Backport 'Rebase' feature from EE to CE

When a project uses fast-forward merging strategy user has
to rebase MRs to target branch before it can be merged.
Now user can do rebase in UI by clicking 'Rebase' button
instead of doing rebase locally.

This feature was already present in EE, this is only backport
of the feature to CE. Couple of changes:
* removed rebase license check
* renamed migration (changed timestamp)

Closes #40301

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

+<script>
+  import simplePoll from '../../../lib/utils/simple_poll';
+  import eventHub from '../../event_hub';
+  import statusIcon from '../mr_widget_status_icon';
+  import loadingIcon from '../../../vue_shared/components/loading_icon.vue';
+  import Flash from '../../../flash';
+
+  export default {
+    name: 'MRWidgetRebase',
+    props: {
+      mr: {
+        type: Object,
+        required: true,
+      },
+      service: {
+        type: Object,
+        required: true,
+      },
+    },
+    components: {
+      statusIcon,
+      loadingIcon,
+    },
+    data() {
+      return {
+        isMakingRequest: false,
+        rebasingError: null,
+      };
+    },
+    computed: {
+      status() {
+        if (this.mr.rebaseInProgress || this.isMakingRequest) {
+          return 'loading';
+        }
+        if (!this.mr.canPushToSourceBranch && !this.mr.rebaseInProgress) {
+          return 'warning';
+        }
+        return 'success';
+      },
+      showDisabledButton() {
+        return ['failed', 'loading'].includes(this.status);
+      },
+    },
+    methods: {
+      rebase() {
+        this.isMakingRequest = true;
+        this.rebasingError = null;
+
+        this.service.rebase()
+          .then(() => {
+            simplePoll(this.checkRebaseStatus);
+          })
+          .catch((error) => {
+            this.rebasingError = error.merge_error;
+            this.isMakingRequest = false;
+            Flash('Something went wrong. Please try again.');
+          });
+      },
+      checkRebaseStatus(continuePolling, stopPolling) {
+        this.service.poll()
+          .then(res => res.data)
+          .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>
+<template>
+  <div class="mr-widget-body media">
+    <status-icon
+      :status="status"
+      :show-disabled-button="showDisabledButton"
+      />
+
+    <div class="rebase-state-find-class-convention media media-body space-children">
+      <template v-if="mr.rebaseInProgress || isMakingRequest">
+        <span class="bold">
+          Rebase in progress
+        </span>
+      </template>
+      <template v-if="!mr.rebaseInProgress && !mr.canPushToSourceBranch">
+        <span class="bold">
+          Fast-forward merge is not possible.
+          Rebase the source branch onto
+          <span class="label-branch">{{mr.targetBranch}}</span>
+          to allow this merge request to be merged.
+        </span>
+      </template>
+      <template v-if="!mr.rebaseInProgress && mr.canPushToSourceBranch && !isMakingRequest">
+        <div class="accept-merge-holder clearfix js-toggle-container accept-action media space-children">
+          <button
+            type="button"
+            class="btn btn-sm btn-reopen btn-success"
+            :disabled="isMakingRequest"
+            @click="rebase">
+            <loading-icon v-if="isMakingRequest" />
+            Rebase
+          </button>
+          <span
+            v-if="!rebasingError"
+            class="bold">
+            Fast-forward merge is not possible.
+            Rebase the source branch onto the target branch or merge target
+            branch into source branch to allow this merge request to be merged.
+          </span>
+          <span
+            v-else
+            class="bold danger">
+            {{rebasingError}}
+          </span>
+        </div>
+      </template>
+    </div>
+  </div>
+</template>

app/assets/javascripts/vue_merge_request_widget/dependencies.js

@@ -32,6 +32,7 @@ export { default as UnresolvedDiscussionsState } from './components/states/mr_wi
 export { default as PipelineBlockedState } from './components/states/mr_widget_pipeline_blocked';
 export { default as PipelineFailedState } from './components/states/mr_widget_pipeline_failed';
 export { default as MergeWhenPipelineSucceedsState } from './components/states/mr_widget_merge_when_pipeline_succeeds';
+export { default as RebaseState } from './components/states/mr_widget_rebase.vue';
 export { default as AutoMergeFailed } from './components/states/mr_widget_auto_merge_failed';
 export { default as CheckingState } from './components/states/mr_widget_checking';
 export { default as MRWidgetStore } from './stores/mr_widget_store';

