Return 404 in project issues API endpoint when project cannot be found

Closes #29631

changelogs/unreleased/api-project-issues-404.yml

+---
+title: Return 404 in project issues API endpoint when project cannot be found
+merge_request: 10093
+author:

lib/api/issues.rb

@@ -91,7 +91,7 @@ module API
         use :issues_params
       end
       get ":id/issues" do
-        project = find_project(params[:id])
+        project = find_project!(params[:id])
 
         issues = find_issues(project_id: project.id)
 

lib/api/v3/issues.rb

@@ -103,7 +103,7 @@ module API
           use :issues_params
         end
         get ":id/issues" do
-          project = find_project(params[:id])
+          project = find_project!(params[:id])
 
           issues = find_issues(project_id: project.id)
 

spec/requests/api/issues_spec.rb

@@ -534,6 +534,12 @@ describe API::Issues, api: true  do
   describe "GET /projects/:id/issues" do
     let(:base_url) { "/projects/#{project.id}" }
 
+    it 'returns 404 when project does not exist' do
+      get api('/projects/1000/issues', non_member)
+
+      expect(response).to have_http_status(404)
+    end
+
     it "returns 404 on private projects for other users" do
       private_project = create(:empty_project, :private)
       create(:issue, project: private_project)

spec/requests/api/v3/issues_spec.rb

@@ -439,6 +439,12 @@ describe API::V3::Issues, api: true  do
   describe "GET /projects/:id/issues" do
     let(:base_url) { "/projects/#{project.id}" }
 
+    it 'returns 404 when project does not exist' do
+      get v3_api('/projects/1000/issues', non_member)
+
+      expect(response).to have_http_status(404)
+    end
+
     it "returns 404 on private projects for other users" do
       private_project = create(:empty_project, :private)
       create(:issue, project: private_project)