Fix git clone failing when default branch is not master

go/kbfs/kbfsgit/runner.go

@@ -682,6 +682,21 @@ func (r *runner) waitForJournal(ctx context.Context) error {
 		r.printStageEndIfNeeded)
 }
 
+// bestBranchFromCandidates selects the best branch for HEAD from a set of
+// candidate branch names (prefer main > master > alphabetically first).
+func bestBranchFromCandidates(best plumbing.ReferenceName, candidate plumbing.ReferenceName) plumbing.ReferenceName {
+	switch {
+	case candidate == "refs/heads/main":
+		return candidate
+	case candidate == "refs/heads/master" && best != "refs/heads/main":
+		return candidate
+	case best == "" || (best != "refs/heads/main" && best != "refs/heads/master" && candidate < best):
+		return candidate
+	default:
+		return best
+	}
+}
+
 // handleList: From https://git-scm.com/docs/git-remote-helpers
 //
 // Lists the refs, one per line, in the format "<value> <name> [<attr>
@@ -708,9 +723,18 @@ func (r *runner) handleList(ctx context.Context, args []string) (err error) {
 	if err != nil {
 		return err
 	}
+	defer refs.Close()
 
-	var symRefs []string
+	type symRefInfo struct {
+		name   plumbing.ReferenceName
+		target plumbing.ReferenceName
+	}
+	var symRefs []symRefInfo
+	hashRefNames := make(map[plumbing.ReferenceName]bool)
 	hashesSeen := false
+	// Track the best fallback branch for HEAD in case its target
+	// doesn't exist (prefer main > master > alphabetically first).
+	var bestBranch plumbing.ReferenceName
 	for {
 		ref, err := refs.Next()
 		if errors.Cause(err) == io.EOF {
@@ -725,6 +749,11 @@ func (r *runner) handleList(ctx context.Context, args []string) (err error) {
 		case plumbing.HashReference:
 			value = ref.Hash().String()
 			hashesSeen = true
+			hashRefNames[ref.Name()] = true
+			// Track best branch for fallback HEAD.
+			if strings.HasPrefix(ref.Name().String(), "refs/heads/") {
+				bestBranch = bestBranchFromCandidates(bestBranch, ref.Name())
+			}
 		case plumbing.SymbolicReference:
 			value = "@" + ref.Target().String()
 		default:
@@ -737,7 +766,10 @@ func (r *runner) handleList(ctx context.Context, args []string) (err error) {
 			// cloning an empty repo will result in an error because
 			// the HEAD symbolic ref points to a ref that doesn't
 			// exist.
-			symRefs = append(symRefs, refStr)
+			symRefs = append(symRefs, symRefInfo{
+				name:   ref.Name(),
+				target: ref.Target(),
+			})
 			continue
 		}
 		r.log.CDebugf(ctx, "Listing ref %s", refStr)
@@ -748,7 +780,30 @@ func (r *runner) handleList(ctx context.Context, args []string) (err error) {
 	}
 
 	if hashesSeen && !forPush {
-		for _, refStr := range symRefs {
+		for _, sr := range symRefs {
+			target := sr.target
+			// If the symref target doesn't exist among hash refs,
+			// rewrite it to point to the best available branch,
+			// but only for HEAD. Other symrefs are emitted as-is.
+			if !hashRefNames[target] {
+				if sr.name == plumbing.HEAD {
+					if bestBranch == "" {
+						r.log.CDebugf(ctx,
+							"Skipping HEAD symref %s -> %s (no branches available)",
+							sr.name, target)
+						continue
+					}
+					r.log.CDebugf(ctx,
+						"Rewriting HEAD symref from %s to %s",
+						target, bestBranch)
+					target = bestBranch
+				} else {
+					r.log.CDebugf(ctx,
+						"Emitting non-HEAD symref %s -> %s with unknown target",
+						sr.name, target)
+				}
+			}
+			refStr := "@" + target.String() + " " + sr.name.String() + "\n"
 			r.log.CDebugf(ctx, "Listing symbolic ref %s", refStr)
 			_, err = r.output.Write([]byte(refStr))
 			if err != nil {
@@ -1830,6 +1885,53 @@ func (r *runner) handlePushBatch(ctx context.Context, args [][]string) (
 		return nil, err
 	}
 