app/assets/javascripts/vue_merge_request_widget/mr_widget_options.js

@@ -10,6 +10,7 @@ import {
   MergedState,
   ClosedState,
   MergingState,
+  RebaseState,
   WipState,
   ArchivedState,
   ConflictsState,
@@ -79,6 +80,7 @@ export default {
         ciEnvironmentsStatusPath: store.ciEnvironmentsStatusPath,
         statusPath: store.statusPath,
         mergeActionsContentPath: store.mergeActionsContentPath,
+        rebasePath: store.rebasePath,
       };
       return new MRWidgetService(endpoints);
     },
@@ -232,6 +234,7 @@ export default {
     'mr-widget-pipeline-failed': PipelineFailedState,
     'mr-widget-merge-when-pipeline-succeeds': MergeWhenPipelineSucceedsState,
     'mr-widget-auto-merge-failed': AutoMergeFailed,
+    'mr-widget-rebase': RebaseState,
   },
   template: `
     <div class="mr-state-widget prepend-top-default">

app/assets/javascripts/vue_merge_request_widget/services/mr_widget_service.js

@@ -37,6 +37,10 @@ export default class MRWidgetService {
     return axios.get(this.endpoints.mergeActionsContentPath);
   }
 
+  rebase() {
+    return axios.post(this.endpoints.rebasePath);
+  }
+
   static stopEnvironment(url) {
     return axios.post(url);
   }

app/assets/javascripts/vue_merge_request_widget/stores/get_state_key.js

@@ -25,6 +25,8 @@ export default function deviseState(data) {
     return this.mergeError ? stateKey.autoMergeFailed : stateKey.mergeWhenPipelineSucceeds;
   } else if (!this.canMerge) {
     return stateKey.notAllowedToMerge;
+  } else if (this.shouldBeRebased) {
+    return stateKey.rebase;
   } else if (this.canBeMerged) {
     return stateKey.readyToMerge;
   }

app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js

@@ -26,6 +26,7 @@ export default class MergeRequestStore {
     this.divergedCommitsCount = data.diverged_commits_count;
     this.pipeline = data.pipeline || {};
     this.deployments = this.deployments || data.deployments || [];
+    this.initRebase(data);
 
     if (data.issues_links) {
       const links = data.issues_links;
@@ -124,6 +125,13 @@ export default class MergeRequestStore {
     return this.state === stateKey.nothingToMerge;
   }
 
+  initRebase(data) {
+    this.canPushToSourceBranch = data.can_push_to_source_branch;
+    this.rebaseInProgress = data.rebase_in_progress;
+    this.approvalsLeft = !data.approved;
+    this.rebasePath = data.rebase_path;
+  }
+
   static buildMetrics(metrics) {
     if (!metrics) {
       return {};

app/assets/javascripts/vue_merge_request_widget/stores/state_maps.js

@@ -17,6 +17,7 @@ const stateToComponentMap = {
   failedToMerge: 'mr-widget-failed-to-merge',
   autoMergeFailed: 'mr-widget-auto-merge-failed',
   shaMismatch: 'mr-widget-sha-mismatch',
+  rebase: 'mr-widget-rebase',
 };
 
 const statesToShowHelpWidget = [
@@ -29,6 +30,7 @@ const statesToShowHelpWidget = [
   'pipelineFailed',
   'pipelineBlocked',
   'autoMergeFailed',
+  'rebase',
 ];
 
 export const stateKey = {
@@ -46,6 +48,7 @@ export const stateKey = {
   mergeWhenPipelineSucceeds: 'mergeWhenPipelineSucceeds',
   notAllowedToMerge: 'notAllowedToMerge',
   readyToMerge: 'readyToMerge',
+  rebase: 'rebase',
 };
 
 export default {

app/controllers/projects/merge_requests_controller.rb

@@ -10,6 +10,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   before_action :authorize_update_issuable!, only: [:close, :edit, :update, :remove_wip, :sort]
   before_action :set_issuables_index, only: [:index]
   before_action :authenticate_user!, only: [:assign_related_issues]
+  before_action :check_user_can_push_to_source_branch!, only: [:rebase]
 
   def index
     @merge_requests = @issuables
@@ -223,6 +224,12 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     render json: environments
   end
 
+  def rebase
+    RebaseWorker.perform_async(@merge_request.id, current_user.id)
+
+    render nothing: true, status: 200
+  end
+
   protected
 
   alias_method :subscribable_resource, :merge_request
@@ -322,4 +329,14 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     @finder_type = MergeRequestsFinder
     super
   end
+
+  def check_user_can_push_to_source_branch!
+    return access_denied! unless @merge_request.source_branch_exists?
+
+    access_check = ::Gitlab::UserAccess
+      .new(current_user, project: @merge_request.source_project)
+      .can_push_to_branch?(@merge_request.source_branch)
+
+    access_denied! unless access_check
+  end
 end

