CallResource: don't set Content-Type header if status is 204 (#50780)

Grafana's HTTPServer ensures that the Content-Type header is always set
in the response to a CallResource call, but when the status code is
204 No Content this shouldn't be done; the body should be empty and no
Content-Type header should be set.

We ran into this in the Grafana ML plugin where we were sending an empty
response with status 204, but the frontend client saw that the content
type was JSON and tried to parse it, resulting in an error that made it
to the JS console.

pkg/api/plugin_resource.go

@@ -193,7 +193,7 @@ func (hs *HTTPServer) flushStream(stream callResourceClientResponseStream, w htt
 		// Expected that headers and status are only part of first stream
 		if processedStreams == 0 && resp.Headers != nil {
 			// Make sure a content type always is returned in response
-			if _, exists := resp.Headers["Content-Type"]; !exists {
+			if _, exists := resp.Headers["Content-Type"]; !exists && resp.Status != http.StatusNoContent {
 				resp.Headers["Content-Type"] = []string{"application/json"}
 			}
 

pkg/api/plugins_test.go

@@ -331,10 +331,35 @@ func TestMakePluginResourceRequest(t *testing.T) {
 		}
 	}
 
+	require.Equal(t, resp.Header().Get("Content-Type"), "application/json")
 	require.Equal(t, "sandbox", resp.Header().Get("Content-Security-Policy"))
 	require.Empty(t, req.Header.Get(customHeader))
 }
 
+func TestMakePluginResourceRequestContentTypeEmpty(t *testing.T) {
+	pluginClient := &fakePluginClient{
+		statusCode: http.StatusNoContent,
+	}
+	hs := HTTPServer{
+		Cfg:          setting.NewCfg(),
+		log:          log.New(),
+		pluginClient: pluginClient,
+	}
+	req := httptest.NewRequest(http.MethodGet, "/", nil)
+	resp := httptest.NewRecorder()
+	pCtx := backend.PluginContext{}
+	err := hs.makePluginResourceRequest(resp, req, pCtx)
+	require.NoError(t, err)
+
+	for {
+		if resp.Flushed {
+			break
+		}
+	}
+
+	require.Zero(t, resp.Header().Get("Content-Type"))
+}
+
 func callGetPluginAsset(sc *scenarioContext) {
 	sc.fakeReqWithParams("GET", sc.url, map[string]string{}).exec()
 }
@@ -366,6 +391,8 @@ type fakePluginClient struct {
 	req *backend.CallResourceRequest
 
 	backend.QueryDataHandlerFunc
+
+	statusCode int
 }
 
 func (c *fakePluginClient) CallResource(_ context.Context, req *backend.CallResourceRequest, sender backend.CallResourceResponseSender) error {
@@ -377,8 +404,13 @@ func (c *fakePluginClient) CallResource(_ context.Context, req *backend.CallReso
 		return err
 	}
 
+	statusCode := http.StatusOK
+	if c.statusCode != 0 {
+		statusCode = c.statusCode
+	}
+
 	return sender.Send(&backend.CallResourceResponse{
-		Status:  http.StatusOK,
+		Status:  statusCode,
 		Headers: make(map[string][]string),
 		Body:    bytes,
 	})