wal: fix panic when decoder not set

Handle the related panic and clarify doc.

wal/wal.go

@@ -61,6 +61,7 @@ var (
 	ErrCRCMismatch      = errors.New("wal: crc mismatch")
 	ErrSnapshotMismatch = errors.New("wal: snapshot mismatch")
 	ErrSnapshotNotFound = errors.New("wal: snapshot not found")
+	ErrDecoderNotFound  = errors.New("wal: decoder not found")
 	crcTable            = crc32.MakeTable(crc32.Castagnoli)
 )
 
@@ -95,7 +96,8 @@ type WAL struct {
 }
 
 // Create creates a WAL ready for appending records. The given metadata is
-// recorded at the head of each WAL file, and can be retrieved with ReadAll.
+// recorded at the head of each WAL file, and can be retrieved with ReadAll
+// after the file is Open.
 func Create(lg *zap.Logger, dirpath string, metadata []byte) (*WAL, error) {
 	if Exist(dirpath) {
 		return nil, os.ErrExist
@@ -434,6 +436,10 @@ func (w *WAL) ReadAll() (metadata []byte, state raftpb.HardState, ents []raftpb.
 	defer w.mu.Unlock()
 
 	rec := &walpb.Record{}
+
+	if w.decoder == nil {
+		return nil, state, nil, ErrDecoderNotFound
+	}
 	decoder := w.decoder
 
 	var match bool

wal/wal_test.go

@@ -1059,3 +1059,24 @@ func TestValidSnapshotEntriesAfterPurgeWal(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+// TestReadAllFail ensure ReadAll error if used without opening the WAL
+func TestReadAllFail(t *testing.T) {
+	dir, err := ioutil.TempDir(os.TempDir(), "waltest")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	// create initial WAL
+	f, err := Create(zap.NewExample(), dir, []byte("metadata"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+	// try to read without opening the WAL
+	_, _, _, err = f.ReadAll()
+	if err == nil || err != ErrDecoderNotFound {
+		t.Fatalf("err = %v, want ErrDecoderNotFound", err)
+	}
+}