Add headers and format flag for WAL segments

wal.go

@@ -3,7 +3,6 @@ package tsdb
 import (
 	"bufio"
 	"encoding/binary"
-	"fmt"
 	"hash/crc32"
 	"io"
 	"math"
@@ -21,8 +20,13 @@ import (
 // WALEntryType indicates what data a WAL entry contains.
 type WALEntryType byte
 
-// The valid WAL entry types.
 const (
+	WALMagic = 0x43AF00EF
+
+	// Format versioning flag of a WAL segment file.
+	WALFormatDefault byte = 1
+
+	// Entry types in a segment file.
 	WALEntrySymbols = 1
 	WALEntrySeries  = 2
 	WALEntrySamples = 3
@@ -93,9 +97,7 @@ type walHandler struct {
 
 // ReadAll consumes all entries in the WAL and triggers the registered handlers.
 func (w *WAL) ReadAll(h *walHandler) error {
-	fmt.Println("readall", w.files)
 	for _, f := range w.files {
-		fmt.Println("  ", f.Name())
 		dec := newWALDecoder(f, h)
 
 		for {
@@ -151,6 +153,24 @@ func (w *WAL) initSegments() error {
 	}
 	w.files = append(w.files, lf)
 
+	// Consume and validate meta headers.
+	for _, f := range w.files {
+		metab := make([]byte, 8)
+
+		if n, err := f.Read(metab); err != nil {
+			return errors.Wrapf(err, "validate meta %q", f.Name())
+		} else if n != 8 {
+			return errors.Errorf("invalid header size %d in %q", n, f.Name())
+		}
+
+		if m := binary.BigEndian.Uint32(metab[:4]); m != WALMagic {
+			return errors.Errorf("invalid magic header %x in %q", m, f.Name())
+		}
+		if metab[4] != WALFormatDefault {
+			return errors.Errorf("unknown WAL segment format %d in %q", metab[4], f.Name())
+		}
+	}
+
 	return nil
 }
 
@@ -190,9 +210,18 @@ func (w *WAL) cut() error {
 		return err
 	}
 
+	// Write header metadata for new file.
+	metab := make([]byte, 8)
+	binary.BigEndian.PutUint32(metab[:4], WALMagic)
+	metab[4] = WALFormatDefault
+
+	if _, err := f.Write(metab); err != nil {
+		return err
+	}
+
 	w.files = append(w.files, f)
 	w.cur = bufio.NewWriterSize(f, 4*1024*1024)
-	w.curN = 0
+	w.curN = len(metab)
 
 	return nil
 }
@@ -391,6 +420,8 @@ type walDecoder struct {
 	buf []byte
 }
 
+// newWALDecoder returns a new decoder for the default WAL format. The meta
+// headers of a segment must already have been consumed.
 func newWALDecoder(r io.Reader, h *walHandler) *walDecoder {
 	return &walDecoder{
 		r:       r,