tx: add missing lock on meta page update

metalock is supposed to protect meta page, but it looks like the only place
where we're modifying it is not protected in fact. Since page update is not
atomic a concurrent reader (RO transaction) can get an inconsistent page. It's
likely to fall back to the other one in this case, but still we better not
allow this to happen.

Signed-off-by: Roman Khimov <roman@nspcc.ru>

Author: roman-khimov

tx.go

@@ -561,10 +561,13 @@ func (tx *Tx) writeMeta() error {
 	tx.meta.Write(p)
 
 	// Write the meta page to file.
+	tx.db.metalock.Lock()
 	if _, err := tx.db.ops.writeAt(buf, int64(p.Id())*int64(tx.db.pageSize)); err != nil {
+		tx.db.metalock.Unlock()
 		lg.Errorf("writeAt failed, pgid: %d, pageSize: %d, error: %v", p.Id(), tx.db.pageSize, err)
 		return err
 	}
+	tx.db.metalock.Unlock()
 	if !tx.db.NoSync || common.IgnoreNoSync {
 		// gofail: var beforeSyncMetaPage struct{}
 		if err := fdatasync(tx.db); err != nil {