store: new function to retrieve underlying file

add a new function to retrieve the underlying big data file for the
layers store.

Signed-off-by: Giuseppe Scrivano <[email protected]>

layers.go

@@ -1798,6 +1798,18 @@ func (r *layerStore) BigData(id, key string) (io.ReadCloser, error) {
 	return os.Open(r.datapath(layer.ID, key))
 }
 
+// Requires startReading or startWriting.
+func (r *layerStore) BigDataFilePath(id, key string) (string, error) {
+	if key == "" {
+		return "", fmt.Errorf("can't retrieve layer big data value for empty name: %w", ErrInvalidBigDataName)
+	}
+	layer, ok := r.lookup(id)
+	if !ok {
+		return "", fmt.Errorf("locating layer with ID %q: %w", id, ErrLayerUnknown)
+	}
+	return r.datapath(layer.ID, key), nil
+}
+
 // Requires startWriting.
 func (r *layerStore) SetBigData(id, key string, data io.Reader) error {
 	if !r.lockfile.IsReadWrite() {

store.go

@@ -127,6 +127,10 @@ type roLayerBigDataStore interface {
 	// with this ID.
 	BigData(id, key string) (io.ReadCloser, error)
 
+	// BigDataFilePath returns the path to the underlying file, if the big data
+	// is backed by a file.  Otherwise it returns the empty string.
+	BigDataFilePath(id, key string) (string, error)
+
 	// BigDataNames() returns a list of the names of previously-stored pieces of
 	// data.
 	BigDataNames(id string) ([]string, error)
@@ -440,6 +444,10 @@ type Store interface {
 	// associated with a layer.
 	LayerBigData(id, key string) (io.ReadCloser, error)
 
+	// LayerBigDataFile retrieves the backing file for the layers big data,
+	// when used.
+	LayerBigDataFilePath(id, key string) (string, error)
+
 	// SetLayerBigData stores a (possibly large) chunk of named data
 	// associated with a layer.
 	SetLayerBigData(id, key string, data io.Reader) error
@@ -2105,6 +2113,28 @@ func (s *store) LayerBigData(id, key string) (io.ReadCloser, error) {
 	return nil, fmt.Errorf("locating layer with ID %q: %w", id, ErrLayerUnknown)
 }
 
+// LayerBigDataFile retrieves the backing file for the layers big data,
+// when used.
+func (s *store) LayerBigDataFilePath(id, key string) (string, error) {
+	foundLayer := false
+	if res, done, err := readAllLayerStores(s, func(store roLayerStore) (string, bool, error) {
+		path, err := store.BigDataFilePath(id, key)
+		if err == nil {
+			return path, true, nil
+		}
+		if store.Exists(id) {
+			foundLayer = true
+		}
+		return "", false, nil
+	}); done {
+		return res, err
+	}
+	if foundLayer {
+		return "", fmt.Errorf("locating item named %q for layer with ID %q: %w", key, id, os.ErrNotExist)
+	}
+	return "", fmt.Errorf("locating layer with ID %q: %w", id, ErrLayerUnknown)
+}
+
 // SetLayerBigData stores a (possibly large) chunk of named data
 // associated with a layer.
 func (s *store) SetLayerBigData(id, key string, data io.Reader) error {