Fixes createDiffNote GraphQL mutation

Fixes `new_line` not allowed to be `null` which was causing
the mutation to not allow for diff notes to be created
on deleted lines.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/282476

app/graphql/mutations/notes/create/diff_note.rb

@@ -11,6 +11,21 @@ module Mutations
                   required: true,
                   description: copy_field_description(Types::Notes::NoteType, :position)
 
+        def ready?(**args)
+          # As both arguments are optional, validate here that one of the
+          # arguments are present.
+          #
+          # This may be able to be done using InputUnions in the future
+          # if this RFC is merged:
+          # https://github.com/graphql/graphql-spec/blob/master/rfcs/InputUnion.md
+          if args[:position].to_hash.values_at(:old_line, :new_line).compact.blank?
+            raise Gitlab::Graphql::Errors::ArgumentError,
+                  'position oldLine or newLine arguments are required'
+          end
+  
+          super(**args)
+        end
+
         private
 
         def create_note_params(noteable, args)

app/graphql/types/notes/diff_position_input_type.rb

@@ -8,7 +8,7 @@ module Types
 
       argument :old_line, GraphQL::INT_TYPE, required: false,
                description: copy_field_description(Types::Notes::DiffPositionType, :old_line)
-      argument :new_line, GraphQL::INT_TYPE, required: true,
+      argument :new_line, GraphQL::INT_TYPE, required: false,
                description: copy_field_description(Types::Notes::DiffPositionType, :new_line)
     end
     # rubocop: enable Graphql/AuthorizeTypes

changelogs/unreleased/ph-282476-fixGraphQLDiffComments.yml

+---
+title: Fixed diff notes GraphQL mutation not allowing comments on deleted lines
+merge_request:
+author:
+type: fixed

spec/requests/api/graphql/mutations/notes/create/diff_note_spec.rb

@@ -41,6 +41,29 @@ RSpec.describe 'Adding a DiffNote' do
 
     it_behaves_like 'a Note mutation that creates a Note'
 
+    context 'add comment to old line' do
+      let(:mutation) do
+        variables = {
+          noteable_id: GitlabSchema.id_from_object(noteable).to_s,
+          body: 'Body text',
+          position: {
+            paths: {
+              old_path: 'files/ruby/popen.rb',
+              new_path: 'files/ruby/popen.rb'
+            },
+            old_line: 14,
+            base_sha: diff_refs.base_sha,
+            head_sha: diff_refs.head_sha,
+            start_sha: diff_refs.start_sha
+          }
+        }
+
+        graphql_mutation(:create_diff_note, variables)
+      end
+
+      it_behaves_like 'a Note mutation that creates a Note'
+    end
+
     it_behaves_like 'a Note mutation when there are active record validation errors', model: DiffNote
 
     it_behaves_like 'a Note mutation when there are rate limit validation errors'