Do not reload diff for merge request made from fork when target branch in fork is updated

The target branch of a merge request has to be a branch in the project
for which the merge request is submitted. When a branch changes in a fork,
it does not make sense to reload diffs of merge requests in the upstream
project that use the same branch name as the target branch.
Please note that it does make sense to reload diffs when the source branch
changes.

app/models/merge_request.rb

@@ -101,7 +101,9 @@ class MergeRequest < ActiveRecord::Base
   validate :validate_branches, unless: [:allow_broken, :importing?, :closed_without_fork?]
   validate :validate_fork, unless: :closed_without_fork?
 
-  scope :by_branch, ->(branch_name) { where("(source_branch LIKE :branch) OR (target_branch LIKE :branch)", branch: branch_name) }
+  scope :by_source_or_target_branch, ->(branch_name) do
+    where("source_branch = :branch OR target_branch = :branch", branch: branch_name)
+  end
   scope :cared, ->(user) { where('assignee_id = :user OR author_id = :user', user: user.id) }
   scope :by_milestone, ->(milestone) { where(milestone_id: milestone) }
   scope :of_projects, ->(ids) { where(target_project_id: ids) }

app/services/merge_requests/refresh_service.rb

@@ -55,8 +55,9 @@ 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 = @project.merge_requests.opened.
+        by_source_or_target_branch(@branch_name).to_a
+      merge_requests += fork_merge_requests
       merge_requests = filter_merge_requests(merge_requests)
 
       merge_requests.each do |merge_request|
@@ -157,13 +158,14 @@ module MergeRequests
     def merge_requests_for_source_branch
       @source_merge_requests ||= begin
         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 += fork_merge_requests
         filter_merge_requests(merge_requests)
       end
     end
 
     def fork_merge_requests
-      @fork_merge_requests ||= @project.fork_merge_requests.opened
+      @fork_merge_requests ||= @project.fork_merge_requests.opened.
+        where(source_branch: @branch_name).to_a
     end
 
     def branch_added?

changelogs/unreleased/do-not-refresh-main-when-fork-target-branch-updated.yml

+---
+title: Do not reload diff for merge request made from fork when target branch in fork is updated
+merge_request: 7973
+author:

spec/services/merge_requests/refresh_service_spec.rb

@@ -126,6 +126,7 @@ describe MergeRequests::RefreshService, services: true do
     end
 
     context 'push to fork repo target branch' do
+      describe 'changes to merge requests' do
         before do
           service.new(@fork_project, @user).execute(@oldrev, @newrev, 'refs/heads/feature')
           reload_mrs
@@ -139,6 +140,15 @@ describe MergeRequests::RefreshService, services: true do
         it { expect(@fork_build_failed_todo).to be_pending }
       end
 
+      describe 'merge request diff' do
+        it 'does not reload the diff of the merge request made from fork' do
+          expect do
+            service.new(@fork_project, @user).execute(@oldrev, @newrev, 'refs/heads/feature')
+          end.not_to change { @fork_merge_request.reload.merge_request_diff }
+        end
+      end
+    end
+
     context 'push to origin repo target branch after fork project was removed' do
       before do
         @fork_project.destroy