metadata: add content lease on existing content

When a writer is requested for an object that already
exists, add that object to the provided any lease to prevent
other operations from affecting the current lease's use of
that content.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

metadata/content.go

@@ -318,12 +318,23 @@ func (cs *contentStore) Writer(ctx context.Context, ref string, size int64, expe
 	cs.l.RLock()
 	defer cs.l.RUnlock()
 
-	var w content.Writer
+	var (
+		w      content.Writer
+		exists bool
+	)
 	if err := update(ctx, cs.db, func(tx *bolt.Tx) error {
 		if expected != "" {
 			cbkt := getBlobBucket(tx, ns, expected)
 			if cbkt != nil {
-				return errors.Wrapf(errdefs.ErrAlreadyExists, "content %v", expected)
+				// Add content to lease to prevent other reference removals
+				// from effecting this object during a provided lease
+				if err := addContentLease(ctx, tx, expected); err != nil {
+					return errors.Wrap(err, "unable to lease content")
+				}
+				// Return error outside of transaction to ensure
+				// commit succeeds with the lease.
+				exists = true
+				return nil
 			}
 		}
 
@@ -363,6 +374,9 @@ func (cs *contentStore) Writer(ctx context.Context, ref string, size int64, expe
 	}); err != nil {
 		return nil, err
 	}
+	if exists {
+		return nil, errors.Wrapf(errdefs.ErrAlreadyExists, "content %v", expected)
+	}
 
 	// TODO: keep the expected in the writer to use on commit
 	// when no expected is provided there.