mvcc: fix panic on *bolt.Tx.Size after commit

Newly created *batchTx from 'newBatchTx' returns after
*batchTx.Commit() where boltdb internally initializes
*bolt.Tx.meta and *bolt.Tx.db as nil. So subsequent *bolt.Tx.Size
call would panic with nil pointer reference.

Fix etcd-io/etcdlabs#30.

mvcc/backend/batch_tx.go

@@ -162,6 +162,10 @@ func (t *batchTx) commit(stop bool) {
 		if t.pending == 0 && !stop {
 			t.backend.mu.RLock()
 			defer t.backend.mu.RUnlock()
+			t.tx, err = t.backend.db.Begin(true)
+			if err != nil {
+				plog.Fatalf("cannot begin tx (%s)", err)
+			}
 			atomic.StoreInt64(&t.backend.size, t.tx.Size())
 			return
 		}

mvcc/backend/batch_tx_test.go

@@ -195,3 +195,15 @@ func TestBatchTxBatchLimitCommit(t *testing.T) {
 		return nil
 	})
 }
+
+// TestBatchTxCommitSize tests *batchTx.tx.Size call after *batchTx.Commit does not panic.
+// *bolt.Tx commit initializes *bolt.Tx.meta and *bolt.Tx.db as nil. And if *batchTx is not
+// properly initialized, it panics with nil pointer reference.
+func TestBatchTxCommitSize(t *testing.T) {
+	b, tmpPath := NewTmpBackend(time.Hour, 100)
+	defer cleanup(b, tmpPath)
+
+	if _, err := b.Hash(make(map[IgnoreKey]struct{})); err != nil {
+		t.Fatal(err)
+	}
+}