Avoid sending an error after 206 HTTP code (#6264)

When a S3 client sends a GET Object with a range header, 206 http
code is returned indicating success, however the call of the object
layer's GetObject() inside the handler can return an error and will lead
to writing an XML error message, which is obviously wrong since
we already sent 206 http code. So in the case, we just stop sending
data to the S3 client, this latter can still detect if there is no
error when comparing received data with Content-Length header
in the Get Object response.

cmd/api-headers.go

@@ -100,6 +100,5 @@ func setObjectHeaders(w http.ResponseWriter, objInfo ObjectInfo, contentRange *h
 		// Override content-length
 		w.Header().Set("Content-Length", strconv.FormatInt(contentRange.getLength(), 10))
 		w.Header().Set("Content-Range", contentRange.String())
-		w.WriteHeader(http.StatusPartialContent)
 	}
 }

cmd/object-handlers.go

@@ -173,16 +173,23 @@ func (api objectAPIHandlers) GetObjectHandler(w http.ResponseWriter, r *http.Req
 
 	setObjectHeaders(w, objInfo, hrange)
 	setHeadGetRespHeaders(w, r.URL.Query())
-	httpWriter := ioutil.WriteOnClose(writer)
 
 	getObject := objectAPI.GetObject
 	if api.CacheAPI() != nil && !hasSSECustomerHeader(r.Header) {
 		getObject = api.CacheAPI().GetObject
 	}
 
+	statusCodeWritten := false
+	httpWriter := ioutil.WriteOnClose(writer)
+
+	if hrange != nil && hrange.offsetBegin > -1 {
+		statusCodeWritten = true
+		w.WriteHeader(http.StatusPartialContent)
+	}
+
 	// Reads the object at startOffset and writes to mw.
 	if err = getObject(ctx, bucket, object, startOffset, length, httpWriter, objInfo.ETag); err != nil {
-		if !httpWriter.HasWritten() { // write error response only if no data has been written to client yet
+		if !httpWriter.HasWritten() && !statusCodeWritten { // write error response only if no data or headers has been written to client yet
 			writeErrorResponse(w, toAPIErrorCode(err), r.URL)
 		}
 		httpWriter.Close()
@@ -190,7 +197,7 @@ func (api objectAPIHandlers) GetObjectHandler(w http.ResponseWriter, r *http.Req
 	}
 
 	if err = httpWriter.Close(); err != nil {
-		if !httpWriter.HasWritten() { // write error response only if no data has been written to client yet
+		if !httpWriter.HasWritten() && !statusCodeWritten { // write error response only if no data or headers has been written to client yet
 			writeErrorResponse(w, toAPIErrorCode(err), r.URL)
 			return
 		}