Fix intermittent 'branch not found' errors when pushing creates an MR

When we push to create a merge request (-omr.create), we kick off an
asynchronous `PostReceive` worker job, then synchronously create a
merge request. This requires the target branch (but not the source
branch) to exist in the repository, so we check when validating the
push options.

The `PostReceive` worker invalidates the branch name cache, but does so
asynchronously, so we may race with it. Until it is invalidated, the
target branch may appear as if it doesn't exist, so we should instead
explicitly invalidate it for ourselves.

This is just dead reckoning at the moment, and is unlikely to solve
cases where the target branch is the project's default branch, but it
seems worth fixing to me.

Changelog: fixed

app/services/merge_requests/push_options_handler_service.rb

@@ -65,7 +65,7 @@ module MergeRequests
       end
 
       if push_options[:target] && !target_project.repository.branch_exists?(push_options[:target])
-        errors << "Branch #{push_options[:target]} does not exist"
+        errors << "Target branch #{target_project.full_path}:#{push_options[:target]} does not exist"
       end
     end
 

app/services/post_receive_service.rb

@@ -17,13 +17,18 @@ class PostReceiveService
     response = Gitlab::InternalPostReceive::Response.new
 
     push_options = Gitlab::PushOptions.new(params[:push_options])
+    mr_options = push_options.get(:merge_request)
 
     response.reference_counter_decreased = Gitlab::ReferenceCounter.new(params[:gl_repository]).decrease
 
+    # The PostReceive worker will normally invalidate the cache. However, it
+    # runs asynchronously. If push options require us to create a new merge
+    # request synchronously, we can't rely on that, so invalidate the cache here
+    repository&.expire_branches_cache if mr_options&.key?(:create)
+
     PostReceive.perform_async(params[:gl_repository], params[:identifier],
                               params[:changes], push_options.as_json)
 
-    mr_options = push_options.get(:merge_request)
     if mr_options.present?
       message = process_mr_push_options(mr_options, params[:changes])
       response.add_alert_message(message)

spec/services/merge_requests/push_options_handler_service_spec.rb

@@ -743,7 +743,7 @@ RSpec.describe MergeRequests::PushOptionsHandlerService do
     it 'records an error' do
       service.execute
 
-      expect(service.errors).to eq(['Branch my-branch does not exist'])
+      expect(service.errors).to eq(["Target branch #{project.full_path}:my-branch does not exist"])
     end
   end
 

spec/services/post_receive_service_spec.rb

@@ -131,6 +131,12 @@ RSpec.describe PostReceiveService do
         project.add_developer(user)
       end
 
+      it 'invalidates the branch name cache' do
+        expect(service.repository).to receive(:expire_branches_cache).and_call_original
+
+        subject
+      end
+
       it 'invokes MergeRequests::PushOptionsHandlerService' do
         expect(MergeRequests::PushOptionsHandlerService).to receive(:new).and_call_original