fix manifest rewriting so unit test will pass using a injected latency on os.file.Sync()

Author: iluminae

manifest.go

@@ -212,19 +212,17 @@ func (mf *manifestFile) addChanges(changesParam []*pb.ManifestChange) error {
 	}
 	// Maybe we could use O_APPEND instead (on certain file systems)
 	mf.appendLock.Lock()
+	defer mf.appendLock.Unlock()
 	if err := applyChangeSet(&mf.manifest, &changes); err != nil {
-		mf.appendLock.Unlock()
 		return err
 	}
 	if mf.inMemory {
-		mf.appendLock.Unlock()
 		return nil
 	}
 	// Rewrite manifest if it'd shrink by 1/10 and it's big enough to care
 	if mf.manifest.Deletions > mf.deletionsRewriteThreshold &&
 		mf.manifest.Deletions > manifestDeletionsRatio*(mf.manifest.Creations-mf.manifest.Deletions) {
 		if err := mf.rewrite(); err != nil {
-			mf.appendLock.Unlock()
 			return err
 		}
 	} else {
@@ -233,12 +231,10 @@ func (mf *manifestFile) addChanges(changesParam []*pb.ManifestChange) error {
 		binary.BigEndian.PutUint32(lenCrcBuf[4:8], crc32.Checksum(buf, y.CastagnoliCrcTable))
 		buf = append(lenCrcBuf[:], buf...)
 		if _, err := mf.fp.Write(buf); err != nil {
-			mf.appendLock.Unlock()
 			return err
 		}
 	}
 
-	mf.appendLock.Unlock()
 	return syncFunc(mf.fp)
 }