Fix when branch creation fails don't post system note

Closes #24347

app/controllers/projects/branches_controller.rb

@@ -46,7 +46,7 @@ class Projects::BranchesController < Projects::ApplicationController
     result = CreateBranchService.new(project, current_user)
         .execute(branch_name, ref)
 
-    if params[:issue_iid]
+    if params[:issue_iid] && result[:status] == :success
       issue = IssuesFinder.new(current_user, project_id: @project.id).find_by(iid: params[:issue_iid])
       SystemNoteService.new_issue_branch(issue, @project, current_user, branch_name) if issue
     end

changelogs/unreleased/24347-dont-post-system-note-when-branch-creation-fails.yml

+---
+title: Fix when branch creation fails don't post system note
+merge_request:
+author: Mateusz Bajorski
+type: fixed

spec/controllers/projects/branches_controller_spec.rb

@@ -148,6 +148,20 @@ describe Projects::BranchesController do
         end
       end
 
+      context 'when create branch service fails' do
+        let(:branch) { "./invalid-branch-name" }
+
+        it "doesn't post a system note" do
+          expect(SystemNoteService).not_to receive(:new_issue_branch)
+
+          post :create,
+            namespace_id: project.namespace,
+            project_id: project,
+            branch_name: branch,
+            issue_iid: issue.iid
+        end
+      end
+
       context 'without issue feature access' do
         before do
           project.update!(visibility_level: Gitlab::VisibilityLevel::PUBLIC)