refactor: improve overlayFS errors

aslilac · aslilac · commit a1f2c0de5e89 · 2025-05-13T21:19:24.000Z
diff --git a/coderd/files/overlay.go b/coderd/files/overlay.go
@@ -8,11 +8,10 @@ import (
 	"golang.org/x/xerrors"
 )
 
-// overlayFS allows you to "join" together the template files tar file fs.FS
-// with the Terraform modules tar file fs.FS. We could potentially turn this
-// into something more parameterized/configurable, but the requirements here are
-// a _bit_ odd, because every file in the modulesFS includes the
-// .terraform/modules/ folder at the beginning of it's path.
+// overlayFS allows you to "join" together multiple fs.FS. Files in any specific
+// overlay will only be accessible if their path starts with the base path
+// provided for the overlay. eg. An overlay at the path .terraform/modules
+// should contain files with paths inside the .terraform/modules folder.
 type overlayFS struct {
 	baseFS   fs.FS
 	overlays []Overlay
@@ -23,64 +22,50 @@ type Overlay struct {
 	fs.FS
 }
 
-func NewOverlayFS(baseFS fs.FS, overlays []Overlay) (fs.FS, error) {
-	if err := valid(baseFS); err != nil {
-		return nil, xerrors.Errorf("baseFS: %w", err)
-	}
-
-	for _, overlay := range overlays {
-		if err := valid(overlay.FS); err != nil {
-			return nil, xerrors.Errorf("overlayFS: %w", err)
-		}
-	}
-
+func NewOverlayFS(baseFS fs.FS, overlays []Overlay) fs.FS {
 	return overlayFS{
 		baseFS:   baseFS,
 		overlays: overlays,
-	}, nil
+	}
 }
 
 func (f overlayFS) Open(p string) (fs.File, error) {
+	target := f.baseFS
 	for _, overlay := range f.overlays {
 		if strings.HasPrefix(path.Clean(p), overlay.Path) {
-			return overlay.FS.Open(p)
+			target = overlay.FS
+			break
 		}
 	}
-	return f.baseFS.Open(p)
+	return target.Open(p)
 }
 
 func (f overlayFS) ReadDir(p string) ([]fs.DirEntry, error) {
+	target := f.baseFS
 	for _, overlay := range f.overlays {
 		if strings.HasPrefix(path.Clean(p), overlay.Path) {
-			//nolint:forcetypeassert
-			return overlay.FS.(fs.ReadDirFS).ReadDir(p)
+			target = overlay.FS
+			break
 		}
 	}
-	//nolint:forcetypeassert
-	return f.baseFS.(fs.ReadDirFS).ReadDir(p)
+	it, ok := target.(fs.ReadDirFS)
+	if !ok {
+		return nil, xerrors.New("provided fs.FS does not implement fs.ReadDirFS")
+	}
+	return it.ReadDir(p)
 }
 
 func (f overlayFS) ReadFile(p string) ([]byte, error) {
+	target := f.baseFS
 	for _, overlay := range f.overlays {
 		if strings.HasPrefix(path.Clean(p), overlay.Path) {
-			//nolint:forcetypeassert
-			return overlay.FS.(fs.ReadFileFS).ReadFile(p)
+			target = overlay.FS
+			break
 		}
 	}
-	//nolint:forcetypeassert
-	return f.baseFS.(fs.ReadFileFS).ReadFile(p)
-}
-
-// valid checks that the fs.FS implements the required interfaces.
-// The fs.FS interface is not sufficient.
-func valid(fsys fs.FS) error {
-	_, ok := fsys.(fs.ReadDirFS)
-	if !ok {
-		return xerrors.New("overlayFS does not implement ReadDirFS")
-	}
-	_, ok = fsys.(fs.ReadFileFS)
+	it, ok := target.(fs.ReadFileFS)
 	if !ok {
-		return xerrors.New("overlayFS does not implement ReadFileFS")
+		return nil, xerrors.New("provided fs.FS does not implement fs.ReadFileFS")
 	}
-	return nil
+	return it.ReadFile(p)
 }
