Support rm src branch on merge w/ push option

MergeRequests::PushOptionsHandlerService has been updated to allow
creating and updating merge requests with the
`remove_source_branch` set using git push options.

To create a new merge request and set it to remove the source branch
when it is merged:

  git push -u origin -o merge_request.create \
    -o merge_request.remove_source_branch

To update an existing merge request and set it to remove the source
branch when it is merged:

  git push -u origin -o merge_request.remove_source_branch

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/64320

app/services/merge_requests/push_options_handler_service.rb

@@ -133,6 +133,10 @@ module MergeRequests
         )
       end
 
+      if push_options.key?(:remove_source_branch)
+        params[:force_remove_source_branch] = push_options[:remove_source_branch]
+      end
+
       params
     end
 
@@ -146,6 +150,10 @@ module MergeRequests
         )
       end
 
+      if push_options.key?(:remove_source_branch)
+        params[:force_remove_source_branch] = push_options[:remove_source_branch]
+      end
+
       if push_options.key?(:target)
         params[:target_branch] = push_options[:target]
       end

changelogs/unreleased/rm-src-branch.yml

+---
+title: Support remove source branch on merge w/ push options
+merge_request: 30728
+author:
+type: added

doc/user/project/merge_requests/index.md

@@ -285,6 +285,7 @@ as pushing changes:
 - Create a new merge request for the pushed branch.
 - Set the target of the merge request to a particular branch.
 - Set the merge request to merge when its pipeline succeeds.
+- Set the merge request to remove the source branch when it's merged.
 
 ### Create a new merge request using git push options
 
@@ -328,6 +329,19 @@ pipeline succeeds at the same time using a `-o` flag per push option:
 git push -o merge_request.create -o merge_request.merge_when_pipeline_succeeds
 ```
 
+### Set removing the source branch using git push options
+
+To set an existing merge request to remove the source branch when the
+merge request is merged, the
+`merge_request.remove_source_branch` push option can be used:
+
+```sh
+git push -o merge_request.remove_source_branch
+```
+
+You can also use this push option in addition to the
+`merge_request.create` push option.
+
 ## Find the merge request that introduced a change
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/2383) in GitLab 10.5.

lib/gitlab/push_options.rb

@@ -4,7 +4,12 @@ module Gitlab
   class PushOptions
     VALID_OPTIONS = HashWithIndifferentAccess.new({
       merge_request: {
-        keys: [:create, :merge_when_pipeline_succeeds, :target]
+        keys: [
+          :create,
+          :merge_when_pipeline_succeeds,
+          :remove_source_branch,
+          :target
+        ]
       },
       ci: {
         keys: [:skip]

spec/services/merge_requests/push_options_handler_service_spec.rb

@@ -90,6 +90,16 @@ describe MergeRequests::PushOptionsHandlerService do
     end
   end
 
+  shared_examples_for 'a service that can remove the source branch when it is merged' do
+    subject(:last_mr) { MergeRequest.last }
+
+    it 'returns true to force_remove_source_branch?' do
+      service.execute
+
+      expect(last_mr.force_remove_source_branch?).to eq(true)
+    end
+  end
+
   shared_examples_for 'a service that does not create a merge request' do
     it do
       expect { service.execute }.not_to change { MergeRequest.count }
@@ -208,6 +218,72 @@ describe MergeRequests::PushOptionsHandlerService do
     end
   end
 
+  describe '`remove_source_branch` push option' do
+    let(:push_options) { { remove_source_branch: true } }
+
+    context 'with a new branch' do
+      let(:changes) { new_branch_changes }
+
+      it_behaves_like 'a service that does not create a merge request'
+
+      it 'adds an error to the service' do
+        error = "A merge_request.create push option is required to create a merge request for branch #{source_branch}"
+
+        service.execute
+
+        expect(service.errors).to include(error)
+      end
+
+      context 'when coupled with the `create` push option' do
+        let(:push_options) { { create: true, remove_source_branch: true } }
+
+        it_behaves_like 'a service that can create a merge request'
+        it_behaves_like 'a service that can remove the source branch when it is merged'
+      end
+    end
+
+    context 'with an existing branch but no open MR' do
+      let(:changes) { existing_branch_changes }
+
+      it_behaves_like 'a service that does not create a merge request'
+
+      it 'adds an error to the service' do
+        error = "A merge_request.create push option is required to create a merge request for branch #{source_branch}"
+
+        service.execute
+
+        expect(service.errors).to include(error)
+      end
+
+      context 'when coupled with the `create` push option' do
+        let(:push_options) { { create: true, remove_source_branch: true } }
+
+        it_behaves_like 'a service that can create a merge request'
+        it_behaves_like 'a service that can remove the source branch when it is merged'
+      end
+    end
+
+    context 'with an existing branch that has a merge request open' do
+      let(:changes) { existing_branch_changes }
+      let!(:merge_request) { create(:merge_request, source_project: project, source_branch: source_branch)}
+
+      it_behaves_like 'a service that does not create a merge request'
+      it_behaves_like 'a service that can remove the source branch when it is merged'
+    end
+
+    context 'with a deleted branch' do
+      let(:changes) { deleted_branch_changes }
+
+      it_behaves_like 'a service that does nothing'
+    end
+
+    context 'with the project default branch' do
+      let(:changes) { default_branch_changes }
+
+      it_behaves_like 'a service that does nothing'
+    end
+  end
+
   describe '`target` push option' do
     let(:push_options) { { target: target_branch } }