Move assignee_usernames issue filter support to resolver

- Issuable finder does not handle assignee_usernames, only assignee_username
- IssueResolver arguments checks mutually exclusive arguments assignee_username(s)
- IssueResolver handles plural version of assignee_username filter

app/finders/issuable_finder/params.rb

@@ -163,8 +163,6 @@ class IssuableFinder
           User.where(id: params[:assignee_id])
         elsif assignee_username?
           User.where(username: params[:assignee_username])
-        elsif assignee_usernames?
-          User.where(username: params[:assignee_usernames])
         else
           User.none
         end

app/graphql/resolvers/concerns/issue_resolver_arguments.rb

@@ -73,11 +73,22 @@ module IssueResolverArguments
     args[:iids] ||= [args.delete(:iid)].compact if args[:iid]
     args[:attempt_project_search_optimizations] = true if args[:search].present?
 
+    prepare_assignee_username_params(args)
+
     finder = IssuesFinder.new(current_user, args)
 
     continue_issue_resolve(parent, finder, **args)
   end
 
+  def ready?(**args)
+    if args.slice(*mutually_exclusive_assignee_username_args).compact.size > 1
+      arg_str = mutually_exclusive_assignee_username_args.map { |x| x.to_s.camelize(:lower) }.join(', ')
+      raise Gitlab::Graphql::Errors::ArgumentError, "only one of [#{arg_str}] arguments is allowed at the same time."
+    end
+
+    super
+  end
+
   class_methods do
     def resolver_complexity(args, child_complexity:)
       complexity = super
@@ -86,4 +97,15 @@ module IssueResolverArguments
       complexity
     end
   end
+
+  private
+
+  def prepare_assignee_username_params(args)
+    args[:assignee_username] = args.delete(:assignee_usernames) if args[:assignee_usernames].present?
+    args[:not][:assignee_username] = args[:not].delete(:assignee_usernames) if args.dig(:not, :assignee_usernames).present?
+  end
+
+  def mutually_exclusive_assignee_username_args
+    [:assignee_usernames, :assignee_username]
+  end
 end

app/graphql/types/issues/negated_issue_filter_input_type.rb

@@ -14,9 +14,6 @@ module Types
       argument :milestone_title, [GraphQL::STRING_TYPE],
                 required: false,
                 description: 'Milestone not applied to this issue.'
-      argument :assignee_username, GraphQL::STRING_TYPE,
-                required: false,
-                description: 'Username of a user not assigned to the issue.'
       argument :assignee_usernames, [GraphQL::STRING_TYPE],
                 required: false,
                 description: 'Usernames of users not assigned to the issue.'

spec/finders/issues_finder_spec.rb

@@ -70,11 +70,6 @@ RSpec.describe IssuesFinder do
             let(:expected_issuables) { [issue2] }
           end
 
-          it_behaves_like 'assignee username filter' do
-            let(:params) { { assignee_usernames: [user2.username] } }
-            let(:expected_issuables) { [issue2] }
-          end
-
           it_behaves_like 'assignee NOT username filter' do
             before do
               issue2.assignees = [user2]

spec/graphql/resolvers/issue_status_counts_resolver_spec.rb

@@ -69,6 +69,14 @@ RSpec.describe Resolvers::IssueStatusCountsResolver do
       expect(result.closed).to eq 1
     end
 
+    context 'when both assignee_username and assignee_usernames are provided' do
+      it 'raises a mutually exclusive filter error' do
+        expect do
+          resolve_issue_status_counts(assignee_usernames: [current_user.username], assignee_username: current_user.username)
+        end.to raise_error(Gitlab::Graphql::Errors::ArgumentError, 'only one of [assigneeUsernames, assigneeUsername] arguments is allowed at the same time.')
+      end
+    end
+
     private
 
     def resolve_issue_status_counts(args = {}, context = { current_user: current_user })

spec/graphql/resolvers/issues_resolver_spec.rb

@@ -46,14 +46,6 @@ RSpec.describe Resolvers::IssuesResolver do
         expect(resolve_issues(milestone_title: [milestone.title])).to contain_exactly(issue1)
       end
 
-      it 'filters by assignee_username' do
-        expect(resolve_issues(assignee_username: [assignee.username])).to contain_exactly(issue2)
-      end
-
-      it 'filters by assignee_usernames' do
-        expect(resolve_issues(assignee_usernames: [assignee.username])).to contain_exactly(issue2)
-      end
-
       it 'filters by two assignees' do
         assignee2 = create(:user)
         issue2.update!(assignees: [assignee, assignee2])
@@ -82,6 +74,24 @@ RSpec.describe Resolvers::IssuesResolver do
         expect(resolve_issues(label_name: [label1.title, label2.title])).to contain_exactly(issue2)
       end
 
+      describe 'filters by assignee_username' do
+        it 'filters by assignee_username' do
+          expect(resolve_issues(assignee_username: [assignee.username])).to contain_exactly(issue2)
+        end
+
+        it 'filters by assignee_usernames' do
+          expect(resolve_issues(assignee_usernames: [assignee.username])).to contain_exactly(issue2)
+        end
+
+        context 'when both assignee_username and assignee_usernames are provided' do
+          it 'raises a mutually exclusive filter error' do
+            expect do
+              resolve_issues(assignee_usernames: [assignee.username], assignee_username: assignee.username)
+            end.to raise_error(Gitlab::Graphql::Errors::ArgumentError, 'only one of [assigneeUsernames, assigneeUsername] arguments is allowed at the same time.')
+          end
+        end
+      end
+
       describe 'filters by created_at' do
         it 'filters by created_before' do
           expect(resolve_issues(created_before: 2.hours.ago)).to contain_exactly(issue1)
@@ -162,10 +172,6 @@ RSpec.describe Resolvers::IssuesResolver do
           expect(resolve_issues(not: { milestone_title: [milestone.title] })).to contain_exactly(issue2)
         end
 
-        it 'returns issues without the specified assignee_username' do
-          expect(resolve_issues(not: { assignee_username: assignee.username })).to contain_exactly(issue1)
-        end
-
         it 'returns issues without the specified assignee_usernames' do
           expect(resolve_issues(not: { assignee_usernames: [assignee.username] })).to contain_exactly(issue1)
         end

spec/requests/api/graphql/project/issues_spec.rb

@@ -12,6 +12,7 @@ RSpec.describe 'getting an issue list for a project' do
   let_it_be(:issues, reload: true) { [issue_a, issue_b] }
 
   let(:issues_data) { graphql_data['project']['issues']['edges'] }
+  let(:issue_filter_params) { {} }
 
   let(:fields) do
     <<~QUERY
@@ -27,7 +28,7 @@ RSpec.describe 'getting an issue list for a project' do
     graphql_query_for(
       'project',
       { 'fullPath' => project.full_path },
-      query_graphql_field('issues', {}, fields)
+      query_graphql_field('issues', issue_filter_params, fields)
     )
   end
 
@@ -50,6 +51,16 @@ RSpec.describe 'getting an issue list for a project' do
     expect(issues_data[1]['node']['discussionLocked']).to eq(true)
   end
 
+  context 'when both assignee_username filters are provided' do
+    let(:issue_filter_params) { { assignee_username: current_user.username, assignee_usernames: [current_user.username] } }
+
+    it 'returns a mutually exclusive param error' do
+      post_graphql(query, current_user: current_user)
+
+      expect_graphql_errors_to_include('only one of [assigneeUsernames, assigneeUsername] arguments is allowed at the same time.')
+    end
+  end
+
   context 'when limiting the number of results' do
     let(:query) do
       <<~GQL