Add file.FSStack implementation

Signed-off-by: Knut Ahlers <knut@ahlers.me>

file/fsstack.go

@@ -0,0 +1,48 @@
+package file
+
+import (
+	"io"
+	"io/fs"
+
+	"github.com/pkg/errors"
+)
+
+// FSStack represents layers of fs.FS to open a file from. The first
+// layer (starting at index 0, going up) responding other than
+// fs.ErrNotExist will determine the response of this stack.
+type FSStack []fs.FS
+
+var _ fs.FS = FSStack{}
+
+// Open iterates the FSStack starting at index 0, going up and returns
+// the first non fs.ErrNotExist response. If all layers responds with
+// fs.ErrNotExist a fs.PathError wrapping fs.ErrNotExist is returned.
+func (f FSStack) Open(name string) (fs.File, error) {
+	for i, fse := range f {
+		file, err := fse.Open(name)
+		if err != nil {
+			if errors.Is(err, fs.ErrNotExist) {
+				continue
+			}
+
+			return nil, errors.Wrapf(err, "opening file from layer %d", i)
+		}
+
+		return file, nil
+	}
+
+	return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+}
+
+// ReadFile is a convenice wrapper around Open and returns the content
+// of the file if any is available.
+func (f FSStack) ReadFile(name string) ([]byte, error) {
+	file, err := f.Open(name)
+	if err != nil {
+		return nil, errors.Wrap(err, "opening file")
+	}
+	defer file.Close()
+
+	content, err := io.ReadAll(file)
+	return content, errors.Wrap(err, "reading content")
+}