app/models/merge_request.rb

@@ -156,6 +156,13 @@ class MergeRequest < ActiveRecord::Base
     '!'
   end
 
+  def rebase_in_progress?
+    # The source project can be deleted
+    return false unless source_project
+
+    source_project.repository.rebase_in_progress?(id)
+  end
+
   # Use this method whenever you need to make sure the head_pipeline is synced with the
   # branch head commit, for example checking if a merge request can be merged.
   # For more information check: https://gitlab.com/gitlab-org/gitlab-ce/issues/40004
@@ -607,7 +614,7 @@ class MergeRequest < ActiveRecord::Base
 
     check_if_can_be_merged
 
-    can_be_merged?
+    can_be_merged? && !should_be_rebased?
   end
 
   def mergeable_state?(skip_ci_check: false)

app/models/repository.rb

@@ -1099,6 +1099,13 @@ class Repository
     @project.repository_storage_path
   end
 
+  def rebase(user, merge_request)
+    raw.rebase(user, merge_request.id, branch: merge_request.source_branch,
+                                       branch_sha: merge_request.source_branch_sha,
+                                       remote_repository: merge_request.target_project.repository.raw,
+                                       remote_branch: merge_request.target_branch)
+  end
+
   private
 
   # TODO Generice finder, later split this on finders by Ref or Oid

app/presenters/merge_request_presenter.rb

@@ -76,6 +76,12 @@ class MergeRequestPresenter < Gitlab::View::Presenter::Delegated
     end
   end
 
+  def rebase_path
+    if !rebase_in_progress? && should_be_rebased? && user_can_push_to_source_branch?
+      rebase_project_merge_request_path(project, merge_request)
+    end
+  end
+
   def target_branch_tree_path
     if target_branch_exists?
       project_tree_path(project, target_branch)
@@ -152,6 +158,10 @@ class MergeRequestPresenter < Gitlab::View::Presenter::Delegated
     user_can_collaborate_with_project? && can_be_cherry_picked?
   end
 
+  def can_push_to_source_branch?
+    source_branch_exists? && user_can_push_to_source_branch?
+  end
+
   private
 
   def conflicts
@@ -174,6 +184,14 @@ class MergeRequestPresenter < Gitlab::View::Presenter::Delegated
     end.sort.to_sentence
   end
 
+  def user_can_push_to_source_branch?
+    return false unless source_branch_exists?
+
+    ::Gitlab::UserAccess
+      .new(current_user, project: source_project)
+      .can_push_to_branch?(source_branch)
+  end
+
   def user_can_collaborate_with_project?
     can?(current_user, :push_code, project) ||
       (current_user && current_user.already_forked?(project))

app/serializers/merge_request_basic_entity.rb

@@ -4,4 +4,5 @@ class MergeRequestBasicEntity < IssuableSidebarEntity
   expose :merge_error
   expose :state
   expose :source_branch_exists?, as: :source_branch_exists
+  expose :rebase_in_progress?, as: :rebase_in_progress
 end

app/serializers/merge_request_widget_entity.rb

@@ -23,6 +23,16 @@ class MergeRequestWidgetEntity < IssuableEntity
     MergeRequestMetricsEntity.new(metrics).as_json
   end
 
+  expose :rebase_commit_sha
+  expose :rebase_in_progress?, as: :rebase_in_progress
+
+  expose :can_push_to_source_branch do |merge_request|
+    presenter(merge_request).can_push_to_source_branch?
+  end
+  expose :rebase_path do |merge_request|
+    presenter(merge_request).rebase_path
+  end
+
   # User entities
   expose :merge_user, using: UserEntity
 

app/services/merge_requests/rebase_service.rb

+module MergeRequests
+  class RebaseService < MergeRequests::WorkingCopyBaseService
+    def execute(merge_request)
+      @merge_request = merge_request
+
+      if rebase
+        success
+      else
+        error('Failed to rebase. Should be done manually')
+      end
+    end
+
+    def rebase
+      if merge_request.rebase_in_progress?
+        log_error('Rebase task canceled: Another rebase is already in progress', save_message_on_model: true)
+        return false
+      end
+
+      rebase_sha = repository.rebase(current_user, merge_request)
+
+      merge_request.update_attributes(rebase_commit_sha: rebase_sha)
+
+      true
+    rescue => e
+      log_error('Failed to rebase branch:')
+      log_error(e.message, save_message_on_model: true)
+      false
+    end
+  end
+end

