Independent Multipart Uploads (#15346)

Do completely independent multipart uploads.

In distributed mode, a lock was held to merge each multipart 
upload as it was added. This lock was highly contested and 
retries are expensive (timewise) in distributed mode.

Instead, each part adds its metadata information uniquely. 
This eliminates the per object lock required for each to merge.
The metadata is read back and merged by "CompleteMultipartUpload" 
without locks when constructing final object.

Co-authored-by: Harshavardhana <harsha@minio.io>

cmd/xl-storage.go

@@ -2565,6 +2565,66 @@ func (s *xlStorage) VerifyFile(ctx context.Context, volume, path string, fi File
 	return nil
 }
 
+// ReadMultiple will read multiple files and send each back as response.
+// Files are read and returned in the given order.
+// The resp channel is closed before the call returns.
+// Only a canceled context will return an error.
+func (s *xlStorage) ReadMultiple(ctx context.Context, req ReadMultipleReq, resp chan<- ReadMultipleResp) error {
+	defer close(resp)
+
+	volumeDir := pathJoin(s.diskPath, req.Bucket)
+
+	for _, f := range req.Files {
+		if contextCanceled(ctx) {
+			return ctx.Err()
+		}
+		r := ReadMultipleResp{
+			Bucket: req.Bucket,
+			Prefix: req.Prefix,
+			File:   f,
+		}
+		var data []byte
+		var mt time.Time
+		var err error
+		fullPath := pathJoin(volumeDir, req.Prefix, f)
+		if req.MetadataOnly {
+			data, mt, err = s.readMetadataWithDMTime(ctx, fullPath)
+		} else {
+			data, mt, err = s.readAllData(ctx, volumeDir, fullPath)
+		}
+
+		if err != nil {
+			if !IsErr(err, errFileNotFound, errVolumeNotFound) {
+				r.Exists = true
+				r.Error = err.Error()
+			}
+			select {
+			case <-ctx.Done():
+				return ctx.Err()
+			case resp <- r:
+			}
+			if req.AbortOn404 && !r.Exists {
+				// We stop at first file not found.
+				// We have already reported the error, return nil.
+				return nil
+			}
+			continue
+		}
+		diskHealthCheckOK(ctx, nil)
+		if req.MaxSize > 0 && int64(len(data)) > req.MaxSize {
+			r.Exists = true
+			r.Error = fmt.Sprintf("max size (%d) exceeded: %d", req.MaxSize, len(data))
+			resp <- r
+			continue
+		}
+		r.Exists = true
+		r.Data = data
+		r.Modtime = mt
+		resp <- r
+	}
+	return nil
+}
+
 func (s *xlStorage) StatInfoFile(ctx context.Context, volume, path string, glob bool) (stat []StatInfo, err error) {
 	volumeDir, err := s.getVolDir(volume)
 	if err != nil {