Add a system note and update relevant merge requests when a branch is deleted or re-added

If a branch is deleted with an open merge request, amended offline, and then pushed again,
GitLab doesn't bother to update the merge request even though the last commit ID and/or
code may have changed. This MR ensures that each push will update any relevant merge
requests and adds a system note if this happens as well.

Closes #2926

8 files changed, 98 insertions, 24 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 39926692147..814a6772cfd 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.1.0 (unreleased)
   - Fix error preventing displaying of commit data for a directory with a leading dot (Stan Hu)
   - Speed up load times of issue detail pages by roughly 1.5x
+  - Add a system note and update relevant merge requests when a branch is deleted or re-added (Stan Hu)
   - Make diff file view easier to use on mobile screens (Stan Hu)
   - Improved performance of finding users by username or Email address
   - Fix bug where merge request comments created by API would not trigger notifications (Stan Hu)
diff --git a/app/models/merge_request_diff.rb b/app/models/merge_request_diff.rb
index c9ef8023aea..bc2d691ece0 100644
--- a/app/models/merge_request_diff.rb
+++ b/app/models/merge_request_diff.rb
@@ -163,7 +163,8 @@ class MergeRequestDiff < ActiveRecord::Base
         merge_request.fetch_ref
 
         # Get latest sha of branch from source project
-        source_sha = merge_request.source_project.commit(source_branch).sha
+        source_commit = merge_request.source_project.commit(source_branch)
+        source_sha = source_commit.try(:sha)
 
         Gitlab::CompareResult.new(
           Gitlab::Git::Compare.new(
diff --git a/app/services/git_push_service.rb b/app/services/git_push_service.rb
index 81d47602f13..e54044365b9 100644
--- a/app/services/git_push_service.rb
+++ b/app/services/git_push_service.rb
@@ -49,10 +49,13 @@ class GitPushService
     elsif push_to_existing_branch?(ref, oldrev)
       # Collect data for this git push
       @push_commits = project.repository.commits_between(oldrev, newrev)
-      project.update_merge_requests(oldrev, newrev, ref, @user)
       process_commit_messages(ref)
     end
 
+    # Update merge requests that may be affected by this push. A new branch
+    # could cause the last commit of a merge request to change.
+    project.update_merge_requests(oldrev, newrev, ref, @user)
+
     @push_data = build_push_data(oldrev, newrev, ref)
 
     # If CI was disabled but .gitlab-ci.yml file was pushed
diff --git a/app/services/merge_requests/refresh_service.rb b/app/services/merge_requests/refresh_service.rb
index e903e48e3cd..802d02b0790 100644
--- a/app/services/merge_requests/refresh_service.rb
+++ b/app/services/merge_requests/refresh_service.rb
@@ -6,12 +6,22 @@ module MergeRequests
       @oldrev, @newrev = oldrev, newrev
       @branch_name = Gitlab::Git.ref_name(ref)
       @fork_merge_requests = @project.fork_merge_requests.opened
-      @commits = @project.repository.commits_between(oldrev, newrev)
+      @commits = []
+      @merge_requests = merge_requests_for_branch
+
+      # Leave a system note if a branch were deleted/added
+      if Gitlab::Git.blank_ref?(oldrev) or Gitlab::Git.blank_ref?(newrev)
+        presence = Gitlab::Git.blank_ref?(oldrev) ? 'added' : 'deleted'
+        comment_mr_branch_presence_changed(presence)
+      else
+        @commits = @project.repository.commits_between(oldrev, newrev)
+
+        close_merge_requests
+        comment_mr_with_commits
+      end
 
-      close_merge_requests
       reload_merge_requests
       execute_mr_web_hooks
-      comment_mr_with_commits
 
       true
     end
@@ -31,7 +41,6 @@ module MergeRequests
         commit_ids.include?(merge_request.last_commit.id)
       end
 
-
       merge_requests.uniq.select(&:source_project).each do |merge_request|
         MergeRequests::PostMergeService.
           new(merge_request.target_project, @current_user).
@@ -46,11 +55,7 @@ module MergeRequests
     # Refresh merge request diff if we push to source or target branch of merge request
     # Note: we should update merge requests from forks too
     def reload_merge_requests
-      merge_requests = @project.merge_requests.opened.by_branch(@branch_name).to_a
-      merge_requests += @fork_merge_requests.by_branch(@branch_name).to_a
-      merge_requests = filter_merge_requests(merge_requests)
-
-      merge_requests.each do |merge_request|
+      @merge_requests.each do |merge_request|
 
         if merge_request.source_branch == @branch_name || force_push?
           merge_request.reload_code
@@ -70,13 +75,20 @@ module MergeRequests
       end
     end
 
-    # Add comment about pushing new commits to merge requests
-    def comment_mr_with_commits
-      merge_requests = @project.origin_merge_requests.opened.where(source_branch: @branch_name).to_a
-      merge_requests += @fork_merge_requests.where(source_branch: @branch_name).to_a
-      merge_requests = filter_merge_requests(merge_requests)
+    # Add comment about branches being deleted or added to merge requests
+    def comment_mr_branch_presence_changed(presence)
+      merge_requests = merge_requests_for_branch
 
       merge_requests.each do |merge_request|
+        SystemNoteService.change_branch_presence(
+            merge_request, merge_request.project, @current_user,
+            'source', @branch_name, presence)
+      end
+    end
+
+    # Add comment about pushing new commits to merge requests
+    def comment_mr_with_commits
+      @merge_requests.each do |merge_request|
         mr_commit_ids = Set.new(merge_request.commits.map(&:id))
 
         new_commits, existing_commits = @commits.partition do |commit|
@@ -91,14 +103,7 @@ module MergeRequests
 