app/services/merge_requests/working_copy_base_service.rb

+module MergeRequests
+  class WorkingCopyBaseService < MergeRequests::BaseService
+    attr_reader :merge_request
+
+    def source_project
+      @source_project ||= merge_request.source_project
+    end
+
+    def target_project
+      @target_project ||= merge_request.target_project
+    end
+
+    def log_error(message, save_message_on_model: false)
+      Gitlab::GitLogger.error("#{self.class.name} error (#{merge_request.to_reference(full: true)}): #{message}")
+
+      merge_request.update(merge_error: message) if save_message_on_model
+    end
+
+    # Don't try to print expensive instance variables.
+    def inspect
+      "#<#{self.class} #{merge_request.to_reference(full: true)}>"
+    end
+  end
+end

app/views/projects/_merge_request_fast_forward_settings.html.haml

@@ -10,4 +10,4 @@
       No merge commits are created and all merges are fast-forwarded, which means that merging is only allowed if the branch could be fast-forwarded.
     %br
     %span.descr
-      When fast-forward merge is not possible, the user must first rebase locally.
+      When fast-forward merge is not possible, the user is given the option to rebase.

app/views/projects/_merge_request_rebase_settings.html.haml

@@ -10,4 +10,4 @@
       This way you could make sure that if this merge request would build, after merging to target branch it would also build.
     %br
     %span.descr
-      When fast-forward merge is not possible, the user must first rebase locally.
+      When fast-forward merge is not possible, the user is given the option to rebase.

app/workers/all_queues.yml

@@ -89,6 +89,7 @@
 - project_service
 - propagate_service_template
 - reactive_caching
+- rebase
 - repository_fork
 - repository_import
 - storage_migrator

app/workers/rebase_worker.rb

+class RebaseWorker
+  include ApplicationWorker
+
+  def perform(merge_request_id, current_user_id)
+    current_user = User.find(current_user_id)
+    merge_request = MergeRequest.find(merge_request_id)
+
+    MergeRequests::RebaseService
+      .new(merge_request.source_project, current_user)
+      .execute(merge_request)
+  end
+end

changelogs/unreleased/40301-rebase.yml

+---
+title: Allow user to rebase merge requests.
+merge_request:
+author:
+type: added

config/routes/project.rb

@@ -96,6 +96,7 @@ constraints(ProjectUrlConstrainer.new) do
           post :toggle_subscription
           post :remove_wip
           post :assign_related_issues
+          post :rebase
 
           scope constraints: { format: nil }, action: :show do
             get :commits, defaults: { tab: 'commits' }

db/migrate/20171230123729_add_rebase_commit_sha_to_merge_requests.rb

+class AddRebaseCommitShaToMergeRequests < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def change
+    add_column :merge_requests, :rebase_commit_sha, :string
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20171229225929) do
+ActiveRecord::Schema.define(version: 20171230123729) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1099,6 +1099,7 @@ ActiveRecord::Schema.define(version: 20171229225929) do
     t.string "merge_jid"
     t.boolean "discussion_locked"
     t.integer "latest_merge_request_diff_id"
+    t.string "rebase_commit_sha"
   end
 
   add_index "merge_requests", ["assignee_id"], name: "index_merge_requests_on_assignee_id", using: :btree

doc/user/project/merge_requests/fast_forward_merge.md

@@ -9,7 +9,7 @@ When the fast-forward merge ([`--ff-only`][ffonly]) setting is enabled, no merge
 commits will be created and all merges are fast-forwarded, which means that
 merging is only allowed if the branch could be fast-forwarded.
 
-When a fast-forward merge is not possible, the user must rebase the branch manually.
+When a fast-forward merge is not possible, the user is given the option to rebase.
 
 ## Use cases
 
@@ -25,7 +25,7 @@ merge commits. In such cases, the fast-forward merge is the perfect candidate.
 Now, when you visit the merge request page, you will be able to accept it
 **only if a fast-forward merge is possible**.
 
-![Fast forward merge request](img/ff_merge_mr.png)
+![Fast forward merge request](img/ff_merge_rebase.png)
 
 If the target branch is ahead of the source branch, you need to rebase the
 source branch locally before you will be able to do a fast-forward merge.

features/project/ff_merge_requests.feature

@@ -22,3 +22,20 @@ Feature: Project Ff Merge Requests
     Then I should see ff-only merge button
     When I accept this merge request
     Then I should see merged request
+
+  @javascript
+  Scenario: I do rebase before ff-only merge
+    Given ff merge enabled
+    And rebase before merge enabled
+    When I visit merge request page "Bug NS-05"
+    Then I should see rebase button
+    When I press rebase button
+    Then I should see rebase in progress message
+
+  @javascript
+  Scenario: I do rebase before regular merge
+    Given rebase before merge enabled
+    When I visit merge request page "Bug NS-05"
+    Then I should see rebase button
+    When I press rebase button
+    Then I should see rebase in progress message

features/steps/project/ff_merge_requests.rb

@@ -17,6 +17,10 @@ class Spinach::Features::ProjectFfMergeRequests < Spinach::FeatureSteps
            author: project.users.first)
   end
 
