Automatically retarget merge requests

This retargets another MR once a one
that it was targeting was merged.

app/services/merge_requests/post_merge_service.rb

@@ -9,6 +9,8 @@ module MergeRequests
   class PostMergeService < MergeRequests::BaseService
     include RemovesRefs
 
+    MAX_RETARGET_MERGE_REQUESTS = 4
+
     def execute(merge_request)
       merge_request.mark_as_merged
       close_issues(merge_request)
@@ -18,6 +20,7 @@ module MergeRequests
       merge_request_activity_counter.track_merge_mr_action(user: current_user)
       notification_service.merge_mr(merge_request, current_user)
       execute_hooks(merge_request, 'merge')
+      retarget_chain_merge_requests(merge_request)
       invalidate_cache_counts(merge_request, users: merge_request.assignees | merge_request.reviewers)
       merge_request.update_project_counter_caches
       delete_non_latest_diffs(merge_request)
@@ -28,6 +31,31 @@ module MergeRequests
 
     private
 
+    def retarget_chain_merge_requests(merge_request)
+      return unless Feature.enabled?(:retarget_merge_requests, merge_request.target_project)
+
+      # we can only retarget MRs that are targeting the same project
+      return unless merge_request.for_same_project?
+
+      # find another merge requests that
+      # - as a target have a current source project and branch
+      other_merge_requests = merge_request.source_project
+        .merge_requests
+        .opened
+        .by_target_branch(merge_request.source_branch)
+        .preload_source_project
+        .at_most(MAX_RETARGET_MERGE_REQUESTS)
+
+      other_merge_requests.find_each do |other_merge_request|
+        # Update only MRs on projects that we have access to
+        next unless can?(current_user, :update_merge_request, other_merge_request.source_project)
+
+        ::MergeRequests::UpdateService
+          .new(other_merge_request.source_project, current_user, target_branch: merge_request.target_branch)
+          .execute(other_merge_request)
+      end
+    end
+
     def close_issues(merge_request)
       return unless merge_request.target_branch == project.default_branch
 

changelogs/unreleased/retarget-branch.yml

+---
+title: Automatically retarget merge requests
+merge_request: 53710
+author:
+type: added

config/feature_flags/development/retarget_merge_requests.yml

+---
+name: retarget_merge_requests
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/53710
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/320895
+milestone: '13.9'
+type: development
+group: group::memory
+default_enabled: false

doc/administration/instance_limits.md

@@ -626,3 +626,12 @@ Set the limit to `0` to allow any file size.
 ### Package versions returned
 
 When asking for versions of a given NuGet package name, the GitLab Package Registry returns a maximum of 300 versions.
+
+## Automatically retargeting merge requests
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/320902) in GitLab 13.9.
+
+Maximum of 4 merge requests point to the currently being merged
+will be automatically retargeted upon merge.
+
+More information can be found in the [Automatically retargeting merge requests](../user/project/merge_requests/getting_started.md#automatically-retargeting-merge-requests).

doc/user/project/merge_requests/getting_started.md

@@ -194,6 +194,33 @@ is set for deletion, the merge request widget displays the
 
 ![Delete source branch status](img/remove_source_branch_status.png)
 
+### Automatically retargeting merge requests
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/320902) in GitLab 13.9.
+> - It's [deployed behind a feature flag](../../feature_flags.md), disabled by default.
+> - It's disabled on GitLab.com.
+> - It's not recommended for production use.
+> - To use it in GitLab self-managed instances, ask a GitLab administrator to [enable it](#enable-or-disable-automatically-retargeting-merge-requests). **(FREE SELF)**
+
+It is common to have a number of merge requests in a chain,
+when one depends on another. For example:
+
+- Merge Request A: merge `feature-A` into `master`
+- Merge Request B: merge `feature-B` into `feature-A`
+
+We can distinguish two workflows:
+
+- the MR A is merged into `master` first, and then MR B is retargeted onto `master`
+- the MR B is merged into `feature-A` branch, and then MR A is merged into `master`
+
+Upon merge of `MR A` does automatically retarget the `MR B` onto `master`.
+This relieves user from having to perform this operation manually.
+
+This feature works only in following cases:
+
+- The MR A needs to be in a main project (forks are not supported), as we, GitLab, cannot change the target project of MR B
+- Only 4 MR's of type B are retargeted automatically this way
+
 ## Recommendations and best practices for Merge Requests
 