     # Call merge request webhook with update branches
     def execute_mr_web_hooks
-      merge_requests = @project.origin_merge_requests.opened
-        .where(source_branch: @branch_name)
-        .to_a
-      merge_requests += @fork_merge_requests.where(source_branch: @branch_name)
-        .to_a
-      merge_requests = filter_merge_requests(merge_requests)
-
-      merge_requests.each do |merge_request|
+      @merge_requests.each do |merge_request|
         execute_hooks(merge_request, 'update')
       end
     end
@@ -106,5 +111,11 @@ module MergeRequests
     def filter_merge_requests(merge_requests)
       merge_requests.uniq.select(&:source_project)
     end
+
+    def merge_requests_for_branch
+      merge_requests = @project.origin_merge_requests.opened.where(source_branch: @branch_name).to_a
+      merge_requests += @fork_merge_requests.where(source_branch: @branch_name).to_a
+      filter_merge_requests(merge_requests)
+    end
   end
 end
diff --git a/app/services/system_note_service.rb b/app/services/system_note_service.rb
index 8253c1f780d..24e06504078 100644
--- a/app/services/system_note_service.rb
+++ b/app/services/system_note_service.rb
@@ -168,6 +168,25 @@ class SystemNoteService
     create_note(noteable: noteable, project: project, author: author, note: body)
   end
 
+  # Called when a branch in Noteable is added or deleted
+  #
+  # noteable    - Noteable object
+  # project     - Project owning noteable
+  # author      - User performing the change
+  # branch_type - 'source' or 'target'
+  # branch      - branch name
+  # presence    - 'deleted' or 'created'
+  #
+  # Example Note text:
+  #
+  #   "Target branch `feature` deleted"
+  #
+  # Returns the created Note object
+  def self.change_branch_presence(noteable, project, author, branch_type, branch, presence)
+    body = "#{branch_type} branch `#{branch}` #{presence}".capitalize
+    create_note(noteable: noteable, project: project, author: author, note: body)
+  end
+
   # Called when a Mentionable references a Noteable
   #
   # noteable  - Noteable object being referenced
diff --git a/spec/services/git_push_service_spec.rb b/spec/services/git_push_service_spec.rb
index fd72905c331..17015d29e51 100644
--- a/spec/services/git_push_service_spec.rb
+++ b/spec/services/git_push_service_spec.rb
@@ -112,6 +112,14 @@ describe GitPushService do
     it { expect(@event.project).to eq(project) }
     it { expect(@event.action).to eq(Event::PUSHED) }
     it { expect(@event.data).to eq(service.push_data) }
+
+    context "Updates merge requests" do
+      it "when pushing a new branch for the first time" do
+        expect(project).to receive(:update_merge_requests).
+                               with(@blankrev, 'newrev', 'refs/heads/master', user)
+        service.execute(project, user, @blankrev, 'newrev', 'refs/heads/master')
+      end
+    end
   end
 
   describe "Web Hooks" do
diff --git a/spec/services/merge_requests/refresh_service_spec.rb b/spec/services/merge_requests/refresh_service_spec.rb
index 9516e7936d8..edb9fd0b43c 100644
--- a/spec/services/merge_requests/refresh_service_spec.rb
+++ b/spec/services/merge_requests/refresh_service_spec.rb
@@ -106,6 +106,25 @@ describe MergeRequests::RefreshService do
       it { expect(@fork_merge_request.notes).to be_empty }
     end
 
+    context 'push new branch that exists in a merge request' do
+      let(:refresh_service) { service.new(@fork_project, @user) }
+      before do
+        allow(refresh_service).to receive(:execute_hooks)
+        refresh_service.execute(Gitlab::Git::BLANK_SHA, @newrev, 'refs/heads/master')
+        reload_mrs
+      end
+
+      it 'should execute hooks with update action' do
+        expect(refresh_service).to have_received(:execute_hooks).
+                                       with(@fork_merge_request, 'update')
+      end
+
+      it { expect(@merge_request.notes).to be_empty }
+      it { expect(@merge_request).to be_open }
+      it { expect(@fork_merge_request.notes.last.note).to include('Source branch `master` added') }
+      it { expect(@fork_merge_request).to be_open }
+    end
+
     def reload_mrs
       @merge_request.reload
       @fork_merge_request.reload
diff --git a/spec/services/system_note_service_spec.rb b/spec/services/system_note_service_spec.rb
index 2658576640c..108bc5995df 100644
--- a/spec/services/system_note_service_spec.rb
+++ b/spec/services/system_note_service_spec.rb
@@ -242,6 +242,18 @@ describe SystemNoteService do
     end
   end
 
+  describe '.change_branch_presence' do
+    subject { described_class.change_branch_presence(noteable, project, author, 'source', 'feature', 'deleted') }
+
+    it_behaves_like 'a system note'
+
+    context 'when source branch deleted' do
+      it 'sets the note text' do
+        expect(subject.note).to eq "Source branch `feature` deleted"
+      end
+    end
+  end
+
   describe '.cross_reference' do
     subject { described_class.cross_reference(noteable, mentioner, author) }