Ensure content writer only unlocks resource once

Make close a no-op after commit and disallow committing after close.

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

content/local/writer.go

@@ -55,6 +55,10 @@ func (w *writer) Write(p []byte) (n int, err error) {
 }
 
 func (w *writer) Commit(size int64, expected digest.Digest, opts ...content.Opt) error {
+	if w.fp == nil {
+		return errors.Wrap(errdefs.ErrFailedPrecondition, "cannot commit on closed writer")
+	}
+
 	if err := w.fp.Sync(); err != nil {
 		return errors.Wrap(err, "sync failed")
 	}
@@ -115,8 +119,8 @@ func (w *writer) Commit(size int64, expected digest.Digest, opts ...content.Opt)
 		return err
 	}
 
-	unlock(w.ref)
 	w.fp = nil
+	unlock(w.ref)
 
 	return nil
 }
@@ -130,12 +134,13 @@ func (w *writer) Commit(size int64, expected digest.Digest, opts ...content.Opt)
 //
 // To abandon a transaction completely, first call close then `Store.Remove` to
 // clean up the associated resources.
-func (cw *writer) Close() (err error) {
-	unlock(cw.ref)
-
-	if cw.fp != nil {
-		cw.fp.Sync()
-		return cw.fp.Close()
+func (w *writer) Close() (err error) {
+	if w.fp != nil {
+		w.fp.Sync()
+		err = w.fp.Close()
+		w.fp = nil
+		unlock(w.ref)
+		return
 	}
 
 	return nil