[CE port] Bypass push rules for merge to ref service

Turns out push rules to validate commit message does
not apply in the context of automatic merge to
the refs/merge-requests/:iid/merge. Mainly because
if it fails to merge to it, we currently can't give enough
preemptive feedback to the user and it'll turn the
merge request unmergeable (given we automatically
mark it as unmergeable if we can't merge to the ref).

In general, it's a systematic operation, which already
bypasses user authorization and git hooks.

Therefore, this commit makes it bypass the push rules
at EE as well.

app/services/merge_requests/merge_base_service.rb

@@ -28,6 +28,17 @@ module MergeRequests
     private
+    def check_source
+      unless source
+        raise_error('No source for merge')
+      end
+    end
+
+    # Overridden in EE.
+    def check_size_limit
+      # No-op
+    end
+
     # Overridden in EE.
     def error_check!
       # No-op

app/services/merge_requests/merge_service.rb

@@ -48,13 +48,13 @@ module MergeRequests
     def error_check!
       super
+      check_source
+
       error =
         if @merge_request.should_be_rebased?
           'Only fast-forward merge is allowed for your project. Please update your source branch'
         elsif !@merge_request.mergeable?
           'Merge request is not mergeable'
-        elsif !source
-          'No source for merge'
         end
       raise_error(error) if error

app/services/merge_requests/merge_to_ref_service.rb

@@ -16,7 +16,7 @@ module MergeRequests
     def execute(merge_request)
       @merge_request = merge_request
-      validate!
+      error_check!
       commit_id = commit
@@ -39,21 +39,9 @@ module MergeRequests
       merge_request.diff_head_sha
     end
-    def validate!
-      error_check!
-    end
-
+    override :error_check!
     def error_check!
-      super
-
-      error =
-        if !hooks_validation_pass?(merge_request)
-          hooks_validation_error(merge_request)
-        elsif source.blank?
-          'No source for merge'
-        end
-
-      raise_error(error) if error
+      check_source
     end
     ##

spec/services/merge_requests/merge_service_spec.rb

@@ -214,6 +214,19 @@ describe MergeRequests::MergeService do
         allow(Rails.logger).to receive(:error)
       end
+      context 'when source is missing' do
+        it 'logs and saves error' do
+          allow(merge_request).to receive(:diff_head_sha) { nil }
+
+          error_message = 'No source for merge'
+
+          service.execute(merge_request)
+
+          expect(merge_request.merge_error).to eq(error_message)
+          expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
+        end
+      end
+
       it 'logs and saves error if there is an exception' do
         error_message = 'error message'

spec/services/merge_requests/merge_to_ref_service_spec.rb

@@ -191,6 +191,19 @@ describe MergeRequests::MergeToRefService do
       it { expect(todo).not_to be_done }
     end
+    context 'when source is missing' do
+      it 'returns error' do
+        allow(merge_request).to receive(:diff_head_sha) { nil }
+
+        error_message = 'No source for merge'
+
+        result = service.execute(merge_request)
+
+        expect(result[:status]).to eq(:error)
+        expect(result[:message]).to eq(error_message)
+      end
+    end
+
     context 'when target ref is passed as a parameter' do
       let(:params) { { commit_message: 'merge train', target_ref: target_ref } }