+  step 'merge request is mergeable' do
+    expect(page).to have_button 'Merge'
+  end
+
   step 'I should see ff-only merge button' do
     expect(page).to have_content "Fast-forward merge without a merge commit"
     expect(page).to have_button 'Merge'
@@ -45,6 +49,10 @@ class Spinach::Features::ProjectFfMergeRequests < Spinach::FeatureSteps
     project.save!
   end
 
+  step 'I should see rebase button' do
+    expect(page).to have_button "Rebase"
+  end
+
   step 'merge request "Bug NS-05" is rebased' do
     merge_request.source_branch = 'flatten-dir'
     merge_request.target_branch = 'improve/awesome'
@@ -59,6 +67,20 @@ class Spinach::Features::ProjectFfMergeRequests < Spinach::FeatureSteps
     merge_request.save!
   end
 
+  step 'rebase before merge enabled' do
+    project = merge_request.target_project
+    project.merge_requests_rebase_enabled = true
+    project.save!
+  end
+
+  step 'I press rebase button' do
+    click_button "Rebase"
+  end
+
+  step "I should see rebase in progress message" do
+    expect(page).to have_content("Rebase in progress")
+  end
+
   def merge_request
     @merge_request ||= MergeRequest.find_by!(title: "Bug NS-05")
   end

lib/gitlab/git/operation_service.rb

@@ -97,6 +97,11 @@ module Gitlab
         end
       end
 
+      def update_branch(branch_name, newrev, oldrev)
+        ref = Gitlab::Git::BRANCH_REF_PREFIX + branch_name
+        update_ref_in_hooks(ref, newrev, oldrev)
+      end
+
       private
 
       # Returns [newrev, should_run_after_create, should_run_after_create_branch]

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -684,4 +684,62 @@ describe Projects::MergeRequestsController do
                             format: :json
     end
   end
+
+  describe 'POST #rebase' do
+    let(:viewer)        { user }
+
+    def post_rebase
+      post :rebase, namespace_id: project.namespace, project_id: project, id: merge_request
+    end
+
+    def expect_rebase_worker_for(user)
+      expect(RebaseWorker).to receive(:perform_async).with(merge_request.id, user.id)
+    end
+
+    context 'successfully' do
+      it 'enqeues a RebaseWorker' do
+        expect_rebase_worker_for(viewer)
+
+        post_rebase
+
+        expect(response.status).to eq(200)
+      end
+    end
+
+    context 'with a forked project' do
+      let(:fork_project) { create(:project, :repository, forked_from_project: project) }
+      let(:fork_owner) { fork_project.owner }
+
+      before do
+        merge_request.update!(source_project: fork_project)
+        fork_project.add_reporter(user)
+      end
+
+      context 'user cannot push to source branch' do
+        it 'returns 404' do
+          expect_rebase_worker_for(viewer).never
+
+          post_rebase
+
+          expect(response.status).to eq(404)
+        end
+      end
+
+      context 'user can push to source branch' do
+        before do
+          project.add_reporter(fork_owner)
+
+          sign_in(fork_owner)
+        end
+
+        it 'returns 200' do
+          expect_rebase_worker_for(fork_owner)
+
+          post_rebase
+
+          expect(response.status).to eq(200)
+        end
+      end
+    end
+  end
 end

spec/fixtures/api/schemas/entities/merge_request_basic.json

@@ -9,6 +9,7 @@
     "human_time_estimate": { "type": ["string", "null"] },
     "human_total_time_spent": { "type": ["string", "null"] },
     "merge_error": { "type": ["string", "null"] },
+    "rebase_in_progress": { "type": "boolean" },
     "assignee_id": { "type": ["integer", "null"] },
     "subscribed": { "type": ["boolean", "null"] },
     "participants": { "type": "array" }

spec/fixtures/api/schemas/entities/merge_request_widget.json

@@ -103,7 +103,11 @@
     "remove_source_branch": { "type": ["boolean", "null"] },
     "merge_ongoing": { "type": "boolean" },
     "ff_only_enabled": { "type": ["boolean", false] },
-    "should_be_rebased": { "type": "boolean" }
+    "should_be_rebased": { "type": "boolean" },
+    "rebase_commit_sha": { "type": ["string", "null"] },
+    "rebase_in_progress": { "type": "boolean" },
+    "can_push_to_source_branch": { "type": "boolean" },
+    "rebase_path": { "type": ["string", "null"] }
   },
   "additionalProperties": false
 }