 - When working locally in your branch, add multiple commits and only push when
@@ -230,3 +257,22 @@ Feature.disable(:reviewer_approval_rules)
 # For a single project
 Feature.disable(:reviewer_approval_rules, Project.find(<project id>))
 ```
+
+### Enable or disable Automatically retargeting merge requests **(FREE SELF)**
+
+Automatically retargeting merge requests is under development but ready for production use.
+It is deployed behind a feature flag that is **enabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can opt to disable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:retarget_merge_requests)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:retarget_merge_requests)
+```

spec/services/merge_requests/post_merge_service_spec.rb

@@ -3,9 +3,11 @@
 require 'spec_helper'
 
 RSpec.describe MergeRequests::PostMergeService do
-  let(:user) { create(:user) }
-  let(:merge_request) { create(:merge_request, assignees: [user]) }
-  let(:project) { merge_request.project }
+  include ProjectForksHelper
+
+  let_it_be(:user) { create(:user) }
+  let_it_be(:merge_request, reload: true) { create(:merge_request, assignees: [user]) }
+  let_it_be(:project) { merge_request.project }
 
   subject { described_class.new(project, user).execute(merge_request) }
 
@@ -128,5 +130,125 @@ RSpec.describe MergeRequests::PostMergeService do
         expect(deploy_job.reload.canceled?).to be false
       end
     end
+
+    context 'for a merge request chain' do
+      context 'when there is another MR' do
+        let!(:another_merge_request) do
+          create(:merge_request,
+            source_project: source_project,
+            source_branch: 'my-awesome-feature',
+            target_project: merge_request.source_project,
+            target_branch: merge_request.source_branch
+          )
+        end
+
+        shared_examples 'retargets merge request' do
+          it 'another merge request is retargeted' do
+            expect(SystemNoteService)
+              .to receive(:change_branch).once
+              .with(another_merge_request, another_merge_request.project, user, 'target',
+                merge_request.source_branch, merge_request.target_branch)
+
+            expect { subject }.to change { another_merge_request.reload.target_branch }
+              .from(merge_request.source_branch)
+              .to(merge_request.target_branch)
+          end
+
+          context 'when FF retarget_merge_requests is disabled' do
+            before do
+              stub_feature_flags(retarget_merge_requests: false)
+            end
+
+            it 'another merge request is unchanged' do
+              expect { subject }.not_to change { another_merge_request.reload.target_branch }
+                .from(merge_request.source_branch)
+            end
+          end
+        end
+
+        shared_examples 'does not retarget merge request' do
+          it 'another merge request is unchanged' do
+            expect { subject }.not_to change { another_merge_request.reload.target_branch }
+              .from(merge_request.source_branch)
+          end
+        end
+
+        context 'in the same project' do
+          let(:source_project) { project }
+
+          it_behaves_like 'retargets merge request'
+
+          context 'and is closed' do
+            before do
+              another_merge_request.close
+            end
+
+            it_behaves_like 'does not retarget merge request'
+          end
+
+          context 'and is merged' do
+            before do
+              another_merge_request.mark_as_merged
+            end
+
+            it_behaves_like 'does not retarget merge request'
+          end
+        end
+
+        context 'in forked project' do
+          let!(:source_project) { fork_project(project) }
+
+          context 'when user has access to source project' do
+            before do
+              source_project.add_developer(user)
+            end
+
+            it_behaves_like 'retargets merge request'
+          end
+
+          context 'when user does not have access to source project' do
+            it_behaves_like 'does not retarget merge request'
+          end
+        end
+
+        context 'and current and another MR is from a fork' do
+          let(:project) { create(:project) }
+          let(:source_project) { fork_project(project) }
+
+          let(:merge_request) do
+            create(:merge_request,
+              source_project: source_project,
+              target_project: project
+            )
+          end
+
+          before do
