Disallow developers to delete builds of protected branches.

app/policies/ci/build_policy.rb

@@ -12,6 +12,14 @@ module Ci
       end
     end
 
+    condition(:unprotected_ref) do
+      if @subject.tag?
+        !ProtectedTag.protected?(@subject.project, @subject.ref)
+      else
+        !ProtectedBranch.protected?(@subject.project, @subject.ref)
+      end
+    end
+
     condition(:owner_of_job) do
       @subject.triggered_by?(@user)
     end
@@ -34,7 +42,7 @@ module Ci
       prevent :erase_build
     end
 
-    rule { can?(:admin_build) | (can?(:update_build) & owner_of_job) }.enable :erase_build
+    rule { can?(:admin_build) | (can?(:update_build) & owner_of_job & unprotected_ref) }.enable :erase_build
 
     rule { can?(:public_access) & branch_allows_collaboration }.policy do
       enable :update_build

changelogs/unreleased/35069-protect-builds.yml

+---
+title: Disallow developers to delete builds of protected branches
+merge_request: 28881
+author: Alexander Kutelev
+type: changed

doc/user/permissions.md

@@ -379,7 +379,9 @@ instance and project. In addition, all admins can use the admin interface under
 | See events in the system              |                 |             |          | ✓      |
 | Admin interface                       |                 |             |          | ✓      |
 
-1. Only if the job was triggered by the user
+1. Only if the job was:
+   - Triggered by the user
+   - [Since GitLab 13.0](https://gitlab.com/gitlab-org/gitlab/-/issues/35069), not run for a protected branch
 
 ### Job permissions
 

spec/policies/ci/build_policy_spec.rb

@@ -176,16 +176,22 @@ describe Ci::BuildPolicy do
         end
 
         context 'when developers can push to the branch' do
+          context 'when the build was created by the developer' do
+            let(:owner) { user }
+
+            context 'when the build was created for a protected ref' do
               before do
                 create(:protected_branch, :developers_can_push,
                        name: build.ref, project: project)
               end
 
-          context 'when the build was created by the developer' do
-            let(:owner) { user }
+              it { expect(policy).to be_disallowed :erase_build }
+            end
 
+            context 'when the build was created for an unprotected ref' do
               it { expect(policy).to be_allowed :erase_build }
             end
+          end
 
           context 'when the build was created by the other' do
             let(:owner) { create(:user) }