Support merge request create with push options

To create a new merge request:

  git push -u origin -o merge_request.create

To create a new merge request setting target branch:

  git push -u origin -o merge_request.create \
    -o merge_request.target=123

To update an existing merge request with a new target branch:

  git push -u origin -o merge_request.target=123

A new Gitlab::PushOptions class handles parsing and validating the push
options array. This can be the start of the standard of GitLab accepting
push options that follow namespacing rules. Rules are discussed in issue
https://gitlab.com/gitlab-org/gitlab-ce/issues/43263.

E.g. these push options:

  -o merge_request.create -o merge_request.target=123

Become parsed as:

  {
    merge_request: {
      create: true,
      target: '123',
    }
  }

And are fetched with the class via:

  push_options.get(:merge_request)
  push_options.get(:merge_request, :create)
  push_options.get(:merge_request, :target)

A new MergeRequests::PushOptionsHandlerService takes the `merge_request`
namespaced push options and handles creating and updating
merge requests.

Any errors encountered are passed to the existing `output` Hash in
Api::Internal's `post_receive` endpoint, and passed to gitlab-shell
where they're output to the user.

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/43263

2 files changed, 34 insertions, 7 deletions

diff --git a/lib/api/helpers/internal_helpers.rb b/lib/api/helpers/internal_helpers.rb
index 3fd824877ae..5014ba51b94 100644
--- a/lib/api/helpers/internal_helpers.rb
+++ b/lib/api/helpers/internal_helpers.rb
@@ -43,6 +43,11 @@ module API
         ::MergeRequests::GetUrlsService.new(project).execute(params[:changes])
       end
 
+      def push_options_warning(warning)
+        options = Array.wrap(params[:push_options]).map { |p| "'#{p}'" }.join(' ')
+        "Error encountered with push options #{options}: #{warning}"
+      end
+
       def redis_ping
         result = Gitlab::Redis::SharedState.with { |redis| redis.ping }
 
diff --git a/lib/api/internal.rb b/lib/api/internal.rb
index 9c7b9146c8f..75202fa953c 100644
--- a/lib/api/internal.rb
+++ b/lib/api/internal.rb
@@ -256,19 +256,41 @@ module API
       post '/post_receive' do
         status 200
 
+        output = {} # Messages to gitlab-shell
+        user = identify(params[:identifier])
+        project = Gitlab::GlRepository.parse(params[:gl_repository]).first
+        push_options = Gitlab::PushOptions.new(params[:push_options])
+
         PostReceive.perform_async(params[:gl_repository], params[:identifier],
           params[:changes], params[:push_options].to_a)
+
+        if (mr_options = push_options.get(:merge_request))
+          begin
+            service = ::MergeRequests::PushOptionsHandlerService.new(
+              project,
+              user,
+              params[:changes],
+              mr_options
+            ).execute
+
+            if service.errors.present?
+              output[:warnings] = push_options_warning(service.errors.join("\n\n"))
+            end
+          rescue ::MergeRequests::PushOptionsHandlerService::Error => e
+            output[:warnings] = push_options_warning(e.message)
+          rescue Gitlab::Access::AccessDeniedError
+            output[:warnings] = push_options_warning('User access was denied')
+          end
+        end
+
         broadcast_message = BroadcastMessage.current&.last&.message
         reference_counter_decreased = Gitlab::ReferenceCounter.new(params[:gl_repository]).decrease
 
-        output = {
-          merge_request_urls: merge_request_urls,
+        output.merge!(
           broadcast_message: broadcast_message,
-          reference_counter_decreased: reference_counter_decreased
-        }
-
-        project = Gitlab::GlRepository.parse(params[:gl_repository]).first
-        user = identify(params[:identifier])
+          reference_counter_decreased: reference_counter_decreased,
+          merge_request_urls: merge_request_urls
+        )
 
         # A user is not guaranteed to be returned; an orphaned write deploy
         # key could be used