break MigrateWAL into two functions, detection and migration (#371)

Signed-off-by: Callum Styan <callumstyan@gmail.com>

db.go

@@ -220,7 +220,7 @@ func Open(dir string, l log.Logger, r prometheus.Registerer, opts *Options) (db
 	if err := repairBadIndexVersion(l, dir); err != nil {
 		return nil, err
 	}
-	// Migrate old WAL.
+	// Migrate old WAL if one exists.
 	if err := MigrateWAL(l, filepath.Join(dir, "wal")); err != nil {
 		return nil, errors.Wrap(err, "migrate WAL")
 	}

wal.go

@@ -1212,38 +1212,44 @@ func (r *walReader) decodeDeletes(flag byte, b []byte, res *[]Stone) error {
 	return nil
 }
 
-// MigrateWAL rewrites the deprecated write ahead log into the new format.
-func MigrateWAL(logger log.Logger, dir string) (err error) {
-	if logger == nil {
-		logger = log.NewNopLogger()
-	}
+func deprecatedWALExists(logger log.Logger, dir string) (bool, error) {
 	// Detect whether we still have the old WAL.
 	fns, err := sequenceFiles(dir)
 	if err != nil && !os.IsNotExist(err) {
-		return errors.Wrap(err, "list sequence files")
+		return false, errors.Wrap(err, "list sequence files")
 	}
 	if len(fns) == 0 {
-		return nil // No WAL at all yet.
+		return false, nil // No WAL at all yet.
 	}
 	// Check header of first segment to see whether we are still dealing with an
 	// old WAL.
 	f, err := os.Open(fns[0])
 	if err != nil {
-		return errors.Wrap(err, "check first existing segment")
+		return false, errors.Wrap(err, "check first existing segment")
 	}
 	defer f.Close()
 
 	var hdr [4]byte
 	if _, err := f.Read(hdr[:]); err != nil && err != io.EOF {
-		return errors.Wrap(err, "read header from first segment")
+		return false, errors.Wrap(err, "read header from first segment")
 	}
 	// If we cannot read the magic header for segments of the old WAL, abort.
 	// Either it's migrated already or there's a corruption issue with which
 	// we cannot deal here anyway. Subsequent attempts to open the WAL will error in that case.
 	if binary.BigEndian.Uint32(hdr[:]) != WALMagic {
-		return nil
+		return false, nil
 	}
+	return true, nil
+}
 
+// MigrateWAL rewrites the deprecated write ahead log into the new format.
+func MigrateWAL(logger log.Logger, dir string) (err error) {
+	if logger == nil {
+		logger = log.NewNopLogger()
+	}
+	if exists, err := deprecatedWALExists(logger, dir); err != nil || !exists {
+		return err
+	}
 	level.Info(logger).Log("msg", "migrating WAL format")
 
 	tmpdir := dir + ".tmp"