Fixes rejected pushes from maintainers

Before the push git would make a call to
`/:namespace/:project/git-receive-pack`. This would perform an access
check without a ref. So the `Project#branch_allows_maintainer_push?`
would return false.

This adjusts `Project#branch_allows_maintainer_push?` to return true
when passing no branch name if there are merge requests open that
would allow the user to push.

The actual check then happens when a call to
`/api/v4/internal/allowed` is made from a git hook.

app/models/project.rb

@@ -2139,10 +2139,14 @@ class Project < ActiveRecord::Base
     check_access = -> do
       next false if empty_repo?
 
-      merge_request = source_of_merge_requests.opened
+      merge_requests = source_of_merge_requests.opened
                          .where(allow_collaboration: true)
-                        .find_by(source_branch: branch_name)
-      merge_request&.can_be_merged_by?(user)
+
+      if branch_name
+        merge_requests.find_by(source_branch: branch_name)&.can_be_merged_by?(user)
+      else
+        merge_requests.any? { |merge_request| merge_request.can_be_merged_by?(user) }
+      end
     end
 
     if RequestStore.active?

changelogs/unreleased/bvl-fix-maintainer-push-rejected.yml

+---
+title: Fix bug where maintainer would not be allowed to push to forks with merge requests
+  that have `Allow maintainer edits` enabled.
+merge_request: 18968
+author:
+type: fixed

spec/models/project_spec.rb

@@ -3658,6 +3658,11 @@ describe Project do
           .to be_truthy
       end
 
+      it 'allows access when there are merge requests open but no branch name is given' do
+        expect(project.branch_allows_collaboration?(user, nil))
+          .to be_truthy
+      end
+
       it 'does not allow guest users access' do
         guest = create(:user)
         target_project.add_guest(guest)

spec/requests/git_http_spec.rb

 require "spec_helper"
 
 describe 'Git HTTP requests' do
+  include ProjectForksHelper
   include TermsHelper
   include GitHttpHelpers
   include WorkhorseHelpers
@@ -305,6 +306,22 @@ describe 'Git HTTP requests' do
                 expect(response.body).to eq(change_access_error(:push_code))
               end
             end
+
+            context 'when merge requests are open that allow maintainer access' do
+              let(:canonical_project) { create(:project, :public, :repository) }
+              let(:project) { fork_project(canonical_project, nil, repository: true) }
+
+              before do
+                canonical_project.add_master(user)
+                create(:merge_request,
+                       source_project: project,
+                       target_project:  canonical_project,
+                       source_branch: 'fixes',
+                       allow_collaboration: true)
+              end
+
+              it_behaves_like 'pushes are allowed'
+            end
           end
         end