+	// If HEAD points to a nonexistent ref, update it to point to the
+	// best available branch in the repo (prefer main > master > alphabetical).
+	// We intentionally repair HEAD even when the target was explicitly
+	// deleted in this batch, because a broken HEAD causes clone failures.
+	// This must happen before waitForJournal so the HEAD update is
+	// included in the same flush.
+	head, headErr := repo.Storer.Reference(plumbing.HEAD)
+	if headErr == nil && head.Type() == plumbing.SymbolicReference {
+		_, targetErr := repo.Storer.Reference(head.Target())
+		if targetErr == plumbing.ErrReferenceNotFound {
+			var bestBranch plumbing.ReferenceName
+			allRefs, refsErr := repo.References()
+			if refsErr == nil {
+				defer allRefs.Close()
+				for {
+					ref, nextErr := allRefs.Next()
+					if errors.Cause(nextErr) == io.EOF {
+						break
+					}
+					if nextErr != nil {
+						break
+					}
+					if ref.Type() != plumbing.HashReference {
+						continue
+					}
+					if !strings.HasPrefix(ref.Name().String(), "refs/heads/") {
+						continue
+					}
+					bestBranch = bestBranchFromCandidates(bestBranch, ref.Name())
+				}
+			}
+			if bestBranch != "" {
+				newHead := plumbing.NewSymbolicReference(
+					plumbing.HEAD, bestBranch)
+				if setErr := repo.Storer.SetReference(
+					newHead); setErr != nil {
+					r.log.CDebugf(ctx,
+						"Error updating HEAD to %s: %+v",
+						bestBranch, setErr)
+				} else {
+					r.log.CDebugf(ctx,
+						"Updated HEAD to point to %s", bestBranch)
+				}
+			}
+		}
+	}
+
 	err = r.waitForJournal(ctx)
 	if err != nil {
 		return nil, err

go/kbfs/kbfsgit/runner_test.go

@@ -27,6 +27,7 @@ import (
 	"github.com/keybase/client/go/protocol/keybase1"
 	"github.com/stretchr/testify/require"
 	gogitcfg "gopkg.in/src-d/go-git.v4/config"
+	"gopkg.in/src-d/go-git.v4/plumbing"
 )
 
 type testErrput struct {
@@ -156,7 +157,7 @@ func makeLocalRepoWithOneFileCustomCommitMsg(t *testing.T,
 	gitExec(t, dotgit, gitDir, "init")
 
 	if branch != "" {
-		gitExec(t, dotgit, gitDir, "checkout", "-b", branch)
+		gitExec(t, dotgit, gitDir, "checkout", "-B", branch)
 	}
 
 	gitExec(t, dotgit, gitDir, "add", filename)
@@ -1289,3 +1290,42 @@ func TestRunnerLFS(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, lfsData, buf)
 }
+
+// Test that when only a non-master branch (e.g. "main") is pushed,
+// HEAD correctly points to that branch instead of the nonexistent
+// "refs/heads/master".
+func TestRunnerListNonMasterDefault(t *testing.T) {
+	ctx, config, tempdir := initConfigForRunner(t)
+	defer func() { _ = os.RemoveAll(tempdir) }()
+	defer libkbfs.CheckConfigAndShutdown(ctx, t, config)
+
+	gitDir, err := os.MkdirTemp(os.TempDir(), "kbfsgittest")
+	require.NoError(t, err)
+	defer func() { _ = os.RemoveAll(gitDir) }()
+
+	makeLocalRepoWithOneFile(t, gitDir, "foo", "hello", "main")
+
+	h, err := tlfhandle.ParseHandle(
+		ctx, config.KBPKI(), config.MDOps(), config, "user1", tlf.Private)
+	require.NoError(t, err)
+	_, err = libgit.CreateRepoAndID(ctx, config, h, "test")
+	require.NoError(t, err)
+
+	testPush(ctx, t, config, gitDir,
+		"refs/heads/main:refs/heads/main")
+
+	// Verify the underlying KBFS repo HEAD symref was actually updated.
+	fs, _, err := libgit.GetRepoAndID(ctx, config, h, "test", "")
+	require.NoError(t, err)
+	storage, err := libgit.NewGitConfigWithoutRemotesStorer(fs)
+	require.NoError(t, err)
+	headRef, err := storage.Reference(plumbing.HEAD)
+	require.NoError(t, err)
+	require.Equal(t, plumbing.SymbolicReference, headRef.Type())
+	require.Equal(t, plumbing.ReferenceName("refs/heads/main"), headRef.Target())
+
+	// List refs and verify HEAD points to refs/heads/main.
+	heads := testListAndGetHeads(ctx, t, config, gitDir,
+		[]string{"refs/heads/main", "HEAD"})
+	require.Equal(t, "@refs/heads/main", heads[1])
+}

go/kbfs/libgit/browser.go

@@ -57,10 +57,12 @@ type Browser struct {
 var _ billy.Filesystem = (*Browser)(nil)
 
 // NewBrowser makes a new Browser instance, browsing the given branch
-// of the given repo.  If `gitBranchName` is empty,
-// "refs/heads/master" is used.  If `gitBranchName` is not empty, but
-// it doesn't begin with "refs/", then "refs/heads/" is prepended to
-// it.
+// of the given repo.  If `gitBranchName` is empty, HEAD is resolved
+// to determine the default branch; if HEAD is missing or points to a
+// nonexistent ref, NewBrowser falls back to "refs/heads/master" if it
+// exists, otherwise the repo is treated as empty.  If `gitBranchName`
+// is not empty but doesn't begin with "refs/", then "refs/heads/" is
+// prepended to it.
 func NewBrowser(
 	repoFS *libfs.FS, clock libkbfs.Clock,
 	gitBranchName plumbing.ReferenceName,
@@ -73,6 +75,7 @@ func NewBrowser(
 	}
 
 	const masterBranch = "refs/heads/master"
+	branchWasEmpty := gitBranchName == ""
 	if gitBranchName == "" {
 		gitBranchName = masterBranch
 	} else if !strings.HasPrefix(string(gitBranchName), "refs/") {
@@ -98,9 +101,23 @@ func NewBrowser(
 		return nil, err
 	}
 
