[carry] Automatically detect default git branch

source/git/gitsource.go

@@ -37,6 +37,7 @@ import (
 )
 
 var validHex = regexp.MustCompile(`^[a-f0-9]{40}$`)
+var defaultBranch = regexp.MustCompile(`refs/heads/(\S+)`)
 
 type Opt struct {
 	CacheAccessor cache.Accessor
@@ -307,14 +308,10 @@ func (gs *gitSourceHandler) mountKnownHosts(ctx context.Context) (string, func()
 
 func (gs *gitSourceHandler) CacheKey(ctx context.Context, g session.Group, index int) (string, solver.CacheOpts, bool, error) {
 	remote := gs.src.Remote
-	ref := gs.src.Ref
-	if ref == "" {
-		ref = "master"
-	}
 	gs.locker.Lock(remote)
 	defer gs.locker.Unlock(remote)
 
-	if isCommitSHA(ref) {
+	if ref := gs.src.Ref; ref != "" && isCommitSHA(ref) {
 		ref = gs.shaToCacheKey(ref)
 		gs.cacheKey = ref
 		return ref, nil, true, nil
@@ -348,6 +345,14 @@ func (gs *gitSourceHandler) CacheKey(ctx context.Context, g session.Group, index
 		defer unmountKnownHosts()
 	}
 
+	ref := gs.src.Ref
+	if ref == "" {
+		ref, err = getDefaultBranch(ctx, gitDir, "", sock, knownHosts, gs.auth, gs.src.Remote)
+		if err != nil {
+			return "", nil, false, err
+		}
+	}
+
 	// TODO: should we assume that remote tag is immutable? add a timer?
 
 	buf, err := gitWithinDir(ctx, gitDir, "", sock, knownHosts, gs.auth, "ls-remote", "origin", ref)
@@ -370,11 +375,6 @@ func (gs *gitSourceHandler) CacheKey(ctx context.Context, g session.Group, index
 }
 
 func (gs *gitSourceHandler) Snapshot(ctx context.Context, g session.Group) (out cache.ImmutableRef, retErr error) {
-	ref := gs.src.Ref
-	if ref == "" {
-		ref = "master"
-	}
-
 	cacheKey := gs.cacheKey
 	if cacheKey == "" {
 		var err error
@@ -426,6 +426,14 @@ func (gs *gitSourceHandler) Snapshot(ctx context.Context, g session.Group) (out
 		defer unmountKnownHosts()
 	}
 
+	ref := gs.src.Ref
+	if ref == "" {
+		ref, err = getDefaultBranch(ctx, gitDir, "", sock, knownHosts, gs.auth, gs.src.Remote)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	doFetch := true
 	if isCommitSHA(ref) {
 		// skip fetch if commit already exists
@@ -686,3 +694,17 @@ func tokenScope(remote string) string {
 	}
 	return remote
 }
+
+// getDefaultBranch gets the default branch of a repository using ls-remote
+func getDefaultBranch(ctx context.Context, gitDir, workDir, sshAuthSock, knownHosts string, auth []string, remoteURL string) (string, error) {
+	buf, err := gitWithinDir(ctx, gitDir, workDir, sshAuthSock, knownHosts, auth, "ls-remote", "--symref", remoteURL, "HEAD")
+	if err != nil {
+		return "", errors.Wrapf(err, "error fetching default branch for repository %s", redactCredentials(remoteURL))
+	}
+
+	ss := defaultBranch.FindAllStringSubmatch(buf.String(), -1)
+	if len(ss) == 0 || len(ss[0]) != 2 {
+		return "", errors.Errorf("could not find default branch for repository: %s", redactCredentials(remoteURL))
+	}
+	return ss[0][1], nil
+}

source/git/gitsource_test.go

@@ -498,6 +498,7 @@ func setupGitRepo(dir string) (string, error) {
 		"git submodule add "+subPath+" sub",
 		"git add -A",
 		"git commit -m withsub",
+		"git checkout master",
 	); err != nil {
 		return "", err
 	}

source/gitidentifier.go

@@ -64,7 +64,7 @@ func isGitTransport(str string) bool {
 
 func getRefAndSubdir(fragment string) (ref string, subdir string) {
 	refAndDir := strings.SplitN(fragment, ":", 2)
-	ref = "master"
+	ref = ""
 	if len(refAndDir[0]) != 0 {
 		ref = refAndDir[0]
 	}

source/gitidentifier_test.go

@@ -10,7 +10,7 @@ func TestNewGitIdentifier(t *testing.T) {
 	gi, err := NewGitIdentifier("ssh://[email protected]:2222/root/my/really/weird/path/foo.git")
 	require.Nil(t, err)
 	require.Equal(t, "ssh://[email protected]:2222/root/my/really/weird/path/foo.git", gi.Remote)
-	require.Equal(t, "master", gi.Ref)
+	require.Equal(t, "", gi.Ref)
 	require.Equal(t, "", gi.Subdir)
 
 	gi, err = NewGitIdentifier("ssh://[email protected]:2222/root/my/really/weird/path/foo.git#main")
@@ -22,13 +22,13 @@ func TestNewGitIdentifier(t *testing.T) {
 	gi, err = NewGitIdentifier("[email protected]:moby/buildkit.git")
 	require.Nil(t, err)
 	require.Equal(t, "[email protected]:moby/buildkit.git", gi.Remote)
-	require.Equal(t, "master", gi.Ref)
+	require.Equal(t, "", gi.Ref)
 	require.Equal(t, "", gi.Subdir)
 
-	gi, err = NewGitIdentifier("github.com/moby/buildkit.git")
+	gi, err = NewGitIdentifier("github.com/moby/buildkit.git#main")
 	require.Nil(t, err)
 	require.Equal(t, "https://github.com/moby/buildkit.git", gi.Remote)
-	require.Equal(t, "master", gi.Ref)
+	require.Equal(t, "main", gi.Ref)
 	require.Equal(t, "", gi.Subdir)
 
 }