mvcc/backend: restore original bolt db options after defrag

Problem: Defrag was implemented before custom bolt options were added.
Currently defrag doesn't restore backend options.
For example BackendFreelistType will be unset after defrag.

Solution: save bolt db options and use them in defrag.

mvcc/backend/backend.go

@@ -95,8 +95,9 @@ type backend struct {
 	// openReadTxN is the number of currently open read transactions in the backend
 	openReadTxN int64
 
-	mu sync.RWMutex
-	db *bolt.DB
+	mu    sync.RWMutex
+	bopts *bolt.Options
+	db    *bolt.DB
 
 	batchInterval time.Duration
 	batchLimit    int
@@ -167,7 +168,8 @@ func newBackend(bcfg BackendConfig) *backend {
 	// In future, may want to make buffering optional for low-concurrency systems
 	// or dynamically swap between buffered/non-buffered depending on workload.
 	b := &backend{
-		db: db,
+		bopts: bopts,
+		db:    db,
 
 		batchInterval: bcfg.BatchInterval,
 		batchLimit:    bcfg.BatchLimit,
@@ -447,7 +449,7 @@ func (b *backend) defrag() error {
 		}
 	}
 
-	b.db, err = bolt.Open(dbp, 0600, boltOpenOptions)
+	b.db, err = bolt.Open(dbp, 0600, b.bopts)
 	if err != nil {
 		if b.lg != nil {
 			b.lg.Fatal("failed to open database", zap.String("path", dbp), zap.Error(err))
@@ -568,18 +570,24 @@ func (b *backend) OpenReadTxN() int64 {
 	return atomic.LoadInt64(&b.openReadTxN)
 }
 
-// NewTmpBackend creates a backend implementation for testing.
-func NewTmpBackend(batchInterval time.Duration, batchLimit int) (*backend, string) {
+// NewTmpBackendFromCfg creates a backend implementation for testing with custom BackendConfig.
+func NewTmpBackendFromCfg(bcfg BackendConfig) (*backend, string) {
 	dir, err := ioutil.TempDir(os.TempDir(), "etcd_backend_test")
 	if err != nil {
 		panic(err)
 	}
 	tmpPath := filepath.Join(dir, "database")
-	bcfg := DefaultBackendConfig()
-	bcfg.Path, bcfg.BatchInterval, bcfg.BatchLimit = tmpPath, batchInterval, batchLimit
+	bcfg.Path = tmpPath
 	return newBackend(bcfg), tmpPath
 }
 
+// NewTmpBackend creates a backend implementation for testing.
+func NewTmpBackend(batchInterval time.Duration, batchLimit int) (*backend, string) {
+	bcfg := DefaultBackendConfig()
+	bcfg.BatchInterval, bcfg.BatchLimit = batchInterval, batchLimit
+	return NewTmpBackendFromCfg(bcfg)
+}
+
 func NewDefaultTmpBackend() (*backend, string) {
 	return NewTmpBackend(defaultBatchInterval, defaultBatchLimit)
 }

mvcc/backend/backend_test.go

@@ -120,7 +120,16 @@ func TestBackendBatchIntervalCommit(t *testing.T) {
 }
 
 func TestBackendDefrag(t *testing.T) {
-	b, tmpPath := NewDefaultTmpBackend()
+	bcfg := DefaultBackendConfig()
+	// Make sure we change BackendFreelistType
+	// The goal is to verify that we restore config option after defrag.
+	if bcfg.BackendFreelistType == bolt.FreelistMapType {
+		bcfg.BackendFreelistType = bolt.FreelistArrayType
+	} else {
+		bcfg.BackendFreelistType = bolt.FreelistMapType
+	}
+
+	b, tmpPath := NewTmpBackendFromCfg(bcfg)
 	defer cleanup(b, tmpPath)
 
 	tx := b.BatchTx()
@@ -167,6 +176,10 @@ func TestBackendDefrag(t *testing.T) {
 		t.Errorf("new size = %v, want < %d", nsize, size)
 	}
 
+	if b.db.FreelistType != bcfg.BackendFreelistType {
+		t.Errorf("db FreelistType = [%v], want [%v]", b.db.FreelistType, bcfg.BackendFreelistType)
+	}
+
 	// try put more keys after shrink.
 	tx = b.BatchTx()
 	tx.Lock()