 3 files changed, 58 insertions(+), 4 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index 094c2f8ea4..78b23d7a9a 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -233,6 +233,13 @@ static void repack_promisor_objects(const struct pack_objects_args *args,
 		/*
 		 * pack-objects creates the .pack and .idx files, but not the
 		 * .promisor file. Create the .promisor file, which is empty.
+		 *
+		 * NEEDSWORK: fetch-pack sometimes generates non-empty
+		 * .promisor files containing the ref names and associated
+		 * hashes at the point of generation of the corresponding
+		 * packfile, but this would not preserve their contents. Maybe
+		 * concatenate the contents of all .promisor files instead of
+		 * just creating a new empty file.
 		 */
 		promisor_name = mkpathdup("%s-%s.promisor", packtmp,
 					  line.buf);
diff --git a/fetch-pack.c b/fetch-pack.c
index 947da545de..b9e63b52ff 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -754,8 +754,33 @@ static int sideband_demux(int in, int out, void *data)
 	return ret;
 }
 
+static void write_promisor_file(const char *keep_name,
+				struct ref **sought, int nr_sought)
+{
+	struct strbuf promisor_name = STRBUF_INIT;
+	int suffix_stripped;
+	FILE *output;
+	int i;
+
+	strbuf_addstr(&promisor_name, keep_name);
+	suffix_stripped = strbuf_strip_suffix(&promisor_name, ".keep");
+	if (!suffix_stripped)
+		BUG("name of pack lockfile should end with .keep (was '%s')",
+		    keep_name);
+	strbuf_addstr(&promisor_name, ".promisor");
+
+	output = xfopen(promisor_name.buf, "w");
+	for (i = 0; i < nr_sought; i++)
+		fprintf(output, "%s %s\n", oid_to_hex(&sought[i]->old_oid),
+			sought[i]->name);
+	fclose(output);
+
+	strbuf_release(&promisor_name);
+}
+
 static int get_pack(struct fetch_pack_args *args,
-		    int xd[2], char **pack_lockfile)
+		    int xd[2], char **pack_lockfile,
+		    struct ref **sought, int nr_sought)
 {
 	struct async demux;
 	int do_keep = args->keep_pack;
@@ -817,7 +842,13 @@ static int get_pack(struct fetch_pack_args *args,
 		}
 		if (args->check_self_contained_and_connected)
 			argv_array_push(&cmd.args, "--check-self-contained-and-connected");
-		if (args->from_promisor)
+		/*
+		 * If we're obtaining the filename of a lockfile, we'll use
+		 * that filename to write a .promisor file with more
+		 * information below. If not, we need index-pack to do it for
+		 * us.
+		 */
+		if (!(do_keep && pack_lockfile) && args->from_promisor)
 			argv_array_push(&cmd.args, "--promisor");
 	}
 	else {
@@ -871,6 +902,14 @@ static int get_pack(struct fetch_pack_args *args,
 		die(_("%s failed"), cmd_name);
 	if (use_sideband && finish_async(&demux))
 		die(_("error in sideband demultiplexer"));
+
+	/*
+	 * Now that index-pack has succeeded, write the promisor file using the
+	 * obtained .keep filename if necessary
+	 */
+	if (do_keep && pack_lockfile && args->from_promisor)
+		write_promisor_file(*pack_lockfile, sought, nr_sought);
+
 	return 0;
 }
 
@@ -1006,7 +1045,7 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 		alternate_shallow_file = setup_temporary_shallow(si->shallow);
 	else
 		alternate_shallow_file = NULL;
-	if (get_pack(args, fd, pack_lockfile))
+	if (get_pack(args, fd, pack_lockfile, sought, nr_sought))
 		die(_("git fetch-pack: fetch failed."));
 
  all_done:
@@ -1453,7 +1492,7 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 
 			/* get the pack */
 			process_section_header(&reader, "packfile", 0);
-			if (get_pack(args, fd, pack_lockfile))
+			if (get_pack(args, fd, pack_lockfile, sought, nr_sought))
 				die(_("git fetch-pack: fetch failed."));
 
 			state = FETCH_DONE;
diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
index 79f7b65f8c..eaa33a852b 100755
--- a/t/t5616-partial-clone.sh
+++ b/t/t5616-partial-clone.sh
@@ -46,6 +46,14 @@ test_expect_success 'do partial clone 1' '
 	test "$(git -C pc1 config --local remote.origin.partialclonefilter)" = "blob:none"
 '
 
+test_expect_success 'verify that .promisor file contains refs fetched' '
+	ls pc1/.git/objects/pack/pack-*.promisor >promisorlist &&
+	test_line_count = 1 promisorlist &&
+	git -C srv.bare rev-list HEAD >headhash &&
+	grep "$(cat headhash) HEAD" $(cat promisorlist) &&
+	grep "$(cat headhash) refs/heads/master" $(cat promisorlist)
+'
+
 # checkout master to force dynamic object fetch of blobs at HEAD.
 test_expect_success 'verify checkout with dynamic object fetch' '
 	git -C pc1 rev-list --quiet --objects --missing=print HEAD >observed &&
-- 
2.23.0.700.g56cf767bdb-goog