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 '~/vue_merge_request_widget/components/states/mr_widget_rebase.vue';
+import mountComponent from '../../helpers/vue_mount_component_helper';
+
+describe('Merge request widget rebase component', () => {
+  let Component;
+  let vm;
+  beforeEach(() => {
+    Component = Vue.extend(component);
+  });
+
+  afterEach(() => {
+    vm.$destroy();
+  });
+
+  describe('While rebasing', () => {
+    it('should show progress message', () => {
+      vm = mountComponent(Component, {
+        mr: { rebaseInProgress: true },
+        service: {},
+      });
+
+      expect(
+        vm.$el.querySelector('.rebase-state-find-class-convention span').textContent.trim(),
+      ).toContain('Rebase in progress');
+    });
+  });
+
+  describe('With permissions', () => {
+    beforeEach(() => {
+      vm = mountComponent(Component, {
+        mr: {
+          rebaseInProgress: false,
+          canPushToSourceBranch: true,
+        },
+        service: {},
+      });
+    });
+
+    it('it should render rebase button and warning message', () => {
+      const text = vm.$el.querySelector('.rebase-state-find-class-convention span').textContent.trim();
+      expect(text).toContain('Fast-forward merge is not possible.');
+      expect(text).toContain('Rebase the source branch onto the target branch or merge target');
+      expect(text).toContain('branch into source branch to allow this merge request to be merged.');
+    });
+
+    it('it should render error message when it fails', (done) => {
+      vm.rebasingError = 'Something went wrong!';
+
+      Vue.nextTick(() => {
+        expect(
+          vm.$el.querySelector('.rebase-state-find-class-convention span').textContent.trim(),
+        ).toContain('Something went wrong!');
+        done();
+      });
+    });
+  });
+
+  describe('Without permissions', () => {
+    it('should render a message explaining user does not have permissions', () => {
+      vm = mountComponent(Component, {
+        mr: {
+          rebaseInProgress: false,
+          canPushToSourceBranch: false,
+          targetBranch: 'foo',
+        },
+        service: {},
+      });
+
+      const text = vm.$el.querySelector('.rebase-state-find-class-convention span').textContent.trim();
+
+      expect(text).toContain('Fast-forward merge is not possible.');
+      expect(text).toContain('Rebase the source branch onto');
+      expect(text).toContain('foo');
+      expect(text).toContain('to allow this merge request to be merged.');
+    });
+  });
+
+  describe('methods', () => {
+    it('checkRebaseStatus', (done) => {
+      spyOn(eventHub, '$emit');
+      vm = mountComponent(Component, {
+        mr: {},
+        service: {
+          rebase() {
+            return Promise.resolve();
+          },
+          poll() {
+            return Promise.resolve({
+              data: {
+                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);
+    });
+  });
+});

spec/models/merge_request_spec.rb

@@ -1903,4 +1903,50 @@ describe MergeRequest do
       end
     end
   end
+
+  describe '#should_be_rebased?' do
+    let(:project) { create(:project, :repository) }
+
+    it 'returns false for the same source and target branches' do
+      merge_request = create(:merge_request, source_project: project, target_project: project)
+
+      expect(merge_request.should_be_rebased?).to be_falsey
+    end
+  end
+
+  describe '#rebase_in_progress?' do
+    # Create merge request and project before we stub file calls
+    before do
+      subject
+    end
+
+    it 'returns true when there is a current rebase directory' do
+      allow(File).to receive(:exist?).and_return(true)
+      allow(File).to receive(:mtime).and_return(Time.now)
+
+      expect(subject.rebase_in_progress?).to be_truthy
+    end
+
+    it 'returns false when there is no rebase directory' do
+      allow(File).to receive(:exist?).and_return(false)
+
+      expect(subject.rebase_in_progress?).to be_falsey
+    end
+
+    it 'returns false when the rebase directory has expired' do
+      allow(File).to receive(:exist?).and_return(true)
+      allow(File).to receive(:mtime).and_return(20.minutes.ago)
+
+      expect(subject.rebase_in_progress?).to be_falsey
+    end
+
+    it 'returns false when the source project has been removed' do
+      allow(subject).to receive(:source_project).and_return(nil)
+      allow(File).to receive(:exist?).and_return(true)
+      allow(File).to receive(:mtime).and_return(Time.now)
+
+      expect(File).not_to have_received(:exist?)
+      expect(subject.rebase_in_progress?).to be_falsey
+    end
+  end
 end

spec/models/project_spec.rb

@@ -418,14 +418,21 @@ describe Project do
   end
 
   describe '#merge_method' do
-    it 'returns "ff" merge_method when ff is enabled' do
-      project = build(:project, merge_requests_ff_only_enabled: true)
-      expect(project.merge_method).to be :ff
+    using RSpec::Parameterized::TableSyntax
+
+    where(:ff, :rebase, :method) do
+      true  | true  | :ff
+      true  | false | :ff
+      false | true  | :rebase_merge
+      false | false | :merge
     end
 
