Add new preparing state

This `preparing` state will be used later on to determine whether
a merge request is mergeable or not
(via `MergeRequest#mergeable?`).

We also won't check mergeability when the `merge_status` is set
to `preparing`.

We set the `merge_status` to `unchecked` in
`MergeRequests::AfterCreateService` (which is called in
`NewMergeRequestWorker`) so we can check for mergeability once
everything's prepared.

Right now, no merge requests will have their `merge_status` to
`preparing` to keep backwards compatibility and to not result to
incidents/downtimes on multi-stage deploy (e.g. canary).

The `MergeRequest#mark_as_preparing` will be later called in
`MergeRequests::CreateService`. This will trigger the behavior
that we want to achieve with the `preparing` state.

app/models/merge_request.rb

@@ -191,8 +191,12 @@ class MergeRequest < ApplicationRecord
   end
 
   state_machine :merge_status, initial: :unchecked do
+    event :mark_as_preparing do
+      transition unchecked: :preparing
+    end
+
     event :mark_as_unchecked do
-      transition [:can_be_merged, :checking] => :unchecked
+      transition [:preparing, :can_be_merged, :checking] => :unchecked
       transition [:cannot_be_merged, :cannot_be_merged_rechecking] => :cannot_be_merged_recheck
     end
 
@@ -209,6 +213,7 @@ class MergeRequest < ApplicationRecord
       transition [:unchecked, :cannot_be_merged_recheck, :checking, :cannot_be_merged_rechecking] => :cannot_be_merged
     end
 
+    state :preparing
     state :unchecked
     state :cannot_be_merged_recheck
     state :checking
@@ -237,7 +242,7 @@ class MergeRequest < ApplicationRecord
   # Returns current merge_status except it returns `cannot_be_merged_rechecking` as `checking`
   # to avoid exposing unnecessary internal state
   def public_merge_status
-    cannot_be_merged_rechecking? ? 'checking' : merge_status
+    cannot_be_merged_rechecking? || preparing? ? 'checking' : merge_status
   end
 
   validates :source_project, presence: true, unless: [:allow_broken, :importing?, :closed_or_merged_without_fork?]

app/services/merge_requests/after_create_service.rb

@@ -3,6 +3,13 @@
 module MergeRequests
   class AfterCreateService < MergeRequests::BaseService
     def execute(merge_request)
+      prepare_merge_request(merge_request)
+      merge_request.mark_as_unchecked if merge_request.preparing?
+    end
+
+    private
+
+    def prepare_merge_request(merge_request)
       event_service.open_mr(merge_request, current_user)
 
       merge_request_activity_counter.track_create_mr_action(user: current_user)

ee/app/services/ee/merge_requests/after_create_service.rb

@@ -5,8 +5,8 @@ module EE
     module AfterCreateService
       extend ::Gitlab::Utils::Override
 
-      override :execute
-      def execute(merge_request)
+      override :prepare_merge_request
+      def prepare_merge_request(merge_request)
         super
 
         schedule_sync_for(merge_request.head_pipeline_id)

spec/models/merge_request_spec.rb

@@ -2899,6 +2899,14 @@ RSpec.describe MergeRequest, factory_default: :keep do
       expect(subject.mergeable?).to be_truthy
     end
 
+    it 'return true if #mergeable_state? is true and the MR #can_be_merged? is false' do
+      allow(subject).to receive(:mergeable_state?) { true }
+      expect(subject).to receive(:check_mergeability)
+      expect(subject).to receive(:can_be_merged?) { false }
+
+      expect(subject.mergeable?).to be_falsey
+    end
+
     context 'with skip_ci_check option' do
       before do
         allow(subject).to receive_messages(check_mergeability: nil,
@@ -3076,6 +3084,7 @@ RSpec.describe MergeRequest, factory_default: :keep do
 
     where(:status, :public_status) do
       'cannot_be_merged_rechecking' | 'checking'
+      'preparing'                   | 'checking'
       'checking'                    | 'checking'
       'cannot_be_merged'            | 'cannot_be_merged'
     end

spec/services/merge_requests/after_create_service_spec.rb

@@ -71,5 +71,27 @@ RSpec.describe MergeRequests::AfterCreateService do
     it_behaves_like 'records an onboarding progress action', :merge_request_created do
       let(:namespace) { merge_request.target_project.namespace }
     end
+
+    context 'when merge request is in unchecked state' do
+      before do
+        merge_request.mark_as_unchecked!
+        execute_service
+      end
+
+      it 'does not change its state' do
+        expect(merge_request.reload).to be_unchecked
+      end
+    end
+
+    context 'when merge request is in preparing state' do
+      before do
+        merge_request.mark_as_preparing!
+        execute_service
+      end
+
+      it 'marks the merge request as unchecked' do
+        expect(merge_request.reload).to be_unchecked
+      end
+    end
   end
 end