Select CI template based on URL param

This allows to select a pre-defined CI template provided by the URL
param `template` with the name of the template.

e.g. `-/new/master?file_name=.gitlab-ci.yml&template=Android`

app/assets/javascripts/blob/file_template_selector.js

@@ -19,6 +19,17 @@ export default class FileTemplateSelector {
     this.$dropdownToggleText = this.$wrapper.find('.dropdown-toggle-text');
 
     this.initDropdown();
+    this.selectInitialTemplate();
+  }
+
+  selectInitialTemplate() {
+    const template = this.$dropdown.data('selected');
+
+    if (!template) {
+      return;
+    }
+
+    this.mediator.selectTemplateFile(this, template);
   }
 
   show() {

app/views/projects/blob/_template_selectors.html.haml

@@ -10,7 +10,7 @@
     .metrics-dashboard-selector.js-metrics-dashboard-selector-wrap.js-template-selector-wrap.hidden
       = dropdown_tag(_("Apply a template"), options: { toggle_class: 'js-metrics-dashboard-selector qa-metrics-dashboard-dropdown', dropdown_class: 'dropdown-menu-selectable', filter: true, placeholder: "Filter", data: { data: metrics_dashboard_ymls(@project) } } )
     #gitlab-ci-yml-selector.gitlab-ci-yml-selector.js-gitlab-ci-yml-selector-wrap.js-template-selector-wrap.hidden
-      = dropdown_tag(_("Apply a template"), options: { toggle_class: 'js-gitlab-ci-yml-selector qa-gitlab-ci-yml-dropdown', dropdown_class: 'dropdown-menu-selectable', filter: true, placeholder: "Filter", data: { data: gitlab_ci_ymls(@project) } } )
+      = dropdown_tag(_("Apply a template"), options: { toggle_class: 'js-gitlab-ci-yml-selector qa-gitlab-ci-yml-dropdown', dropdown_class: 'dropdown-menu-selectable', filter: true, placeholder: "Filter", data: { data: gitlab_ci_ymls(@project), selected: params[:template] } } )
     - if experiment_enabled?(:ci_syntax_templates_b, subject: current_user) && @project.namespace.recent?
       .gitlab-ci-syntax-yml-selector.js-gitlab-ci-syntax-yml-selector-wrap.js-template-selector-wrap.hidden
         = dropdown_tag(_("Learn CI/CD syntax"), options: { toggle_class: 'js-gitlab-ci-syntax-yml-selector qa-gitlab-ci-syntax-yml-dropdown', dropdown_class: 'dropdown-menu-selectable', filter: true, placeholder: "Filter", data: { data: gitlab_ci_syntax_ymls(@project) } } )

changelogs/unreleased/nicolasdular-apply-ci-template-via-param.yml

+---
+title: Allow selecting a CI template by providing the template name as a URL param gitlab_ci_yml
+merge_request: 56861
+author:
+type: added

spec/features/projects/files/gitlab_ci_yml_dropdown_spec.rb

@@ -5,10 +5,14 @@ require 'spec_helper'
 RSpec.describe 'Projects > Files > User wants to add a .gitlab-ci.yml file', :js do
   include Spec::Support::Helpers::Features::EditorLiteSpecHelpers
 
+  let(:params) { {} }
+  let(:filename) { '.gitlab-ci.yml' }
+
+  let_it_be(:project) { create(:project, :repository) }
+
   before do
-    project = create(:project, :repository)
     sign_in project.owner
-    visit project_new_blob_path(project, 'master', file_name: '.gitlab-ci.yml')
+    visit project_new_blob_path(project, 'master', file_name: filename, **params)
   end
 
   it 'user can pick a template from the dropdown' do
@@ -29,4 +33,38 @@ RSpec.describe 'Projects > Files > User wants to add a .gitlab-ci.yml file', :js
     expect(editor_get_value).to have_content('This file is a template, and might need editing before it works on your project')
     expect(editor_get_value).to have_content('jekyll build -d test')
   end
+
+  context 'when template param is provided' do
+    let(:params) { { template: 'Jekyll' } }
+
+    it 'uses the given template' do
+      wait_for_requests
+
+      expect(page).to have_css('.gitlab-ci-yml-selector .dropdown-toggle-text', text: 'Apply a template')
+      expect(editor_get_value).to have_content('This file is a template, and might need editing before it works on your project')
+      expect(editor_get_value).to have_content('jekyll build -d test')
+    end
+  end
+
+  context 'when provided template param is not a valid template name' do
+    let(:params) { { template: 'non-existing-template' } }
+
+    it 'leaves the editor empty' do
+      wait_for_requests
+
+      expect(page).to have_css('.gitlab-ci-yml-selector .dropdown-toggle-text', text: 'Apply a template')
+      expect(editor_get_value).to have_content('')
+    end
+  end
+
+  context 'when template is not available for the given file' do
+    let(:filename) { 'Dockerfile' }
+    let(:params) { { template: 'Jekyll' } }
+
+    it 'leaves the editor empty' do
+      wait_for_requests
+
+      expect(editor_get_value).to have_content('')
+    end
+  end
 end