-    it 'returns "merge" merge_method when ff is disabled' do
-      project = build(:project, merge_requests_ff_only_enabled: false)
-      expect(project.merge_method).to be :merge
+    with_them do
+      let(:project) { build(:project, merge_requests_rebase_enabled: rebase, merge_requests_ff_only_enabled: ff) }
+
+      subject { project.merge_method }
+
+      it { is_expected.to eq(method) }
     end
   end
 

spec/presenters/merge_request_presenter_spec.rb

@@ -404,4 +404,67 @@ describe MergeRequestPresenter do
         .to eq("<a href=\"/#{resource.source_project.full_path}/tree/#{resource.source_branch}\">#{resource.source_branch}</a>")
     end
   end
+
+  describe '#rebase_path' do
+    before do
+      allow(resource).to receive(:rebase_in_progress?) { rebase_in_progress }
+      allow(resource).to receive(:should_be_rebased?) { should_be_rebased }
+
+      allow_any_instance_of(Gitlab::UserAccess::RequestCacheExtension)
+        .to receive(:can_push_to_branch?)
+        .with(resource.source_branch)
+        .and_return(can_push_to_branch)
+    end
+
+    subject do
+      described_class.new(resource, current_user: user).rebase_path
+    end
+
+    context 'when can rebase' do
+      let(:rebase_in_progress) { false }
+      let(:can_push_to_branch) { true }
+      let(:should_be_rebased) { true }
+
+      before do
+        allow(resource).to receive(:source_branch_exists?) { true }
+      end
+
+      it 'returns path' do
+        is_expected
+          .to eq("/#{project.full_path}/merge_requests/#{resource.iid}/rebase")
+      end
+    end
+
+    context 'when cannot rebase' do
+      context 'when rebase in progress' do
+        let(:rebase_in_progress) { true }
+        let(:can_push_to_branch) { true }
+        let(:should_be_rebased) { true }
+
+        it 'returns nil' do
+          is_expected.to be_nil
+        end
+      end
+
+      context 'when user cannot merge' do
+        let(:rebase_in_progress) { false }
+        let(:can_push_to_branch) { false }
+        let(:should_be_rebased) { true }
+
+        it 'returns nil' do
+          is_expected.to be_nil
+        end
+      end
+
+      context 'should not be rebased' do
+        let(:rebase_in_progress) { false }
+        let(:can_push_to_branch) { true }
+        let(:should_be_rebased) { false }
+
+        it 'returns nil' do
+          is_expected.to be_nil
+        end
+      end
+    end
+  end
 end

spec/serializers/merge_request_widget_entity_spec.rb

@@ -190,4 +190,20 @@ describe MergeRequestWidgetEntity do
       end
     end
   end
+
+  describe 'when source project is deleted' do
+    let(:project) { create(:project, :repository) }
+    let(:fork_project) { create(:project, :repository, forked_from_project: project) }
+    let(:merge_request) { create(:merge_request, source_project: fork_project, target_project: project) }
+
+    it 'returns a blank rebase_path' do
+      allow(merge_request).to receive(:should_be_rebased?).and_return(true)
+      fork_project.destroy
+      merge_request.reload
+
+      entity = described_class.new(merge_request, request: request).as_json
+
+      expect(entity[:rebase_path]).to be_nil
+    end
+  end
 end

