binaryHeader: Fixed partial write issue for index-header.

Fixes #2213

This caused was indicated as regression of latency, and also causes potential critical issue
for store GW, where manual delete of index-header from local storage was required.

This might be considered as blocker for 0.12, so it would be worth to port it to 0.12 TBH @squat.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

pkg/block/indexheader/binary_reader.go

@@ -71,20 +71,21 @@ type BinaryTOC struct {
 }
 
 // WriteBinary build index-header file from the pieces of index in object storage.
-func WriteBinary(ctx context.Context, bkt objstore.BucketReader, id ulid.ULID, fn string) (err error) {
+func WriteBinary(ctx context.Context, bkt objstore.BucketReader, id ulid.ULID, filename string) (err error) {
 	ir, indexVersion, err := newChunkedIndexReader(ctx, bkt, id)
 	if err != nil {
 		return errors.Wrap(err, "new index reader")
 	}
+	tmpFilename := filename + ".tmp"
 
 	// Buffer for copying and encbuffers.
 	// This also will control the size of file writer buffer.
 	buf := make([]byte, 32*1024)
-	bw, err := newBinaryWriter(fn, buf)
+	bw, err := newBinaryWriter(tmpFilename, buf)
 	if err != nil {
 		return errors.Wrap(err, "new binary index header writer")
 	}
-	defer runutil.CloseWithErrCapture(&err, bw, "close binary writer for %s", fn)
+	defer runutil.CloseWithErrCapture(&err, bw, "close binary writer for %s", tmpFilename)
 
 	if err := bw.AddIndexMeta(indexVersion, ir.toc.PostingsTable); err != nil {
 		return errors.Wrap(err, "add index meta")
@@ -109,7 +110,9 @@ func WriteBinary(ctx context.Context, bkt objstore.BucketReader, id ulid.ULID, f
 	if err := bw.WriteTOC(); err != nil {
 		return errors.Wrap(err, "write index header TOC")
 	}
-	return nil
+
+	// Create index-header in atomic way, to avoid partial writes (e.g during restart or crash of store GW).
+	return os.Rename(tmpFilename, filename)
 }
 
 type chunkedIndexReader struct {