Adds /target_branch slash command functionality for merge requests

Allows to use slash command /target_branch <target_branch_name> in merge requests notes and description.
Command allows to specify target branch for current merge request.

Proposed in #23619

app/services/slash_commands/interpret_service.rb

@@ -304,6 +304,18 @@ module SlashCommands
     params '@user'
     command :cc
 
+    desc 'Defines target branch for MR'
+    params '<Local branch name>'
+    condition do
+      issuable.respond_to?(:target_branch) &&
+        (current_user.can?(:"update_#{issuable.to_ability_name}", issuable) ||
+          issuable.new_record?)
+    end
+    command :target_branch do |target_branch_param|
+      branch_name = target_branch_param.strip
+      @updates[:target_branch] = branch_name if project.repository.branch_names.include?(branch_name)
+    end
+
     def find_label_ids(labels_param)
       label_ids_by_reference = extract_references(labels_param, :label).map(&:id)
       labels_ids_by_name = LabelsFinder.new(current_user, project_id: project.id, name: labels_param.split).execute.select(:id)

changelogs/unreleased/Add-a-shash-command-for-target-merge-request-branch.yml

+---
+title: Adds /target_branch slash command functionality for merge requests
+merge_request:
+author: YarNayar

doc/user/project/slash_commands.md

@@ -34,3 +34,4 @@ do.
 | `/remove_estimate`       | Remove estimated time |
 | <code>/spend &lt;1h 30m &#124; -1h 5m&gt;</code> | Add or substract spent time |
 | `/remove_time_spent`       | Remove time spent |
+| `/target_branch <Branch Name>` | Set target branch for current merge request |

spec/features/merge_requests/user_uses_slash_commands_spec.rb

@@ -120,5 +120,79 @@ feature 'Merge Requests > User uses slash commands', feature: true, js: true do
         expect(page).not_to have_content '/due 2016-08-28'
       end
     end
+
+    describe '/target_branch command in merge request' do
+      let(:another_project) { create(:project, :public) }
+      let(:new_url_opts) { { merge_request: { source_branch: 'feature' } } }
+
+      before do
+        logout
+        another_project.team << [user, :master]
+        login_with(user)
+      end
+
+      it 'changes target_branch in new merge_request' do
+        visit new_namespace_project_merge_request_path(another_project.namespace, another_project, new_url_opts)
+        fill_in "merge_request_title", with: 'My brand new feature'
+        fill_in "merge_request_description", with: "le feature \n/target_branch fix\nFeature description:"
+        click_button "Submit merge request"
+
+        merge_request = another_project.merge_requests.first
+        expect(merge_request.description).to eq "le feature \nFeature description:"
+        expect(merge_request.target_branch).to eq 'fix'
+      end
+
+      it 'does not change target branch when merge request is edited' do
+        new_merge_request = create(:merge_request, source_project: another_project)
+
+        visit edit_namespace_project_merge_request_path(another_project.namespace, another_project, new_merge_request)
+        fill_in "merge_request_description", with: "Want to update target branch\n/target_branch fix\n"
+        click_button "Save changes"
+
+        new_merge_request = another_project.merge_requests.first
+        expect(new_merge_request.description).to include('/target_branch')
+        expect(new_merge_request.target_branch).not_to eq('fix')
+      end
+    end
+
+    describe '/target_branch command from note' do
+      context 'when the current user can change target branch' do
+        it 'changes target branch from a note' do
+          write_note("message start \n/target_branch merge-test\n message end.")
+
+          expect(page).not_to have_content('/target_branch')
+          expect(page).to have_content('message start')
+          expect(page).to have_content('message end.')
+
+          expect(merge_request.reload.target_branch).to eq 'merge-test'
+        end
+
+        it 'does not fail when target branch does not exists' do
+          write_note('/target_branch totally_not_existing_branch')
+
+          expect(page).not_to have_content('/target_branch')
+
+          expect(merge_request.target_branch).to eq 'feature'
+        end
+      end
+
+      context 'when current user can not change target branch' do
+        let(:guest) { create(:user) }
+        before do
+          project.team << [guest, :guest]
+          logout
+          login_with(guest)
+          visit namespace_project_merge_request_path(project.namespace, project, merge_request)
+        end
+
+        it 'does not change target branch' do
+          write_note('/target_branch merge-test')
+
+          expect(page).not_to have_content '/target_branch merge-test'
+
+          expect(merge_request.target_branch).to eq 'feature'
+        end
+      end
+    end
   end
 end

spec/services/slash_commands/interpret_service_spec.rb

@@ -653,5 +653,37 @@ describe SlashCommands::InterpretService, services: true do
         let(:issuable) { issue }
       end
     end
+
+    context '/target_branch command' do
+      let(:non_empty_project) { create(:project) }
+      let(:another_merge_request) { create(:merge_request, author: developer, source_project: non_empty_project) }
+      let(:service) { described_class.new(non_empty_project, developer)}
+
+      it 'updates target_branch if /target_branch command is executed' do
+        _, updates = service.execute('/target_branch merge-test', merge_request)
+
+        expect(updates).to eq(target_branch: 'merge-test')
+      end
+
+      it 'handles blanks around param' do
+        _, updates = service.execute('/target_branch  merge-test     ', merge_request)
+
+        expect(updates).to eq(target_branch: 'merge-test')
+      end
+
+      context 'ignores command with no argument' do
+        it_behaves_like 'empty command' do
+          let(:content) { '/target_branch' }
+          let(:issuable) { another_merge_request }
+        end
+      end
+
+      context 'ignores non-existing target branch' do
+        it_behaves_like 'empty command' do
+          let(:content) { '/target_branch totally_non_existing_branch' }
+          let(:issuable) { another_merge_request }
+        end
+      end
+    end
   end
 end