Only create unmergeable todos once

Previously, we created an unmergeable todo when a merge request:

1. Had merge when pipeline succeeds set.
2. Became unmergeable.

However, when merge when pipeline succeeds fails due to unmergeability,
the flag isn't actually removed. And a merge request can become
unmergeable multiple times, as every time the target branch is updated
we need to re-check the mergeable status. This means that if the todo
was marked done, and the MR was checked again, a new todo would be
created for the same event.

Instead of checking this, we should create the todo from the service
responsible for merging when the pipeline succeeds. That way the todo is
guaranteed to only be created when we care about it.

app/models/merge_request.rb

@@ -91,10 +91,6 @@ class MergeRequest < ActiveRecord::Base
     around_transition do |merge_request, transition, block|
       Gitlab::Timeless.timeless(merge_request, &block)
     end
-
-    after_transition unchecked: :cannot_be_merged do |merge_request, transition|
-      TodoService.new.merge_request_became_unmergeable(merge_request)
-    end
   end
 
   validates :source_project, presence: true, unless: [:allow_broken, :importing?, :closed_without_fork?]

app/services/merge_requests/merge_when_pipeline_succeeds_service.rb

@@ -24,7 +24,11 @@ module MergeRequests
 
       pipeline_merge_requests(pipeline) do |merge_request|
         next unless merge_request.merge_when_build_succeeds?
-        next unless merge_request.mergeable?
+
+        unless merge_request.mergeable?
+          todo_service.merge_request_became_unmergeable(merge_request)
+          next
+        end
 
         MergeWorker.perform_async(merge_request.id, merge_request.merge_user_id, merge_request.merge_params)
       end

changelogs/unreleased/only-create-unmergeable-todo-once.yml

+---
+title: Only create unmergeable todos once when MR fails to merge
+merge_request:
+author:

spec/models/merge_request_spec.rb

@@ -833,12 +833,6 @@ describe MergeRequest, models: true do
       it 'becomes unmergeable' do
         expect { subject.check_if_can_be_merged }.to change { subject.merge_status }.to('cannot_be_merged')
       end
-
-      it 'creates Todo on unmergeability' do
-        expect_any_instance_of(TodoService).to receive(:merge_request_became_unmergeable).with(subject)
-
-        subject.check_if_can_be_merged
-      end
     end
 
     context 'when it has conflicts' do

spec/services/merge_requests/merge_when_pipeline_succeeds_service_spec.rb

@@ -111,6 +111,31 @@ describe MergeRequests::MergeWhenPipelineSucceedsService do
         service.trigger(unrelated_pipeline)
       end
     end
+
+    context 'when the merge request is not mergeable' do
+      let(:mr_conflict) do
+        create(:merge_request, merge_when_build_succeeds: true, merge_user: user,
+                               source_branch: 'master', target_branch: 'feature-conflict',
+                               source_project: project, target_project: project)
+      end
+
+      let(:conflict_pipeline) do
+        create(:ci_pipeline, project: project, ref: mr_conflict.source_branch,
+                             sha: mr_conflict.diff_head_sha, status: 'success')
+      end
+
+      it 'does not merge the merge request' do
+        expect(MergeWorker).not_to receive(:perform_async)
+
+        service.trigger(conflict_pipeline)
+      end
+
+      it 'creates todos for unmergeability' do
+        expect_any_instance_of(TodoService).to receive(:merge_request_became_unmergeable).with(mr_conflict)
+
+        service.trigger(conflict_pipeline)
+      end
+    end
   end
 
   describe "#cancel" do