Only create a protected branch upon a push to a new branch if a rule for that branch doesn't exist

A customer ran into an issue where a Sidekiq task retried over and over, leading to duplicate
master branches in their protected branch list.

Closes #22177

CHANGELOG

@@ -6,6 +6,7 @@ v 8.12.0 (unreleased)
   - Bump fog-aws to v0.11.0 to support ap-south-1 region
   - Add ability to fork to a specific namespace using API. (ritave)
   - Cleanup misalignments in Issue list view !6206
+  - Only create a protected branch upon a push to a new branch if a rule for that branch doesn't exist
   - Prune events older than 12 months. (ritave)
   - Prepend blank line to `Closes` message on merge request linked to issue (lukehowell)
   - Fix issues/merge-request templates dropdown for forked projects

app/services/git_push_service.rb

@@ -87,7 +87,7 @@ class GitPushService < BaseService
     project.change_head(branch_name)
 
     # Set protection on the default branch if configured
-    if current_application_settings.default_branch_protection != PROTECTION_NONE
+    if current_application_settings.default_branch_protection != PROTECTION_NONE && !@project.protected_branch?(@project.default_branch)
 
       params = {
         name: @project.default_branch,

spec/services/git_push_service_spec.rb

@@ -253,6 +253,21 @@ describe GitPushService, services: true do
         expect(project.protected_branches.last.merge_access_levels.map(&:access_level)).to eq([Gitlab::Access::MASTER])
       end
 
+      it "when pushing a branch for the first time with an existing branch permission configured" do
+        stub_application_setting(default_branch_protection: Gitlab::Access::PROTECTION_DEV_CAN_PUSH)
+
+        create(:protected_branch, :no_one_can_push, :developers_can_merge, project: project, name: 'master')
+        expect(project).to receive(:execute_hooks)
+        expect(project.default_branch).to eq("master")
+        expect_any_instance_of(ProtectedBranches::CreateService).not_to receive(:execute)
+
+        execute_service(project, user, @blankrev, 'newrev', 'refs/heads/master' )
+
+        expect(project.protected_branches).not_to be_empty
+        expect(project.protected_branches.last.push_access_levels.map(&:access_level)).to eq([Gitlab::Access::NO_ACCESS])
+        expect(project.protected_branches.last.merge_access_levels.map(&:access_level)).to eq([Gitlab::Access::DEVELOPER])
+      end
+
       it "when pushing a branch for the first time with default branch protection set to 'developers can merge'" do
         stub_application_setting(default_branch_protection: Gitlab::Access::PROTECTION_DEV_CAN_MERGE)