Check for empty position object before stringify

Updates specs to check for correct string parsing

app/assets/javascripts/batch_comments/stores/modules/batch_comments/actions.js

+import { isEmpty } from 'lodash';
 import { deprecatedCreateFlash as flash } from '~/flash';
 import { scrollToElement } from '~/lib/utils/common_utils';
 import { __ } from '~/locale';
@@ -88,18 +89,23 @@ export const updateDiscussionsAfterPublish = async ({ dispatch, getters, rootGet
 export const updateDraft = (
   { commit, getters },
   { note, noteText, resolveDiscussion, position, callback },
-) =>
-  service
-    .update(getters.getNotesData.draftsPath, {
-      draftId: note.id,
-      note: noteText,
-      resolveDiscussion,
-      position: JSON.stringify(position),
-    })
+) => {
+  const params = {
+    draftId: note.id,
+    note: noteText,
+    resolveDiscussion,
+  };
+  // Stringifying an empty object yields `{}` which breaks graphql queries
+  // https://gitlab.com/gitlab-org/gitlab/-/issues/298827
+  if (!isEmpty(position)) params.position = JSON.stringify(position);
+
+  return service
+    .update(getters.getNotesData.draftsPath, params)
     .then((res) => res.data)
     .then((data) => commit(types.RECEIVE_DRAFT_UPDATE_SUCCESS, data))
     .then(callback)
     .catch(() => flash(__('An error occurred while updating the comment')));
+};
 
 export const scrollToDraft = ({ dispatch, rootGetters }, draft) => {
   const discussion = draft.discussion_id && rootGetters.getDiscussion(draft.discussion_id);

app/assets/javascripts/notes/components/noteable_note.vue

 <script>
 import { GlSprintf, GlSafeHtmlDirective as SafeHtml } from '@gitlab/ui';
 import $ from 'jquery';
-import { escape } from 'lodash';
+import { escape, isEmpty } from 'lodash';
 import { mapGetters, mapActions } from 'vuex';
 import { INLINE_DIFF_LINES_KEY } from '~/diffs/constants';
 import httpStatusCodes from '~/lib/utils/http_status';
@@ -282,9 +282,13 @@ export default {
         note: {
           target_type: this.getNoteableData.targetType,
           target_id: this.note.noteable_id,
-          note: { note: noteText, position: JSON.stringify(position) },
+          note: { note: noteText },
         },
       };
+
+      // Stringifying an empty object yields `{}` which breaks graphql queries
+      // https://gitlab.com/gitlab-org/gitlab/-/issues/298827
+      if (!isEmpty(position)) data.note.note.position = JSON.stringify(position);
       this.isRequesting = true;
       this.oldContent = this.note.note_html;
       // eslint-disable-next-line vue/no-mutating-props

changelogs/unreleased/298827-graphql-getting-mr-diff-discussions-often-returns-500.yml

+---
+title: fix stringify empty position object
+merge_request: 56037
+author:
+type: fixed

spec/frontend/batch_comments/stores/modules/batch_comments/actions_spec.js

 import MockAdapter from 'axios-mock-adapter';
 import { TEST_HOST } from 'helpers/test_constants';
 import testAction from 'helpers/vuex_action_helper';
+import service from '~/batch_comments/services/drafts_service';
 import * as actions from '~/batch_comments/stores/modules/batch_comments/actions';
 import axios from '~/lib/utils/axios_utils';
 
@@ -201,6 +202,12 @@ describe('Batch comments store actions', () => {
 
   describe('updateDraft', () => {
     let getters;
+    service.update = jest.fn();
+    service.update.mockResolvedValue({ data: { id: 1 } });
+
+    const commit = jest.fn();
+    let context;
+    let params;
 
     beforeEach(() => {
       getters = {
@@ -208,43 +215,43 @@ describe('Batch comments store actions', () => {
           draftsPath: TEST_HOST,
         },
       };
-    });
 
-    it('commits RECEIVE_DRAFT_UPDATE_SUCCESS with returned data', (done) => {
-      const commit = jest.fn();
-      const context = {
+      context = {
         getters,
         commit,
       };
       res = { id: 1 };
       mock.onAny().reply(200, res);
+      params = { note: { id: 1 }, noteText: 'test' };
+    });
 
-      actions
-        .updateDraft(context, { note: { id: 1 }, noteText: 'test', callback() {} })
-        .then(() => {
-          expect(commit).toHaveBeenCalledWith('RECEIVE_DRAFT_UPDATE_SUCCESS', { id: 1 });
-        })
-        .then(done)
-        .catch(done.fail);
+    afterEach(() => jest.clearAllMocks());
+
+    it('commits RECEIVE_DRAFT_UPDATE_SUCCESS with returned data', () => {
+      return actions.updateDraft(context, { ...params, callback() {} }).then(() => {
+        expect(commit).toHaveBeenCalledWith('RECEIVE_DRAFT_UPDATE_SUCCESS', { id: 1 });
+      });
     });
 
-    it('calls passed callback', (done) => {
-      const commit = jest.fn();
-      const context = {
-        getters,
-        commit,
-      };
+    it('calls passed callback', () => {
       const callback = jest.fn();
-      res = { id: 1 };
-      mock.onAny().reply(200, res);
+      return actions.updateDraft(context, { ...params, callback }).then(() => {
+        expect(callback).toHaveBeenCalled();
+      });
+    });
 
-      actions
-        .updateDraft(context, { note: { id: 1 }, noteText: 'test', callback })
-        .then(() => {
-          expect(callback).toHaveBeenCalled();
-        })
-        .then(done)
-        .catch(done.fail);
+    it('does not stringify empty position', () => {
+      return actions.updateDraft(context, { ...params, position: {}, callback() {} }).then(() => {
+        expect(service.update.mock.calls[0][1].position).toBeUndefined();
+      });
+    });
+
+    it('stringifies a non-empty position', () => {
+      const position = { test: true };
+      const expectation = JSON.stringify(position);
+      return actions.updateDraft(context, { ...params, position, callback() {} }).then(() => {
+        expect(service.update.mock.calls[0][1].position).toBe(expectation);
+      });
     });
   });