+	// If no branch was specified, try to resolve HEAD to find the
+	// default branch instead of hardcoding master.
+	if branchWasEmpty {
+		headRef, headErr := repo.Reference(plumbing.HEAD, false)
+		if headErr == nil && headRef.Type() == plumbing.SymbolicReference {
+			gitBranchName = headRef.Target()
+		}
+	}
+
 	ref, err := repo.Reference(gitBranchName, true)
-	if err == plumbing.ErrReferenceNotFound && gitBranchName == masterBranch {
-		// This branch has no commits, so pretend it's empty.
+	if err == plumbing.ErrReferenceNotFound && branchWasEmpty && gitBranchName != masterBranch {
+		// HEAD points to a nonexistent ref; fall back to master.
+		gitBranchName = masterBranch
+		ref, err = repo.Reference(gitBranchName, true)
+	}
+	if err == plumbing.ErrReferenceNotFound && (gitBranchName == masterBranch || branchWasEmpty) {
+		// No commits on this branch, so pretend it's empty.
 		return &Browser{
 			root:        string(gitBranchName),
 			sharedCache: sharedCache,

go/kbfs/libgit/browser_test.go

@@ -19,6 +19,7 @@ import (
 	"github.com/keybase/client/go/protocol/keybase1"
 	"github.com/stretchr/testify/require"
 	gogit "gopkg.in/src-d/go-git.v4"
+	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/object"
 )
 
@@ -176,3 +177,68 @@ func TestBrowserWithCache(t *testing.T) {
 	require.NoError(t, err)
 	testBrowser(t, cache)
 }
+
+func TestBrowserHeadResolution(t *testing.T) {
+	ctx, config, cancel, tempdir := initConfigForAutogit(t)
+	defer cancel()
+	defer func() { _ = os.RemoveAll(tempdir) }()
+	defer libkbfs.CheckConfigAndShutdown(ctx, t, config)
+
+	h, err := tlfhandle.ParseHandle(
+		ctx, config.KBPKI(), config.MDOps(), nil, "user1", tlf.Private)
+	require.NoError(t, err)
+	rootFS, err := libfs.NewFS(
+		ctx, config, h, data.MasterBranch, "", "", keybase1.MDPriorityNormal)
+	require.NoError(t, err)
+
+	t.Log("Init a new repo directly into KBFS.")
+	dotgitFS, _, err := GetOrCreateRepoAndID(ctx, config, h, "test-head", "")
+	require.NoError(t, err)
+
+	err = rootFS.MkdirAll("worktree-head", 0o600)
+	require.NoError(t, err)
+	worktreeFS, err := rootFS.Chroot("worktree-head")
+	require.NoError(t, err)
+	dotgitStorage, err := NewGitConfigWithoutRemotesStorer(dotgitFS)
+	require.NoError(t, err)
+	repo, err := gogit.Init(dotgitStorage, worktreeFS)
+	require.NoError(t, err)
+
+	t.Log("Set HEAD to point to refs/heads/main instead of master.")
+	newHead := plumbing.NewSymbolicReference(
+		plumbing.HEAD, "refs/heads/main")
+	err = repo.Storer.SetReference(newHead)
+	require.NoError(t, err)
+
+	t.Log("Commit a file — go-git creates the main branch since HEAD points there.")
+	addFileToWorktreeAndCommit(
+		ctx, t, config, h, repo, worktreeFS, "hello.txt", "world")
+
+	t.Log("NewBrowser with empty branch should resolve HEAD to main.")
+	b, err := NewBrowser(dotgitFS, config.Clock(), "", noopSharedInBrowserCache{})
+	require.NoError(t, err)
+	require.NotNil(t, b.tree, "browser should have a non-nil tree")
+	fi, err := b.Stat("hello.txt")
+	require.NoError(t, err)
+	require.Equal(t, "hello.txt", fi.Name())
+
+	f, err := b.Open("hello.txt")
+	require.NoError(t, err)
+	defer func() { _ = f.Close() }()
+	fileData, err := io.ReadAll(f)
+	require.NoError(t, err)
+	require.Equal(t, "world", string(fileData))
+
+	t.Log("Test stale HEAD: point HEAD to a nonexistent ref.")
+	staleHead := plumbing.NewSymbolicReference(
+		plumbing.HEAD, "refs/heads/nonexistent")
+	err = repo.Storer.SetReference(staleHead)
+	require.NoError(t, err)
+
+	t.Log("NewBrowser should return an empty browser without error.")
+	b, err = NewBrowser(dotgitFS, config.Clock(), "", noopSharedInBrowserCache{})
+	require.NoError(t, err)
+	fis, err := b.ReadDir("")
+	require.NoError(t, err)
+	require.Len(t, fis, 0)
+}