wal: never leave a corrupted wal file

If the process dies during wal.cut(), it might leave a corrupted wal
file. This commit solves the problem by creating a temp wal file first,
then atomically rename it to a wal file when we are sure it is vaild.

wal/wal.go

@@ -284,31 +284,28 @@ func (w *WAL) ReadAll() (metadata []byte, state raftpb.HardState, ents []raftpb.
 }
 
 // cut closes current file written and creates a new one ready to append.
+// cut first creates a temp wal file and writes necessary headers into it.
+// Then cut atomtically rename temp wal file to a wal file.
 func (w *WAL) cut() error {
-	// create a new wal file with name sequence + 1
-	fpath := path.Join(w.dir, walName(w.seq+1, w.enti+1))
-	f, err := os.OpenFile(fpath, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0600)
-	if err != nil {
+	// close old wal file
+	if err := w.sync(); err != nil {
 		return err
 	}
-	log.Printf("wal: segmented wal file %v is created", fpath)
-	l, err := fileutil.NewLock(f.Name())
-	if err != nil {
+	if err := w.f.Close(); err != nil {
 		return err
 	}
-	err = l.Lock()
+
+	fpath := path.Join(w.dir, walName(w.seq+1, w.enti+1))
+	ftpath := fpath + ".tmp"
+
+	// create a temp wal file with name sequence + 1
+	ft, err := os.OpenFile(ftpath, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0600)
 	if err != nil {
 		return err
 	}
-	w.locks = append(w.locks, l)
-	if err = w.sync(); err != nil {
-		return err
-	}
-	w.f.Close()
 
 	// update writer and save the previous crc
-	w.f = f
-	w.seq++
+	w.f = ft
 	prevCrc := w.encoder.crc.Sum32()
 	w.encoder = newEncoder(w.f, prevCrc)
 	if err := w.saveCrc(prevCrc); err != nil {
@@ -320,7 +317,45 @@ func (w *WAL) cut() error {
 	if err := w.saveState(&w.state); err != nil {
 		return err
 	}
-	return w.sync()
+	// close temp wal file
+	if err := w.sync(); err != nil {
+		return err
+	}
+	if err := w.f.Close(); err != nil {
+		return err
+	}
+
+	// atomically move temp wal file to wal file
+	if err := os.Rename(ftpath, fpath); err != nil {
+		return err
+	}
+
+	// open the wal file and update writer again
+	f, err := os.OpenFile(fpath, os.O_WRONLY|os.O_APPEND, 0600)
+	if err != nil {
+		return err
+	}
+	w.f = f
+	prevCrc = w.encoder.crc.Sum32()
+	w.encoder = newEncoder(w.f, prevCrc)
+
+	// lock the new wal file
+	l, err := fileutil.NewLock(f.Name())
+	if err != nil {
+		return err
+	}
+	err = l.Lock()
+	if err != nil {
+		return err
+	}
+	w.locks = append(w.locks, l)
+
+	// increase the wal seq
+	w.seq++
+
+	log.Printf("wal: segmented wal file %v is created", fpath)
+
+	return nil
 }
 
 func (w *WAL) sync() error {