Write meta.json file atomically

block.go

@@ -94,7 +94,11 @@ func readMetaFile(dir string) (*BlockMeta, error) {
 }
 
 func writeMetaFile(dir string, meta *BlockMeta) error {
-	f, err := os.Create(filepath.Join(dir, metaFilename))
+	// Make any changes to the file appear atomic.
+	path := filepath.Join(dir, metaFilename)
+	tmp := path + ".tmp"
+
+	f, err := os.Create(tmp)
 	if err != nil {
 		return err
 	}
@@ -108,8 +112,7 @@ func writeMetaFile(dir string, meta *BlockMeta) error {
 	if err := f.Close(); err != nil {
 		return err
 	}
-
-	return nil
+	return renameFile(tmp, path)
 }
 
 func newPersistedBlock(dir string) (*persistedBlock, error) {

compact.go

@@ -406,7 +406,7 @@ func (c *compactionMerger) At() (labels.Labels, []ChunkMeta) {
 	return c.l, c.c
 }
 
-func renameDir(from, to string) error {
+func renameFile(from, to string) error {
 	if err := os.RemoveAll(to); err != nil {
 		return err
 	}

db.go

@@ -300,7 +300,7 @@ func (db *DB) compact(i, j int) error {
 		}
 	}
 
-	if err := renameDir(tmpdir, dir); err != nil {
+	if err := renameFile(tmpdir, dir); err != nil {
 		return errors.Wrap(err, "rename dir")
 	}
 	pb.dir = dir