Fix successful rebase throwing flash error message

Fix https://gitlab.com/gitlab-org/gitlab-ee/issues/4304

changelogs/unreleased-ee/4304-fix-successful-rebase.yml

+---
+title: Fix successful rebase throwing flash error message
+merge_request: 3727
+author:
+type: fixed

ee/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_rebase.vue

@@ -50,30 +50,7 @@
 
         this.service.rebase()
           .then(() => {
-            simplePoll((continuePolling, stopPolling) => {
-              this.service.poll()
-                .then(res => res.json())
-                .then((res) => {
-                  if (res.rebase_in_progress) {
-                    continuePolling();
-                  } else {
-                    this.isMakingRequest = false;
-
-                    if (res.merge_error.length) {
-                      this.rebasingError = res.merge_error;
-                      Flash('Something went wrong. Please try again.');
-                    }
-
-                    eventHub.$emit('MRWidgetUpdateRequested');
-                    stopPolling();
-                  }
-                })
-                .catch(() => {
-                  this.isMakingRequest = false;
-                  Flash('Something went wrong. Please try again.');
-                  stopPolling();
-                });
-            });
+            simplePoll(this.checkRebaseStatus);
           })
           .catch((error) => {
             this.rebasingError = error.merge_error;
@@ -81,6 +58,30 @@
             Flash('Something went wrong. Please try again.');
           });
       },
+      checkRebaseStatus(continuePolling, stopPolling) {
+        this.service.poll()
+          .then(res => res.json())
+          .then((res) => {
+            if (res.rebase_in_progress) {
+              continuePolling();
+            } else {
+              this.isMakingRequest = false;
+
+              if (res.merge_error && res.merge_error.length) {
+                this.rebasingError = res.merge_error;
+                Flash('Something went wrong. Please try again.');
+              }
+
+              eventHub.$emit('MRWidgetUpdateRequested');
+              stopPolling();
+            }
+          })
+          .catch(() => {
+            this.isMakingRequest = false;
+            Flash('Something went wrong. Please try again.');
+            stopPolling();
+          });
+      },
     },
   };
 </script>

spec/javascripts/vue_mr_widget/components/mr_widget_rebase_spec.js

 import Vue from 'vue';
+import eventHub from '~/vue_merge_request_widget/event_hub';
 import component from 'ee/vue_merge_request_widget/components/states/mr_widget_rebase.vue';
 import mountComponent from '../../helpers/vue_mount_component_helper';
 
@@ -75,4 +76,42 @@ describe('Merge request widget rebase component', () => {
       expect(text).toContain('to allow this merge request to be merged.');
     });
   });
+
+  describe('methods', () => {
+    it('checkRebaseStatus', (done) => {
+      spyOn(eventHub, '$emit');
+      vm = mountComponent(Component, {
+        mr: {},
+        service: {
+          rebase() {
+            return Promise.resolve();
+          },
+          poll() {
+            return Promise.resolve({
+              json() {
+                return {
+                  rebase_in_progress: false,
+                  merge_error: null,
+                };
+              },
+            });
+          },
+        },
+      });
+
+      vm.rebase();
+
+      // Wait for the rebase request
+      vm.$nextTick()
+        // Wait for the polling request
+        .then(vm.$nextTick())
+        // Wait for the eventHub to be called
+        .then(vm.$nextTick())
+        .then(() => {
+          expect(eventHub.$emit).toHaveBeenCalledWith('MRWidgetUpdateRequested');
+        })
+        .then(done)
+        .catch(done.fail);
+    });
+  });
 });