git: Always disable use of alternate refs for git-fetch(1)

While we know that git-fetch(1) is having issues when using alternate
refs, we currently only disable it when using `FetchInternal()`. Let's
globally disable usage of alternate refs so that we don't ever hit the
performance edge case when fetching into repositories which have an
object pool connected to them.

This fixes a performance regression in the FetchRemote RPC in case the
external configuration doesn't ask us to ignore alternate refs. In a
recent incident, latency of the RPC regressed from ~1s to ~90s per
invocation for forks of `gitlab-org/gitlab` when the external Git
configuration was removed.

Changelog: fixed

2 files changed, 10 insertions, 8 deletions

diff --git a/internal/git/command_description.go b/internal/git/command_description.go
index 6ca1ce76c..c0939a704 100644
--- a/internal/git/command_description.go
+++ b/internal/git/command_description.go
@@ -86,6 +86,16 @@ var commandDescriptions = map[string]commandDescription{
 		flags: 0,
 
 		opts: append([]GlobalOption{
+			// We've observed performance issues when fetching into big repositories
+			// part of an object pool. The root cause of this seems to be the
+			// connectivity check, which by default will also include references of any
+			// alternates. Given that object pools often have hundreds of thousands of
+			// references, this is quite expensive to compute. Below config entry will
+			// disable listing of alternate refs: they shouldn't even be included in the
+			// negotiation phase, so they aren't going to matter in the connectivity
+			// check either.
+			ConfigPair{Key: "core.alternateRefsCommand", Value: "exit 0 #"},
+
 			// While git-fetch(1) by default won't write commit graphs, both CNG and
 			// Omnibus set this value to true. This has caused performance issues when
 			// doing internal fetches, and furthermore it's not encouraged to run such
diff --git a/internal/git/localrepo/remote.go b/internal/git/localrepo/remote.go
index b9d4428aa..53b1ce67e 100644
--- a/internal/git/localrepo/remote.go
+++ b/internal/git/localrepo/remote.go
@@ -128,14 +128,6 @@ func (repo *Repo) FetchInternal(
 	commandOptions := []git.CmdOpt{
 		git.WithEnv(opts.Env...),
 		git.WithStderr(opts.Stderr),
-		// We've observed performance issues when fetching into big repositories part of an
-		// object pool. The root cause of this seems to be the connectivity check, which by
-		// default will also include references of any alternates. Given that object pools
-		// often have hundreds of thousands of references, this is quite expensive to
-		// compute. Below config entry will disable listing of alternate refs: they
-		// shouldn't even be included in the negotiation phase, so they aren't going to
-		// matter in the connectivity check either.
-		git.WithConfig(git.ConfigPair{Key: "core.alternateRefsCommand", Value: "exit 0 #"}),
 		git.WithInternalFetchWithSidechannel(
 			&gitalypb.SSHUploadPackWithSidechannelRequest{
 				Repository:       remoteRepo,