+            source_project.add_developer(user)
+          end
+
+          it_behaves_like 'does not retarget merge request'
+        end
+      end
+
+      context 'when many merge requests are to be retargeted' do
+        let!(:many_merge_requests) do
+          create_list(:merge_request, 10, :unique_branches,
+            source_project: merge_request.source_project,
+            target_project: merge_request.source_project,
+            target_branch: merge_request.source_branch
+          )
+        end
+
+        it 'retargets only 4 of them' do
+          subject
+
+          expect(many_merge_requests.each(&:reload).pluck(:target_branch).tally)
+            .to eq(
+              merge_request.source_branch => 6,
+              merge_request.target_branch => 4
+            )
+        end
+      end
+    end
   end
 end

spec/services/merge_requests/refresh_service_spec.rb

@@ -633,31 +633,37 @@ RSpec.describe MergeRequests::RefreshService do
     end
 
     context 'merge request metrics' do
-      let(:issue) { create :issue, project: @project }
-      let(:commit_author) { create :user }
+      let(:user) { create(:user) }
+      let(:project) { create(:project, :repository) }
+      let(:issue) { create(:issue, project: project) }
       let(:commit) { project.commit }
 
       before do
-        project.add_developer(commit_author)
         project.add_developer(user)
 
         allow(commit).to receive_messages(
           safe_message: "Closes #{issue.to_reference}",
           references: [issue],
-          author_name: commit_author.name,
-          author_email: commit_author.email,
+          author_name: user.name,
+          author_email: user.email,
           committed_date: Time.current
         )
-
-        allow_any_instance_of(MergeRequest).to receive(:commits).and_return(CommitCollection.new(@project, [commit], 'feature'))
       end
 
       context 'when the merge request is sourced from the same project' do
         it 'creates a `MergeRequestsClosingIssues` record for each issue closed by a commit' do
-          merge_request = create(:merge_request, target_branch: 'master', source_branch: 'feature', source_project: @project)
-          refresh_service = service.new(@project, @user)
+          allow_any_instance_of(MergeRequest).to receive(:commits).and_return(
+            CommitCollection.new(project, [commit], 'close-by-commit')
+          )
+
+          merge_request = create(:merge_request,
+                                 target_branch: 'master',
+                                 source_branch: 'close-by-commit',
+                                 source_project: project)
+
+          refresh_service = service.new(project, user)
           allow(refresh_service).to receive(:execute_hooks)
-          refresh_service.execute(@oldrev, @newrev, 'refs/heads/feature')
+          refresh_service.execute(@oldrev, @newrev, 'refs/heads/close-by-commit')
 
           issue_ids = MergeRequestsClosingIssues.where(merge_request: merge_request).pluck(:issue_id)
           expect(issue_ids).to eq([issue.id])
@@ -666,16 +672,21 @@ RSpec.describe MergeRequests::RefreshService do
 
       context 'when the merge request is sourced from a different project' do
         it 'creates a `MergeRequestsClosingIssues` record for each issue closed by a commit' do
-          forked_project = fork_project(@project, @user, repository: true)
+          forked_project = fork_project(project, user, repository: true)
+
+          allow_any_instance_of(MergeRequest).to receive(:commits).and_return(
+            CommitCollection.new(forked_project, [commit], 'close-by-commit')
+          )
 
           merge_request = create(:merge_request,
                                  target_branch: 'master',
-                                 source_branch: 'feature',
-                                 target_project: @project,
+                                 target_project: project,
+                                 source_branch: 'close-by-commit',
                                  source_project: forked_project)
-          refresh_service = service.new(@project, @user)
+
+          refresh_service = service.new(forked_project, user)
           allow(refresh_service).to receive(:execute_hooks)
-          refresh_service.execute(@oldrev, @newrev, 'refs/heads/feature')
+          refresh_service.execute(@oldrev, @newrev, 'refs/heads/close-by-commit')
 
           issue_ids = MergeRequestsClosingIssues.where(merge_request: merge_request).pluck(:issue_id)
           expect(issue_ids).to eq([issue.id])