Fix 404 errors when "No epic" selected

If a user clicked on "No epic" in creating an issuable, a 404 would
return, and the user would lose all the information entered.  This
happened because when the button were selected, the frontend would make
the hidden element active and send the backend a "0" value. To avoid
this, we now ignore zero values.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/226999

ee/app/services/ee/issues/base_service.rb

@@ -26,7 +26,7 @@ module EE
 
       def find_epic(issue)
         id = params.delete(:epic_id)
-        return unless id.present?
+        return if id.to_i.zero?
 
         group = issue.project.group
         return unless group.present?

ee/changelogs/unreleased/sh-fix-issue-404-epics.yml

+---
+title: Fix 404 errors when "No epic" selected
+merge_request: 36137
+author:
+type: fixed

ee/spec/features/projects/issues/user_creates_issue_spec.rb

@@ -3,16 +3,20 @@
 require "spec_helper"
 
 RSpec.describe "User creates issue", :js do
-  let(:user) { create(:user) }
-  let(:group) { create(:group, :public) }
-  let(:project) { create(:project_empty_repo, :public, namespace: group) }
-  let!(:epic) { create(:epic, group: group, title: 'Sample epic', author: user) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:group) { create(:group, :public) }
+  let_it_be(:project) { create(:project_empty_repo, :public, namespace: group) }
+  let_it_be(:epic) { create(:epic, group: group, title: 'Sample epic', author: user) }
+  let(:issue_title) { '500 error on profile' }
+
+  before_all do
+    group.add_developer(user)
+  end
 
   before do
     stub_licensed_features(issue_weights: true)
     stub_licensed_features(epics: true)
 
-    group.add_developer(user)
     sign_in(user)
 
     visit(new_project_issue_path(project))
@@ -20,7 +24,6 @@ RSpec.describe "User creates issue", :js do
 
   context "with weight set" do
     it "creates issue" do
-      issue_title = "500 error on profile"
       weight = "7"
 
       fill_in("Title", with: issue_title)
@@ -36,19 +39,30 @@ RSpec.describe "User creates issue", :js do
     end
   end
 
-  context "with epic set" do
-    it "creates issue" do
-      issue_title = "500 error on profile"
-
+  context 'with epics' do
+    before do
       fill_in("Title", with: issue_title)
-      page.within('.issue-epic .js-epic-block') do
-        page.find('.js-epic-select').click
-        wait_for_requests
+      scroll_to(page.find('.epic-dropdown-container', visible: false))
+    end
+
+    it 'creates an issue with no epic' do
+      click_button 'Select epic'
+      click_link('No Epic')
+      click_button('Submit issue')
 
-        page.find('.dropdown-content .gl-link', text: epic.title).click
+      wait_for_all_requests
+
+      page.within(".js-epic-block .js-epic-label") do
+        expect(page).to have_content('None')
       end
 
-      click_button("Submit issue")
+      expect(page).to have_content(issue_title)
+    end
+
+    it 'credates an issue with an epic' do
+      click_button 'Select epic'
+      click_link(epic.title)
+      click_button('Submit issue')
 
       wait_for_all_requests
 

ee/spec/support/shared_examples/services/issue_epic_shared_examples.rb

@@ -13,6 +13,17 @@ RSpec.shared_examples 'issue with epic_id parameter' do
     end
   end
 
+  context 'when epic_id is 0' do
+    let(:params) { { title: 'issue1', epic_id: 0 } }
+
+    it 'ignores epic_id' do
+      issue = execute
+
+      expect(issue).to be_persisted
+      expect(issue.epic).to be_nil
+    end
+  end
+
   context 'when user can not add issues to the epic' do
     before do
       project.add_maintainer(user)