spokes-receive-pack: narrow the refs used for the connectivity check

.github/workflows/go-test.yml

@@ -17,7 +17,7 @@ jobs:
           go-version: "1.21"
       - name: Install git.
         run: |
-          sudo apt-get install -y libcurl4-openssl-dev
+          sudo apt-get install -y libcurl4-openssl-dev cargo
           git clone https://github.com/git/git.git
           git -C git checkout next
           sudo make -j 16 -C git prefix=/usr NO_GETTEXT=YesPlease all install

internal/spokes/spokes.go

@@ -1078,18 +1078,16 @@ func (r *spokesReceivePack) performCheckConnectivity(ctx context.Context, comman
 		_ = devNull.Close()
 	}()
 
-	cmd := exec.CommandContext(
-		ctx,
-		"git",
+	args := []string{
 		"rev-list",
 		"--objects",
 		"--no-object-names",
 		"--stdin",
 		"--not",
-		"--exclude-hidden=receive",
-		"--all",
-		"--alternate-refs",
-	)
+	}
+	args = append(args, r.connectivityStopperArgs(ctx)...)
+
+	cmd := exec.CommandContext(ctx, "git", args...)
 	cmd.Stderr = devNull
 	cmd.Env = os.Environ()
 	cmd.Env = append(cmd.Env, r.getAlternateObjectDirsEnv()...)
@@ -1124,6 +1122,34 @@ func (r *spokesReceivePack) performCheckConnectivity(ctx context.Context, comman
 	return nil
 }
 
+// connectivityStopperArgs returns the rev-list arguments that mark the
+// pre-existing object graph as uninteresting for the connectivity check.
+func (r *spokesReceivePack) connectivityStopperArgs(ctx context.Context) []string {
+	if !sockstat.GetBool("spokes_receive_pack_narrow_connectivity_stopper") {
+		return []string{"--exclude-hidden=receive", "--all", "--alternate-refs"}
+	}
+
+	args := []string{"--exclude=*/*", "--branches"}
+	if r.headResolves(ctx) {
+		args = append(args, "HEAD")
+	}
+	return args
+}
+
+// headResolves reports whether `git rev-parse --verify --quiet HEAD`
+// succeeds in the current directory.
+func (r *spokesReceivePack) headResolves(ctx context.Context) bool {
+	cmd := exec.CommandContext(ctx, "git", "rev-parse", "--verify", "--quiet", "HEAD")
+	cmd.Env = os.Environ()
+	cmd.Env = append(cmd.Env, r.getAlternateObjectDirsEnv()...)
+	// Discard stdout (the resolved OID on success) and stderr so we
+	// neither corrupt the receive-pack protocol stream on stdout nor
+	// leak diagnostics on stderr.
+	cmd.Stdout = io.Discard
+	cmd.Stderr = io.Discard
+	return cmd.Run() == nil
+}
+
 func commandsForConnectivityCheck(commands []command) []command {
 	var res []command
 	for _, c := range commands {

internal/spokes/spokes_test.go

@@ -5,9 +5,12 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"os/exec"
+	"path/filepath"
 	"testing"
 
 	"github.com/github/spokes-receive-pack/internal/config"
+	"github.com/github/spokes-receive-pack/internal/sockstat"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -271,3 +274,50 @@ func TestPerformReferenceDiscovery(t *testing.T) {
 	assert.NoError(t, r.performReferenceDiscovery(context.Background()))
 	assert.Equal(t, expectedReferenceList, buf.String())
 }
+
+func TestConnectivityStopperArgs(t *testing.T) {
+	origwd, err := os.Getwd()
+	require.NoError(t, err)
+	// A valid quarantine directory is required so that
+	// getAlternateObjectDirsEnv produces well-formed environment
+	// variables (an empty GIT_OBJECT_DIRECTORY would make git refuse
+	// to operate).
+	quarantine := t.TempDir()
+
+	t.Run("flag off uses legacy stoppers", func(t *testing.T) {
+		t.Setenv(sockstat.Prefix+"spokes_receive_pack_narrow_connectivity_stopper", "")
+		// No filesystem operations are required: the legacy path
+		// never invokes git.
+		r := &spokesReceivePack{}
+		got := r.connectivityStopperArgs(context.Background())
+		assert.Equal(t, []string{"--exclude-hidden=receive", "--all", "--alternate-refs"}, got)
+	})
+
+	t.Run("flag on uses --exclude=*/* --branches HEAD when HEAD resolves", func(t *testing.T) {
+		t.Setenv(sockstat.Prefix+"spokes_receive_pack_narrow_connectivity_stopper", "bool:true")
+		require.NoError(t, os.Chdir("testdata/lots-of-refs.git"))
+		t.Cleanup(func() { _ = os.Chdir(origwd) })
+		wd, _ := os.Getwd()
+		r := &spokesReceivePack{repoPath: wd, quarantineFolder: quarantine}
+		got := r.connectivityStopperArgs(context.Background())
+		assert.Equal(t, []string{"--exclude=*/*", "--branches", "HEAD"}, got)
+	})
+
+	t.Run("flag on omits HEAD when HEAD does not resolve", func(t *testing.T) {
+		t.Setenv(sockstat.Prefix+"spokes_receive_pack_narrow_connectivity_stopper", "bool:true")
+		// A fresh repo with no commits has HEAD pointing at an
+		// unborn branch, so rev-parse --verify HEAD fails.  Use a
+		// non-bare repo to side-step any safe.bareRepository
+		// configuration the caller may have set.
+		dir := t.TempDir()
+		require.NoError(t, exec.Command("git", "init", dir).Run())
+		require.NoError(t, os.Chdir(dir))
+		t.Cleanup(func() { _ = os.Chdir(origwd) })
+		r := &spokesReceivePack{
+			repoPath:         filepath.Join(dir, ".git"),
+			quarantineFolder: quarantine,
+		}
+		got := r.connectivityStopperArgs(context.Background())
+		assert.Equal(t, []string{"--exclude=*/*", "--branches"}, got)
+	})
+}