Preserve discovery failures for damaged checkouts

internal/git/source.go

@@ -80,8 +80,20 @@ func DiscoverRepositoryRoot(ctx context.Context, directory string) (string, bool
 		}
 	}
 	var exitErr *exec.ExitError
-	if errors.As(err, &exitErr) && exitErr.ExitCode() == 128 && noRepositoryDiagnostic(stderr.String()) {
-		return "", false, nil
+	if errors.As(err, &exitErr) && exitErr.ExitCode() == 128 {
+		if boundary, noRepository := noRepositoryBoundary(stderr.String()); noRepository {
+			boundaries := discoveryCeilings()
+			if boundary != "" {
+				boundaries = append(boundaries, boundary)
+			}
+			standalone, inspectErr := standaloneDirectory(directory, boundaries)
+			if inspectErr != nil {
+				return "", false, errors.Join(inspectErr, err)
+			}
+			if standalone {
+				return "", false, nil
+			}
+		}
 	}
 	if message := strings.TrimSpace(stderr.String()); message != "" {
 		return "", false, fmt.Errorf("resolve Git repository root: %s: %w", message, err)
@@ -89,14 +101,41 @@ func DiscoverRepositoryRoot(ctx context.Context, directory string) (string, bool
 	return "", false, fmt.Errorf("resolve Git repository root: %w", err)
 }
 
-func noRepositoryDiagnostic(message string) bool {
+func noRepositoryBoundary(message string) (string, bool) {
 	message = strings.TrimSpace(message)
 	if message == "fatal: not a git repository (or any of the parent directories): .git" {
-		return true
+		return "", true
 	}
 	const prefix = "fatal: not a git repository (or any parent up to mount point "
 	const suffix = ")\nStopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set)."
-	return strings.HasPrefix(message, prefix) && strings.HasSuffix(message, suffix) && len(message) > len(prefix)+len(suffix)
+	if strings.HasPrefix(message, prefix) && strings.HasSuffix(message, suffix) && len(message) > len(prefix)+len(suffix) {
+		return filepath.Clean(message[len(prefix) : len(message)-len(suffix)]), true
+	}
+	return "", false
+}
+
+func discoveryCeilings() []string {
+	var ceilings []string
+	resolveLinks := true
+	for _, entry := range filepath.SplitList(os.Getenv("GIT_CEILING_DIRECTORIES")) {
+		// Git uses an empty entry to mark subsequent paths as already physical.
+		if entry == "" {
+			resolveLinks = false
+			continue
+		}
+		if !filepath.IsAbs(entry) {
+			continue
+		}
+		if resolveLinks {
+			resolved, err := filepath.EvalSymlinks(entry)
+			if err != nil {
+				continue // Git ignores ceiling entries it cannot resolve.
+			}
+			entry = resolved
+		}
+		ceilings = append(ceilings, filepath.Clean(entry))
+	}
+	return ceilings
 }
 
 func standaloneWithoutGit(directory string) (bool, error) {
@@ -105,6 +144,12 @@ func standaloneWithoutGit(directory string) (bool, error) {
 	if gitDirSet || workTreeSet {
 		return false, nil
 	}
+	return standaloneDirectory(directory, nil)
+}
+
+// standaloneDirectory checks only locations Git could have searched. Boundaries
+// are excluded parents; the supplied directory itself is always inspected.
+func standaloneDirectory(directory string, boundaries []string) (bool, error) {
 	abs, err := filepath.Abs(directory)
 	if err != nil {
 		return false, fmt.Errorf("inspect repository directory: %w", err)
@@ -128,9 +173,15 @@ func standaloneWithoutGit(directory string) (bool, error) {
 		if !errors.Is(err, os.ErrNotExist) {
 			return false, fmt.Errorf("inspect Git marker in %s: %w", dir, err)
 		}
-		if filepath.Dir(dir) == dir {
+		parent := filepath.Dir(dir)
+		if parent == dir {
 			return true, nil
 		}
+		for _, boundary := range boundaries {
+			if parent == boundary {
+				return true, nil
+			}
+		}
 	}
 }