Finish output directory follow-up work

internal/app/output.go

@@ -67,14 +67,54 @@ func resolveOutputDir(workingDir, override string) (string, error) {
 		directory = filepath.Join(workingDir, directory)
 	}
 	directory = filepath.Clean(directory)
-	if info, err := os.Stat(directory); err == nil && !info.IsDir() {
-		return "", fmt.Errorf("output directory %q is not a directory", directory)
-	} else if err != nil && !os.IsNotExist(err) {
-		return "", fmt.Errorf("inspect output directory %q: %w", directory, err)
+	if err := preflightOutputDirectory(directory); err != nil {
+		return "", err
 	}
 	return directory, nil
 }
 
+func preflightOutputDirectory(directory string) error {
+	info, err := os.Stat(directory)
+	if err == nil {
+		if !info.IsDir() {
+			return fmt.Errorf("output directory %q is not a directory", directory)
+		}
+		return nil
+	}
+	if !os.IsNotExist(err) {
+		return fmt.Errorf("inspect output directory %q: %w", directory, err)
+	}
+
+	// A missing directory is valid, but os.Stat also reports ErrNotExist for a
+	// dangling symlink. Walk to the first existing component so invalid links
+	// fail preflight instead of being discovered only during publication.
+	for component := directory; ; component = filepath.Dir(component) {
+		componentInfo, componentErr := os.Lstat(component)
+		if componentErr == nil {
+			if componentInfo.Mode()&os.ModeSymlink != 0 {
+				targetInfo, targetErr := os.Stat(component)
+				if targetErr != nil {
+					return fmt.Errorf("inspect output directory %q at %q: %w", directory, component, targetErr)
+				}
+				if !targetInfo.IsDir() {
+					return fmt.Errorf("output directory %q has non-directory path component %q", directory, component)
+				}
+				return nil
+			}
+			if !componentInfo.IsDir() {
+				return fmt.Errorf("output directory %q has non-directory path component %q", directory, component)
+			}
+			return nil
+		}
+		if !os.IsNotExist(componentErr) {
+			return fmt.Errorf("inspect output directory %q at %q: %w", directory, component, componentErr)
+		}
+		if filepath.Dir(component) == component {
+			return fmt.Errorf("inspect output directory %q: no existing directory ancestor", directory)
+		}
+	}
+}
+
 func resolveOutputPath(workingDir, override, defaultName string) (string, error) {
 	workingDir, err := validateWorkingDir(workingDir)
 	if err != nil {