Move the containerStore locking inside putLayer

There's no need for the callers to deal with this,
and now the API of the function is much less surprising.

Also fix the documentation about locking rlstores.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

store.go

@@ -1437,8 +1437,15 @@ func (s *store) canUseShifting(uidmap, gidmap []idtools.IDMap) bool {
 	return true
 }
 
-// putLayer requires the rlstore, rlstores, as well as s.containerStore (even if not an argument to this function) to be locked for write.
+// On entry:
+// - rlstore must be locked for writing
+// - rlstores MUST NOT be locked
 func (s *store) putLayer(rlstore rwLayerStore, rlstores []roLayerStore, id, parent string, names []string, mountLabel string, writeable bool, lOptions *LayerOptions, diff io.Reader, slo *stagedLayerOptions) (*Layer, int64, error) {
+	if err := s.containerStore.startWriting(); err != nil {
+		return nil, -1, err
+	}
+	defer s.containerStore.stopWriting()
+
 	var parentLayer *Layer
 	var options LayerOptions
 	if lOptions != nil {
@@ -1525,10 +1532,6 @@ func (s *store) PutLayer(id, parent string, names []string, mountLabel string, w
 		return nil, -1, err
 	}
 	defer rlstore.stopWriting()
-	if err := s.containerStore.startWriting(); err != nil {
-		return nil, -1, err
-	}
-	defer s.containerStore.stopWriting()
 	return s.putLayer(rlstore, rlstores, id, parent, names, mountLabel, writeable, lOptions, diff, nil)
 }
 
@@ -3017,11 +3020,6 @@ func (s *store) ApplyStagedLayer(args ApplyStagedLayerOptions) (*Layer, error) {
 
 	// if the layer doesn't exist yet, try to create it.
 
-	if err := s.containerStore.startWriting(); err != nil {
-		return nil, err
-	}
-	defer s.containerStore.stopWriting()
-
 	slo := stagedLayerOptions{
 		DiffOutput:  args.DiffOutput,
 		DiffOptions: args.DiffOptions,