send-pack.c: add config push.useBitmaps

Reachability bitmaps are designed to speed up the "counting objects"
phase of generating a pack during a clone or fetch. They are not
optimized for Git clients sending a small topic branch via "git push".
In some cases (see [1]), using reachability bitmaps during "git push"
can cause significant performance regressions.

Add a new "push.useBitmaps" configuration variable to allow users to
tell "git push" not to use bitmaps. We already have "pack.bitmaps"
that controls the use of bitmaps, but a separate configuration variable
allows the reachability bitmaps to still be used in other areas,
such as "git upload-pack", while disabling it only for "git push".

[1]: https://lore.kernel.org/git/87zhoz8b9o.fsf@evledraar.gmail.com/

Signed-off-by: Kyle Zhao <kylezhao@tencent.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/push.txt

@@ -137,3 +137,8 @@ push.negotiate::
 	server attempt to find commits in common. If "false", Git will
 	rely solely on the server's ref advertisement to find commits
 	in common.
+
+push.useBitmaps::
+	If set to "false", disable use of bitmaps for "git push" even if
+	`pack.useBitmaps` is "true", without preventing other git operations
+	from using bitmaps. Default is true.

send-pack.c

@@ -84,6 +84,8 @@ static int pack_objects(int fd, struct ref *refs, struct oid_array *advertised,
 		strvec_push(&po.args, "--progress");
 	if (is_repository_shallow(the_repository))
 		strvec_push(&po.args, "--shallow");
+	if (args->disable_bitmaps)
+		strvec_push(&po.args, "--no-use-bitmap-index");
 	po.in = -1;
 	po.out = args->stateless_rpc ? -1 : fd;
 	po.git_cmd = 1;
@@ -487,6 +489,7 @@ int send_pack(struct send_pack_args *args,
 	struct async demux;
 	const char *push_cert_nonce = NULL;
 	struct packet_reader reader;
+	int use_bitmaps;
 
 	if (!remote_refs) {
 		fprintf(stderr, "No refs in common and none specified; doing nothing.\n"
@@ -498,6 +501,9 @@ int send_pack(struct send_pack_args *args,
 	if (push_negotiate)
 		get_commons_through_negotiation(args->url, remote_refs, &commons);
 
+	if (!git_config_get_bool("push.usebitmaps", &use_bitmaps))
+		args->disable_bitmaps = !use_bitmaps;
+
 	git_config_get_bool("transfer.advertisesid", &advertise_sid);
 
 	/* Does the other end support the reporting? */

send-pack.h

@@ -26,7 +26,8 @@ struct send_pack_args {
 		/* One of the SEND_PACK_PUSH_CERT_* constants. */
 		push_cert:2,
 		stateless_rpc:1,
-		atomic:1;
+		atomic:1,
+		disable_bitmaps:1;
 	const struct string_list *push_options;
 };
 

t/t5516-fetch-push.sh

@@ -1865,4 +1865,26 @@ test_expect_success 'push warns or fails when using username:password' '
 	test_line_count = 1 warnings
 '
 
+test_expect_success 'push with config push.useBitmaps' '
+	mk_test testrepo heads/main &&
+	git checkout main &&
+	test_unconfig push.useBitmaps &&
+	GIT_TRACE2_EVENT="$PWD/default" \
+	git push testrepo main:test &&
+	test_subcommand git pack-objects --all-progress-implied --revs --stdout \
+		--thin --delta-base-offset -q <default &&
+
+	test_config push.useBitmaps true &&
+	GIT_TRACE2_EVENT="$PWD/true" \
+	git push testrepo main:test2 &&
+	test_subcommand git pack-objects --all-progress-implied --revs --stdout \
+		--thin --delta-base-offset -q <true &&
+
+	test_config push.useBitmaps false &&
+	GIT_TRACE2_EVENT="$PWD/false" \
+	git push testrepo main:test3 &&
+	test_subcommand git pack-objects --all-progress-implied --revs --stdout \
+		--thin --delta-base-offset -q --no-use-bitmap-index <false
+'
+
 test_done