Show an error when failling to save a merge request dependency

Merge Request: gitlab.com/gitlab-org/gitlab/-/merge_requests/48237
Issue: gitlab.com/gitlab-org/gitlab/-/issues/207395

ee/app/services/merge_requests/update_blocks_service.rb

@@ -33,17 +33,24 @@ module MergeRequests
       return unless update?
       return unless merge_request.target_project.feature_available?(:blocking_merge_requests)
 
-      merge_request
-        .blocks_as_blockee
-        .with_blocking_mr_ids(ids_to_del)
-        .delete_all
+      valid_references, invalid_references = extract_references
 
-      # If the block is invalid, silently fail to add it
-      ids_to_add.each do |blocking_id|
-        ::MergeRequestBlock.create(
-          blocking_merge_request_id: blocking_id,
-          blocked_merge_request_id: merge_request.id
-        )
+      delete_old_blockers!(valid_references)
+
+      errors = create_blockers(valid_references)
+
+      if invalid_references.present?
+        merge_request.errors.add(:dependencies, 'failed to save: ' + invalid_references.join(', '))
+      end
+
+      if errors.present?
+        merge_request.errors.add(:dependencies, 'failed to save: ' + errors.join(', '))
+      end
+
+      if invalid_references.present? || errors.present?
+        # When there are invalid references, we need to reset the associations
+        # so that the latest blocking merge requests are shown in the UI
+        merge_request.blocking_merge_requests.reset
       end
 
       true
@@ -65,36 +72,68 @@ module MergeRequests
       params.fetch(:references, [])
     end
 
-    def requested_ids
-      strong_memoize(:requested_ids) do
-        next [] unless references.present?
+    # Returns two lists of references separating valid from invalid ones
+    #
+    # @return [Array<Array>] an array of valid and an array of invalid references
+    def extract_references
+      invalid_references = []
+      valid_references = []
 
-        # The analyzer will only return references the current user can see
+      return [], [] unless references.present?
+
+      # The analyzer will only return references the current user can see
+      references.each do |reference|
         analyzer = ::Gitlab::ReferenceExtractor.new(merge_request.target_project, current_user)
-        analyzer.analyze(references.join(" "))
+        analyzer.analyze(reference)
 
-        analyzer.merge_requests.map(&:id)
+        if analyzer.merge_requests.any?
+          valid_references << analyzer.merge_requests
+        else
+          invalid_references << reference
+        end
       end
+
+      [valid_references.flatten.map(&:id), invalid_references]
     end
 
-    def visible_ids
-      strong_memoize(:visible_ids) { visible_blocks.map(&:blocking_merge_request_id) }
+    def delete_old_blockers!(valid_references)
+      merge_request
+        .blocks_as_blockee
+        .with_blocking_mr_ids(ids_to_delete(valid_references))
+        .delete_all
     end
 
-    def hidden_ids
-      strong_memoize(:hidden_ids) { hidden_blocks.map(&:blocking_merge_request_id) }
+    def create_blockers(valid_references)
+      new_ids = ids_to_add(valid_references)
+
+      new_ids.each_with_object([]) do |blocking_id, errors|
+        blocked = ::MergeRequestBlock.create(
+          blocking_merge_request_id: blocking_id,
+          blocked_merge_request_id: merge_request.id
+        )
+
+        unless blocked.persisted?
+          errors << blocked.errors.full_messages
+        end
+      end
     end
 
-    def ids_to_add
-      strong_memoize(:ids_to_add) { requested_ids - visible_ids }
+    def ids_to_add(valid_references)
+      valid_references - visible_ids
     end
 
-    def ids_to_del
-      strong_memoize(:ids_to_del) do
-        (visible_ids - requested_ids).tap do |ary|
-          ary.push(*hidden_ids) if remove_hidden?
-        end
+    def ids_to_delete(valid_references)
+      (visible_ids - valid_references).tap do |ary|
+        ary.push(*hidden_ids) if remove_hidden?
       end
     end
+
+    def visible_ids
+      strong_memoize(:visible_ids) { visible_blocks.map(&:blocking_merge_request_id) }
+    end
+
+    def hidden_ids
+      hidden_blocks.map(&:blocking_merge_request_id)
+    end
   end
 end

ee/changelogs/unreleased/207395-merge-request-dependency-silently-fails-if-complexity-is-unsupport.yml

+---
+title: Show an error when failing to save a merge request dependency
+merge_request: 48237
+author:
+type: changed

ee/spec/services/merge_requests/update_blocks_service_spec.rb

@@ -88,10 +88,20 @@ RSpec.describe MergeRequests::UpdateBlocksService do
           context 'with a self-referential block' do
             let(:mr_to_add) { merge_request }
 
-            it 'ignores the addition' do
+            it 'has an error on the merge request' do
               service.execute
 
-              expect(merge_request.blocking_merge_requests).not_to include(mr_to_add)
+              expect(merge_request.reload.blocking_merge_requests).not_to include(mr_to_add)
+              expect(merge_request.errors[:dependencies]).to include(/cannot itself be blocked/)
+            end
+          end
+
+          context 'when an invalid reference' do
+            it 'has an error on the merge request' do
+              refs << 'notavalid'
+              service.execute
+
+              expect(merge_request.errors[:dependencies]).to include(/notavalid/)
             end
           end
         end