Port semi-linear merge strategy to CE (mostly FE)

app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_conflicts.js

@@ -15,7 +15,7 @@ export default {
         showDisabledButton />
       <div class="media-body space-children">
         <span
-          v-if="mr.ffOnlyEnabled"
+          v-if="mr.shouldBeRebased"
           class="bold">
           Fast-forward merge is not possible.
           To merge this request, first rebase locally.

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

@@ -58,6 +58,7 @@ export default class MergeRequestStore {
     this.mergeWhenPipelineSucceeds = data.merge_when_pipeline_succeeds || false;
     this.mergePath = data.merge_path;
     this.ffOnlyEnabled = data.ff_only_enabled;
+    this.shouldBeRebased = !!data.should_be_rebased;
     this.statusPath = data.status_path;
     this.emailPatchesPath = data.email_patches_path;
     this.plainDiffPath = data.plain_diff_path;

app/models/merge_request.rb

@@ -524,6 +524,14 @@ class MergeRequest < ActiveRecord::Base
     true
   end
+  def ff_merge_possible?
+    project.repository.ancestor?(target_branch_sha, diff_head_sha)
+  end
+
+  def should_be_rebased?
+    project.ff_merge_must_be_possible? && !ff_merge_possible?
+  end
+
   def can_cancel_merge_when_pipeline_succeeds?(current_user)
     can_be_merged_by?(current_user) || self.author == current_user
   end

app/serializers/merge_request_entity.rb

@@ -13,6 +13,7 @@ class MergeRequestEntity < IssuableEntity
   expose :target_branch
   expose :target_project_id
+  expose :should_be_rebased?, as: :should_be_rebased
   expose :ff_only_enabled do |merge_request|
     merge_request.project.merge_requests_ff_only_enabled
   end

app/views/projects/_merge_request_rebase_settings.html.haml

+- form = local_assigns.fetch(:form)
+
+.radio
+  = label_tag :project_merge_method_rebase_merge do
+    = form.radio_button :merge_method, :rebase_merge, class: "js-merge-method-radio"
+    %strong Merge commit with semi-linear history
+    %br
+    %span.descr
+      A merge commit is created for every merge, but merging is only allowed if fast-forward merge is possible.
+      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.

app/views/projects/_merge_request_settings.html.haml

@@ -11,6 +11,8 @@
       %span.descr
         A merge commit is created for every merge, and merging is allowed as long as there are no conflicts.
+  = render 'merge_request_rebase_settings', form: form
+
   = render 'merge_request_fast_forward_settings', project: @project, form: form
 = render 'projects/merge_request_merge_settings', form: form

spec/features/projects/project_settings_spec.rb

@@ -41,6 +41,14 @@ describe 'Edit Project Settings' do
       end
     end
+    it 'shows "Merge commit with semi-linear history " strategy' do
+      visit edit_project_path(project)
+
+      page.within '.merge-requests-feature' do
+        expect(page).to have_content 'Merge commit with semi-linear history'
+      end
+    end
+
     it 'shows "Fast-forward merge" strategy' do
       visit edit_project_path(project)

spec/javascripts/vue_mr_widget/components/states/mr_widget_conflicts_spec.js

@@ -81,13 +81,13 @@ describe('MRWidgetConflicts', () => {
       });
     });
-    describe('when fast-forward merge enabled', () => {
+    describe('when fast-forward or semi-linear merge enabled', () => {
       let vm;
       beforeEach(() => {
         vm = mountComponent(ConflictsComponent, {
           mr: {
-            ffOnlyEnabled: true,
+            shouldBeRebased: true,
           },
         });
       });