[#60] Implement periodic white space XML writer

Periodic white space XML writer sends XML header
and white spaces to the io.Writer.

Signed-off-by: Alex Vanin <a.vanin@yadro.com>

api/handler/multipart_upload.go

@@ -2,6 +2,7 @@ package handler
 
 import (
 	"encoding/xml"
+	"io"
 	"net/http"
 	"net/url"
 	"strconv"
@@ -681,3 +682,53 @@ func encodeListPartsToResponse(info *layer.ListPartsInfo, params *layer.ListPart
 		Parts:            info.Parts,
 	}
 }
+
+// periodicXMLWriter creates go routine to write xml header and whitespaces
+// over time to avoid connection drop from the client. To work properly,
+// pass `http.ResponseWriter` with implemented `http.Flusher` interface.
+// Returns stop function which returns boolean if writer has been used
+// during goroutine execution. To disable writer, pass 0 duration value.
+func periodicXMLWriter(w io.Writer, dur time.Duration) (stop func() bool) {
+	if dur == 0 { // 0 duration disables periodic writer
+		return func() bool { return false }
+	}
+
+	whitespaceChar := []byte(" ")
+	closer := make(chan struct{})
+	done := make(chan struct{})
+	headerWritten := false
+
+	go func() {
+		defer close(done)
+
+		tick := time.NewTicker(dur)
+		defer tick.Stop()
+
+		for {
+			select {
+			case <-tick.C:
+				if !headerWritten {
+					_, err := w.Write([]byte(xml.Header))
+					headerWritten = err == nil
+				}
+				_, err := w.Write(whitespaceChar)
+				if err != nil {
+					return // is there anything we can do better than ignore error?
+				}
+				if buffered, ok := w.(http.Flusher); ok {
+					buffered.Flush()
+				}
+			case <-closer:
+				return
+			}
+		}
+	}()
+
+	stop = func() bool {
+		close(closer)
+		<-done // wait for goroutine to stop
+		return headerWritten
+	}
+
+	return stop
+}