store: get exclusive access to store with Diff/Changes

when NaiveDiff is used, the Diff/Changes operations can trigger the
mount of the layer.  Prevent that multiple processes step on each
other and one of them performs an unmount while the other one is still
accessing the mount.

Closes: #2033

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

store.go

@@ -2930,14 +2930,40 @@ func (s *store) Unmount(id string, force bool) (bool, error) {
 }
 
 func (s *store) Changes(from, to string) ([]archive.Change, error) {
-	if res, done, err := readAllLayerStores(s, func(store roLayerStore) ([]archive.Change, bool, error) {
+	// NaiveDiff could cause mounts to happen without a lock, so be safe
+	// and treat the .Diff operation as a Mount.
+	// We need to make sure the home mount is present when the Mount is done, which happens by possibly reinitializing the graph driver
+	// in startUsingGraphDriver().
+	if err := s.startUsingGraphDriver(); err != nil {
+		return nil, err
+	}
+	defer s.stopUsingGraphDriver()
+
+	rlstore, lstores, err := s.bothLayerStoreKindsLocked()
+	if err != nil {
+		return nil, err
+	}
+	if err := rlstore.startWriting(); err != nil {
+		return nil, err
+	}
+	if rlstore.Exists(to) {
+		res, err := rlstore.Changes(from, to)
+		rlstore.stopWriting()
+		return res, err
+	}
+	rlstore.stopWriting()
+
+	for _, s := range lstores {
+		store := s
+		if err := store.startReading(); err != nil {
+			return nil, err
+		}
 		if store.Exists(to) {
 			res, err := store.Changes(from, to)
-			return res, true, err
+			store.stopReading()
+			return res, err
 		}
-		return nil, false, nil
-	}); done {
-		return res, err
+		store.stopReading()
 	}
 	return nil, ErrLayerUnknown
 }
@@ -2968,12 +2994,30 @@ func (s *store) Diff(from, to string, options *DiffOptions) (io.ReadCloser, erro
 	}
 	defer s.stopUsingGraphDriver()
 
-	layerStores, err := s.allLayerStoresLocked()
+	rlstore, lstores, err := s.bothLayerStoreKindsLocked()
 	if err != nil {
 		return nil, err
 	}
 
-	for _, s := range layerStores {
+	if err := rlstore.startWriting(); err != nil {
+		return nil, err
+	}
+	if rlstore.Exists(to) {
+		rc, err := rlstore.Diff(from, to, options)
+		if rc != nil && err == nil {
+			wrapped := ioutils.NewReadCloserWrapper(rc, func() error {
+				err := rc.Close()
+				rlstore.stopWriting()
+				return err
+			})
+			return wrapped, nil
+		}
+		rlstore.stopWriting()
+		return rc, err
+	}
+	rlstore.stopWriting()
+
+	for _, s := range lstores {
 		store := s
 		if err := store.startReading(); err != nil {
 			return nil, err