spec/services/merge_requests/rebase_service_spec.rb

+require 'spec_helper'
+
+describe MergeRequests::RebaseService do
+  include ProjectForksHelper
+
+  let(:user) { create(:user) }
+  let(:merge_request) do
+    create(:merge_request,
+           source_branch: 'feature_conflict',
+           target_branch: 'master')
+  end
+  let(:project) { merge_request.project }
+  let(:repository) { project.repository.raw }
+
+  subject(:service) { described_class.new(project, user, {}) }
+
+  before do
+    project.add_master(user)
+  end
+
+  describe '#execute' do
+    context 'when another rebase is already in progress' do
+      before do
+        allow(merge_request).to receive(:rebase_in_progress?).and_return(true)
+      end
+
+      it 'saves the error message' do
+        subject.execute(merge_request)
+
+        expect(merge_request.reload.merge_error).to eq 'Rebase task canceled: Another rebase is already in progress'
+      end
+
+      it 'returns an error' do
+        expect(service.execute(merge_request)).to match(status: :error,
+                                                        message: 'Failed to rebase. Should be done manually')
+      end
+    end
+
+    context 'when unexpected error occurs' do
+      before do
+        allow(repository).to receive(:run_git!).and_raise('Something went wrong')
+      end
+
+      it 'saves the error message' do
+        subject.execute(merge_request)
+
+        expect(merge_request.reload.merge_error).to eq 'Something went wrong'
+      end
+
+      it 'returns an error' do
+        expect(service.execute(merge_request)).to match(status: :error,
+                                                        message: 'Failed to rebase. Should be done manually')
+      end
+    end
+
+    context 'with git command failure' do
+      before do
+        allow(repository).to receive(:run_git!).and_raise(Gitlab::Git::Repository::GitError, 'Something went wrong')
+      end
+
+      it 'saves the error message' do
+        subject.execute(merge_request)
+
+        expect(merge_request.reload.merge_error).to eq 'Something went wrong'
+      end
+
+      it 'returns an error' do
+        expect(service.execute(merge_request)).to match(status: :error,
+                                                        message: 'Failed to rebase. Should be done manually')
+      end
+    end
+
+    context 'valid params' do
+      before do
+        service.execute(merge_request)
+      end
+
+      it 'rebases source branch' do
+        parent_sha = merge_request.source_project.repository.commit(merge_request.source_branch).parents.first.sha
+        target_branch_sha = merge_request.target_project.repository.commit(merge_request.target_branch).sha
+        expect(parent_sha).to eq(target_branch_sha)
+      end
+
+      it 'records the new SHA on the merge request' do
+        head_sha = merge_request.source_project.repository.commit(merge_request.source_branch).sha
+        expect(merge_request.reload.rebase_commit_sha).to eq(head_sha)
+      end
+
+      it 'logs correct author and commiter' do
+        head_commit = merge_request.source_project.repository.commit(merge_request.source_branch)
+
+        expect(head_commit.author_email).to eq('dmitriy.zaporozhets@gmail.com')
+        expect(head_commit.author_name).to eq('Dmitriy Zaporozhets')
+        expect(head_commit.committer_email).to eq(user.email)
+        expect(head_commit.committer_name).to eq(user.name)
+      end
+
+      context 'git commands' do
+        it 'sets GL_REPOSITORY env variable when calling git commands' do
+          expect(repository).to receive(:popen).exactly(3)
+            .with(anything, anything, hash_including('GL_REPOSITORY'))
+            .and_return(['', 0])
+
+          service.execute(merge_request)
+        end
+      end
+
+      context 'fork' do
+        let(:forked_project) do
+          fork_project(project, user, repository: true)
+        end
+
+        let(:merge_request_from_fork) do
+          forked_project.repository.create_file(
+            user,
+            'new-file-to-target',
+            '',
+            message: 'Add new file to target',
+            branch_name: 'master')
+
+          create(:merge_request,
+                 source_branch: 'master', source_project: forked_project,
+                 target_branch: 'master', target_project: project)
+        end
+
+        it 'rebases source branch' do
+          parent_sha = forked_project.repository.commit(merge_request_from_fork.source_branch).parents.first.sha
+          target_branch_sha = project.repository.commit(merge_request_from_fork.target_branch).sha
+          expect(parent_sha).to eq(target_branch_sha)
+        end
+      end
+    end
+  end
+end

spec/views/projects/merge_requests/show.html.haml_spec.rb

@@ -54,6 +54,8 @@ describe 'projects/merge_requests/show.html.haml' do
     it 'closes the merge request if the source project does not exist' do
       closed_merge_request.update_attributes(state: 'open')
       forked_project.destroy
+      # Reload merge request so MergeRequest#source_project turns to `nil`
+      closed_merge_request.reload
 
       render
 

spec/workers/rebase_worker_spec.rb

+require 'spec_helper'
+
+describe RebaseWorker, '#perform' do
+  context 'when rebasing an MR from a fork where upstream has protected branches' do
+    let(:upstream_project) { create(:project, :repository) }
+    let(:fork_project) { create(:project, :repository) }
+
+    let(:merge_request) do
+      create(:merge_request,
+             source_project: fork_project,
+             source_branch: 'feature_conflict',
+             target_project: upstream_project,
+             target_branch: 'master')
+    end
+
+    before do
+      create(:forked_project_link, forked_to_project: fork_project, forked_from_project: upstream_project)
+    end
+
+    it 'sets the correct project for running hooks' do
+      expect(MergeRequests::RebaseService)
+        .to receive(:new).with(fork_project, merge_request.author).and_call_original
+
+      subject.perform(merge_request, merge_request.author)
+    end
+  end
+end