Discover checkout roots for optional preparation

internal/git/source.go

@@ -3,8 +3,11 @@ package git
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"io"
+	"os"
+	"os/exec"
 	"path"
 	"path/filepath"
 	"strconv"
@@ -41,6 +44,10 @@ func RepositoryRoot(ctx context.Context, directory string) (string, error) {
 	if err != nil {
 		return "", fmt.Errorf("resolve Git repository root: %w", err)
 	}
+	return normalizeRepositoryRoot(root)
+}
+
+func normalizeRepositoryRoot(root string) (string, error) {
 	root = strings.TrimSuffix(root, "\n")
 	resolved, err := filepath.EvalSymlinks(root)
 	if err != nil {
@@ -49,6 +56,84 @@ func RepositoryRoot(ctx context.Context, directory string) (string, error) {
 	return resolved, nil
 }
 
+// DiscoverRepositoryRoot finds a containing checkout or confirms standalone use.
+func DiscoverRepositoryRoot(ctx context.Context, directory string) (string, bool, error) {
+	if directory == "" {
+		directory = "."
+	}
+	var stdout, stderr strings.Builder
+	err := process.Run(ctx, process.Invocation{
+		Args: []string{"git", "rev-parse", "--show-toplevel"}, Dir: directory,
+		Env: map[string]string{"LC_ALL": "C"}, Stdout: &stdout, Stderr: &stderr,
+	})
+	if err == nil {
+		root, err := normalizeRepositoryRoot(stdout.String())
+		return root, err == nil, err
+	}
+	if errors.Is(err, exec.ErrNotFound) {
+		standalone, inspectErr := standaloneWithoutGit(directory)
+		if inspectErr != nil {
+			return "", false, inspectErr
+		}
+		if standalone {
+			return "", false, nil
+		}
+	}
+	var exitErr *exec.ExitError
+	if errors.As(err, &exitErr) && exitErr.ExitCode() == 128 && noRepositoryDiagnostic(stderr.String()) {
+		return "", false, nil
+	}
+	if message := strings.TrimSpace(stderr.String()); message != "" {
+		return "", false, fmt.Errorf("resolve Git repository root: %s: %w", message, err)
+	}
+	return "", false, fmt.Errorf("resolve Git repository root: %w", err)
+}
+
+func noRepositoryDiagnostic(message string) bool {
+	message = strings.TrimSpace(message)
+	if message == "fatal: not a git repository (or any of the parent directories): .git" {
+		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)
+}
+
+func standaloneWithoutGit(directory string) (bool, error) {
+	_, gitDirSet := os.LookupEnv("GIT_DIR")
+	_, workTreeSet := os.LookupEnv("GIT_WORK_TREE")
+	if gitDirSet || workTreeSet {
+		return false, nil
+	}
+	abs, err := filepath.Abs(directory)
+	if err != nil {
+		return false, fmt.Errorf("inspect repository directory: %w", err)
+	}
+	physical, err := filepath.EvalSymlinks(abs)
+	if err != nil {
+		return false, fmt.Errorf("inspect repository directory: %w", err)
+	}
+	info, err := os.Stat(physical)
+	if err != nil {
+		return false, fmt.Errorf("inspect repository directory: %w", err)
+	}
+	if !info.IsDir() {
+		return false, fmt.Errorf("inspect repository directory %q: not a directory", directory)
+	}
+	for dir := physical; ; dir = filepath.Dir(dir) {
+		_, err := os.Lstat(filepath.Join(dir, ".git"))
+		if err == nil {
+			return false, nil
+		}
+		if !errors.Is(err, os.ErrNotExist) {
+			return false, fmt.Errorf("inspect Git marker in %s: %w", dir, err)
+		}
+		if filepath.Dir(dir) == dir {
+			return true, nil
+		}
+	}
+}
+
 // CaptureHead records the checkout commit before identity derived from files
 // in that commit is inspected.
 func CaptureHead(ctx context.Context, repositoryDir string) (string, error) {