Update `CI_OPEN_MERGE_REQUESTS` env

This changes the `CI_HAS_OPEN_MERGE_REQUEST`
to `CI_OPEN_MERGE_REQUESTS`.

app/models/ci/pipeline.rb

@@ -7,6 +7,7 @@ module Ci
     include Importable
     include AfterCommitQueue
     include Presentable
+    include Gitlab::Allowable
     include Gitlab::OptimisticLocking
     include Gitlab::Utils::StrongMemoize
     include AtomicInternalId
@@ -16,6 +17,8 @@ module Ci
     include FromUnion
     include UpdatedAtFilterable
 
+    MAX_OPEN_MERGE_REQUESTS_REFS = 4
+
     PROJECT_ROUTE_AND_NAMESPACE_ROUTE = {
       project: [:project_feature, :route, { namespace: :route }]
     }.freeze
@@ -801,6 +804,10 @@ module Ci
           variables.concat(merge_request.predefined_variables)
         end
 
+        if Gitlab::Ci::Features.pipeline_open_merge_requests?(project) && open_merge_requests_refs.any?
+          variables.append(key: 'CI_OPEN_MERGE_REQUESTS', value: open_merge_requests_refs.join(','))
+        end
+
         variables.append(key: 'CI_KUBERNETES_ACTIVE', value: 'true') if has_kubernetes_active?
         variables.append(key: 'CI_DEPLOY_FREEZE', value: 'true') if freeze_period?
 
@@ -867,6 +874,36 @@ module Ci
       all_merge_requests.order(id: :desc)
     end
 
+    # This returns a list of MRs that point
+    # to the same source project/branch
+    def related_merge_requests
+      if merge_request?
+        # We look for all other MRs that this branch might be pointing to
+        MergeRequest.where(
+          source_project_id: merge_request.source_project_id,
+          source_branch: merge_request.source_branch)
+      else
+        MergeRequest.where(
+          source_project_id: project_id,
+          source_branch: ref)
+      end
+    end
+
+    # We cannot use `all_merge_requests`, due to race condition
+    # This returns a list of at most 4 open MRs
+    def open_merge_requests_refs
+      strong_memoize(:open_merge_requests_refs) do
+        # We ensure that triggering user can actually read the pipeline
+        related_merge_requests
+          .opened
+          .limit(MAX_OPEN_MERGE_REQUESTS_REFS)
+          .order(id: :desc)
+          .preload(:target_project)
+          .select { |mr| can?(user, :read_merge_request, mr) }
+          .map { |mr| mr.to_reference(project, full: true) }
+      end
+    end
+
     def same_family_pipeline_ids
       ::Gitlab::Ci::PipelineObjectHierarchy.new(
         self.class.where(id: root_ancestor), options: { same_project: true }

changelogs/unreleased/38673-CI_HAS_OPEN_MERGE_REQUEST.yml

+---
+title: Add CI_OPEN_MERGE_REQUESTS environment variable
+merge_request: 38673
+author: Ben Bodenmiller @bbodenmiller
+type: added

config/feature_flags/development/ci_pipeline_open_merge_requests.yml

+---
+name: ci_pipeline_open_merge_requests
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38673
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/292727
+group: group::memory
+type: development
+default_enabled: false

doc/ci/variables/predefined_variables.md

@@ -68,6 +68,7 @@ Kubernetes-specific environment variables are detailed in the
 | `CI_EXTERNAL_PULL_REQUEST_TARGET_BRANCH_NAME` | 12.3   | all    | The target branch name of the pull request if [the pipelines are for external pull requests](../ci_cd_for_external_repos/index.md#pipelines-for-external-pull-requests). Available only if `only: [external_pull_requests]` or [`rules`](../yaml/README.md#rules) syntax is used and the pull request is open.                                                                                          |
 | `CI_EXTERNAL_PULL_REQUEST_TARGET_BRANCH_SHA`  | 12.3   | all    | The HEAD SHA of the target branch of the pull request if [the pipelines are for external pull requests](../ci_cd_for_external_repos/index.md#pipelines-for-external-pull-requests). Available only if `only: [external_pull_requests]` or [`rules`](../yaml/README.md#rules) syntax is used and the pull request is open.                                                                               |
 | `CI_HAS_OPEN_REQUIREMENTS`                    | 13.1   | all    | Included with the value `true` only if the pipeline's project has any open [requirements](../../user/project/requirements/index.md). Not included if there are no open requirements for the pipeline's project.                                                                                                                                                                                        |
+| `CI_OPEN_MERGE_REQUESTS`                    | 13.7   | all    | Contains a comma-delimited list of up to 4 Merge Requests from the current source project and branch in the form `gitlab-org/gitlab!333,gitlab-org/gitlab-foss!11`                                                                                                                                                                                 |
 | `CI_JOB_ID`                                   | 9.0    | all    | The unique ID of the current job that GitLab CI/CD uses internally                                                                                                                                                                                                                                                                                            |
 | `CI_JOB_IMAGE`                                | 12.9   | 12.9   | The name of the image running the CI job                                                                                                                                                                                                                                                                                                                   |
 | `CI_JOB_MANUAL`                               | 8.12   | all    | The flag to indicate that job was manually started                                                                                                                                                                                                                                                                                                         |

lib/gitlab/ci/features.rb

@@ -55,6 +55,10 @@ module Gitlab
         ::Feature.enabled?(:ci_trace_log_invalid_chunks, project, type: :ops, default_enabled: false)
       end
 
+      def self.pipeline_open_merge_requests?(project)
+        ::Feature.enabled?(:ci_pipeline_open_merge_requests, project, default_enabled: false)
+      end
+
       def self.seed_block_run_before_workflow_rules_enabled?(project)
         ::Feature.enabled?(:ci_seed_block_run_before_workflow_rules, project, default_enabled: true)
       end

spec/services/ci/create_pipeline_service/merge_requests_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Ci::CreatePipelineService do
+  context 'merge requests handling' do
+    let_it_be(:project)  { create(:project, :repository) }
+    let_it_be(:user)     { project.owner }
+
+    let(:ref)      { 'refs/heads/feature' }
+    let(:source)   { :push }
+    let(:service)  { described_class.new(project, user, { ref: ref }) }
+    let(:pipeline) { service.execute(source) }
+
+    before do
+      stub_ci_pipeline_yaml_file <<-EOS
+        workflow:
+          rules:
+            # do not create pipelines if merge requests are opened
+            - if: $CI_OPEN_MERGE_REQUESTS
+              when: never
+
+            - if: $CI_COMMIT_BRANCH
+
+        rspec:
+          script: echo Hello World
+      EOS
+    end
+
+    context 'when pushing a change' do
+      context 'when a merge request already exists' do
+        let!(:merge_request) do
+          create(:merge_request,
+                 source_project: project,
+                 source_branch: 'feature',
+                 target_project: project,
+                 target_branch: 'master')
+        end
+
+        it 'does not create a pipeline' do
+          expect(pipeline).not_to be_persisted
+        end
+      end
+
+      context 'when no merge request exists' do
+        it 'does create a pipeline' do
+          expect(pipeline.errors).to be_empty
+          expect(pipeline).to be_persisted
+        end
+      end
+    end
+  end
+end