[PATCH 8/8] receive-pack: check connectivity via quarantined objects

[Patrick Steinhardt <ps@pks.im>]

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Convert git-receive-pack(1) to use the new connectivity check which
uses the quarantine directory to enumerate new objects instead of using
references. This change significantly decreases the time required to
accept reference updates due to the fact that we don't need to do a
graph walk anymore. The following measurements have been executed on
linux.git:

Test                                     v2.32.0-rc0             HEAD
--------------------------------------------------------------------------------------------
5400.3: receive-pack clone create        1.27(1.11+0.16)         0.02(0.01+0.01) -98.4%
5400.5: receive-pack clone update        1.27(1.13+0.13)         0.02(0.02+0.00) -98.4%
5400.7: receive-pack clone reset         0.13(0.11+0.02)         0.02(0.01+0.01) -84.6%
5400.9: receive-pack clone delete        0.02(0.01+0.01)         0.03(0.02+0.01) +50.0%
5400.11: receive-pack extrarefs create   33.01(18.80+14.43)      9.00(4.30+4.65) -72.7%
5400.13: receive-pack extrarefs update   33.13(18.85+14.50)      9.01(4.28+4.67) -72.8%
5400.15: receive-pack extrarefs reset    32.90(18.82+14.32)      9.04(4.26+4.77) -72.5%
5400.17: receive-pack extrarefs delete   9.13(4.35+4.77)         8.94(4.29+4.64) -2.1%
5400.19: receive-pack empty create       223.35(640.63+127.74)   227.55(651.75+130.94) +1.9%

Interestingly enough, the normal "clone" repository shows a more extreme
improvement compared to the "extrarefs" repository, which has one
reference per commit. While I haven't yet dived into the root cause, I
expect that there is another location besides the connectivity check
which does scale with the number of references.

Notably, the connectivity check when pushing into an empty repository
shows comparable performance with the previous implementation. This is
because in both cases we're actually performing a complete graph walk to
determine whether there are any unreachable commits.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 builtin/receive-pack.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index b9263cec15..23087acad0 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -1937,8 +1937,12 @@ static void execute_commands(struct command *commands,
 	 * first place. As a result, we do not set up a quarantine environment
 	 * because we know no new objects will be received. And that in turn
 	 * means that we can skip connectivity checks here.
+	 *
+	 * Using the quarantine directory to determine connectivity may not
+	 * work with shallow clones, so let's let's play it safe for now and
+	 * fall back to the old connectivity check.
 	 */
-	if (tmp_objdir) {
+	if (tmp_objdir && (si->nr_ours || si->nr_theirs)) {
 		struct check_connected_options opt = CHECK_CONNECTED_INIT;
 		struct async muxer;
 		int err_fd = 0;
@@ -1960,6 +1964,10 @@ static void execute_commands(struct command *commands,
 
 		if (use_sideband)
 			finish_async(&muxer);
+	} else if (tmp_objdir) {
+		if (check_connected_quarantine(the_repository, tmp_objdir,
+					       iterate_receive_command_list, &data))
+			set_connectivity_errors(commands, si);
 	}
 
 	reject_updates_to_hidden(commands);
-- 
2.31.1


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