Support merge on pipeline success w/ push options

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

To create a new merge request and set it to merge when the pipeline
succeeds:

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

To update an existing merge request and set it to merge when the
pipeline succeeds:

  git push -u origin -o merge_request.merge_when_pipeline_succeeds

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/53198

5 files changed, 149 insertions, 13 deletions

diff --git a/app/services/merge_requests/push_options_handler_service.rb b/app/services/merge_requests/push_options_handler_service.rb
index c810340c636..73b4869cc5d 100644
--- a/app/services/merge_requests/push_options_handler_service.rb
+++ b/app/services/merge_requests/push_options_handler_service.rb
@@ -7,7 +7,7 @@ module MergeRequests
     LIMIT = 10
 
     attr_reader :branches, :changes_by_branch, :current_user, :errors,
-                :project, :push_options, :target
+                :project, :push_options
 
     def initialize(project, current_user, changes, push_options)
       @project = project
@@ -16,7 +16,6 @@ module MergeRequests
       @push_options = push_options
       @errors = []
       @branches = @changes_by_branch.keys
-      @target = @push_options[:target] || @project.default_branch
 
       raise Error, 'User is required' if @current_user.nil?
 
@@ -28,8 +27,8 @@ module MergeRequests
         raise Error, "Too many branches pushed (#{@branches.size} were pushed, limit is #{LIMIT})"
       end
 
-      if @push_options[:target] && !@project.repository.branch_exists?(@target)
-        raise Error, "Branch #{@target} does not exist"
+      if @push_options[:target] && !@project.repository.branch_exists?(@push_options[:target])
+        raise Error, "Branch #{@push_options[:target]} does not exist"
       end
     end
 
@@ -93,12 +92,10 @@ module MergeRequests
     end
 
     def update!(merge_request)
-      return if target == merge_request.target_branch
-
       merge_request = ::MergeRequests::UpdateService.new(
         project,
         current_user,
-        { target_branch: target }
+        update_params
       ).execute(merge_request)
 
       collect_errors_from_merge_request(merge_request) unless merge_request.valid?
@@ -111,13 +108,39 @@ module MergeRequests
       commits = CommitCollection.new(project, commits)
       commit = commits.without_merge_commits.first
 
-      {
+      params = {
         assignee: current_user,
         source_branch: branch,
-        target_branch: target,
+        target_branch: push_options[:target] || project.default_branch,
         title: commit&.title&.strip || 'New Merge Request',
         description: commit&.description&.strip
       }
+
+      if push_options.key?(:merge_when_pipeline_succeeds)
+        params.merge!(
+          merge_when_pipeline_succeeds: push_options[:merge_when_pipeline_succeeds],
+          merge_user: current_user
+        )
+      end
+
+      params
+    end
+
+    def update_params
+      params = {}
+
+      if push_options.key?(:merge_when_pipeline_succeeds)
+        params.merge!(
+          merge_when_pipeline_succeeds: push_options[:merge_when_pipeline_succeeds],
+          merge_user: current_user
+        )
+      end
+
+      if push_options.key?(:target)
+        params[:target_branch] = push_options[:target]
+      end
+
+      params
     end
 
     def collect_errors_from_merge_request(merge_request)
diff --git a/changelogs/unreleased/53198-git-push-option-merge-when-pipeline-succeeds.yml b/changelogs/unreleased/53198-git-push-option-merge-when-pipeline-succeeds.yml
new file mode 100644
index 00000000000..6fefd05049c
--- /dev/null
+++ b/changelogs/unreleased/53198-git-push-option-merge-when-pipeline-succeeds.yml
@@ -0,0 +1,6 @@
+---
+title: Allow merge requests to be set to merge when pipeline succeeds via git push
+  options
+merge_request: 26842
+author:
+type: added
diff --git a/doc/user/project/merge_requests/index.md b/doc/user/project/merge_requests/index.md
index 678fc3dd196..ba7d05a7ad7 100644
--- a/doc/user/project/merge_requests/index.md
+++ b/doc/user/project/merge_requests/index.md
@@ -223,9 +223,17 @@ branch already exists, the patches will be applied on top of it.
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26752) in GitLab 11.10.
 
-GitLab supports using [Git push options](https://git-scm.com/docs/git-push#Documentation/git-push.txt--oltoptiongt) to create merge requests and set the target
-branch during a push. Note that git push options are only available with
-Git 2.10 or newer.
+NOTE: **Note:**
+Git push options are only available with Git 2.10 or newer.
+
+GitLab supports using
+[Git push options](https://git-scm.com/docs/git-push#Documentation/git-push.txt--oltoptiongt)
+to perform the following actions against merge requests at the same time
+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.
 
 ### Create a new merge request using git push options
 
@@ -252,6 +260,23 @@ same time using a `-o` flag per push option:
 git push -o merge_request.create -o merge_request.target=branch_name
 ```
 
+### Set merge when pipeline succeeds using git push options
+
+To set an existing merge request to
+[merge when its pipeline succeeds](merge_when_pipeline_succeeds.md), use
+the `merge_request.merge_when_pipeline_succeeds` push option:
+
+```sh
+git push -o merge_request.merge_when_pipeline_succeeds
+```
+
+You can also create a merge request and set it to merge when its
+pipeline succeeds at the same time using a `-o` flag per push option:
+
+```sh
+git push -o merge_request.create -o merge_request.merge_when_pipeline_succeeds
+```
+
 ## Find the merge request that introduced a change
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/2383) in GitLab 10.5.
diff --git a/lib/gitlab/push_options.rb b/lib/gitlab/push_options.rb
index 7c655c12995..810aba436cc 100644
--- a/lib/gitlab/push_options.rb
+++ b/lib/gitlab/push_options.rb
@@ -4,7 +4,7 @@ module Gitlab
   class PushOptions
     VALID_OPTIONS = HashWithIndifferentAccess.new({
       merge_request: {
-        keys: [:create, :target]
+        keys: [:create, :merge_when_pipeline_succeeds, :target]
       },
       ci: {
         keys: [:skip]
diff --git a/spec/services/merge_requests/push_options_handler_service_spec.rb b/spec/services/merge_requests/push_options_handler_service_spec.rb
index 96becfcae26..eafe7a34617 100644
--- a/spec/services/merge_requests/push_options_handler_service_spec.rb
+++ b/spec/services/merge_requests/push_options_handler_service_spec.rb
@@ -66,6 +66,22 @@ describe MergeRequests::PushOptionsHandlerService do
     end
   end
 
+  shared_examples_for 'a service that can set the merge request to merge when pipeline succeeds' do
+    subject(:last_mr) { MergeRequest.last }
+
+    it 'sets merge_when_pipeline_succeeds' do
+      service.execute
+
+      expect(last_mr.merge_when_pipeline_succeeds).to eq(true)
+    end
+
+    it 'sets merge_user to the user' do
+      service.execute
+
+      expect(last_mr.merge_user).to eq(user)
+    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 }
@@ -118,6 +134,72 @@ describe MergeRequests::PushOptionsHandlerService do
     end
   end
 
+  describe '`merge_when_pipeline_succeeds` push option' do
+    let(:push_options) { { merge_when_pipeline_succeeds: 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, merge_when_pipeline_succeeds: true } }
+
+        it_behaves_like 'a service that can create a merge request'
+        it_behaves_like 'a service that can set the merge request to merge when pipeline succeeds'
+      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, merge_when_pipeline_succeeds: true } }
+
+        it_behaves_like 'a service that can create a merge request'
+        it_behaves_like 'a service that can set the merge request to merge when pipeline succeeds'
+      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 set the merge request to merge when pipeline succeeds'
+    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 } }