test: Add test for incrementing number atomically

spec/ParseObject.spec.js

@@ -2055,4 +2055,27 @@ describe('Parse.Object testing', () => {
     const object = new Parse.Object('CloudCodeIsNew');
     await object.save();
   });
+
+  it('increments atomically', async () => {
+    Parse.Object.disableSingleInstance();
+
+    Parse.Cloud.beforeSave('Parent', (req) => {});
+    Parse.Cloud.beforeSave('Child', async (req) => {
+      await req.object.get("parent").increment("num_child").save(null, {useMasterKey:true})
+    });
+
+    let parent = await new Parse.Object('Parent').save(null, {useMasterKey:true});
+    const child = () => new Parse.Object('Child').set("parent",parent).save();
+
+    // add synchronously
+    await child()
+    await child()
+    parent = await parent.fetch();
+    expect(parent.get('num_child')).toBe(2);
+
+    // add asynchronously
+    await Promise.all(Array.from({length: 40}, () => child()))
+    parent = await parent.fetch();
+    expect(parent.get('num_child')).toBe(42);
+  });
 });