Add validations and errors handling for SSE

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/212558

* Add permissions check for user role
* Add validations
* Expose `is_supported_content` boolean field to frontend.

`is_supported_content` shows if there are no validation errors
detected by backend.

Validation error messages are not used for now. Frontend will show a
general error message for all validation failures.

app/controllers/projects/static_site_editor_controller.rb

@@ -2,10 +2,13 @@
 
 class Projects::StaticSiteEditorController < Projects::ApplicationController
   include ExtractsPath
+  include CreatesCommit
+
   layout 'fullscreen'
 
   prepend_before_action :authenticate_user!, only: [:show]
   before_action :assign_ref_and_path, only: [:show]
+  before_action :authorize_edit_tree!, only: [:show]
 
   def show
     @config = Gitlab::StaticSiteEditor::Config.new(@repository, @ref, @path, params[:return_url])

lib/gitlab/static_site_editor/config.rb

@@ -3,33 +3,49 @@
 module Gitlab
   module StaticSiteEditor
     class Config
+      SUPPORTED_EXTENSIONS = %w[.md].freeze
+
       def initialize(repository, ref, file_path, return_url)
         @repository = repository
         @ref = ref
         @file_path = file_path
         @return_url = return_url
+        @commit_id = repository.commit(ref)&.id if ref
       end
 
       def payload
         {
           branch: ref,
           path: file_path,
-          commit: commit.id,
+          commit_id: commit_id,
           project_id: project.id,
           project: project.path,
           namespace: project.namespace.path,
-          return_url: return_url
+          return_url: return_url,
+          is_supported_content: supported_content?
         }
       end
 
       private
 
-      attr_reader :repository, :ref, :file_path, :return_url
+      attr_reader :repository, :ref, :file_path, :return_url, :commit_id
 
       delegate :project, to: :repository
 
-      def commit
-        repository.commit(ref)
+      def supported_content?
+        master_branch? && extension_supported? && file_exists?
+      end
+
+      def master_branch?
+        ref == 'master'
+      end
+
+      def extension_supported?
+        File.extname(file_path).in?(SUPPORTED_EXTENSIONS)
+      end
+
+      def file_exists?
+        commit_id.present? && repository.blob_at(commit_id, file_path).present?
       end
     end
   end

spec/controllers/projects/static_site_editor_controller_spec.rb

@@ -26,7 +26,21 @@ describe Projects::StaticSiteEditorController do
         end
       end
 
-      %w[guest developer maintainer].each do |role|
+      context 'as guest' do
+        let(:user) { create(:user) }
+
+        before do
+          project.add_guest(user)
+          sign_in(user)
+          get :show, params: default_params
+        end
+
+        it 'responds with 404 page' do
+          expect(response).to have_gitlab_http_status(:not_found)
+        end
+      end
+
+      %w[developer maintainer].each do |role|
         context "as #{role}" do
           let(:user) { create(:user) }
 

spec/lib/gitlab/static_site_editor/config_spec.rb

@@ -18,13 +18,44 @@ describe Gitlab::StaticSiteEditor::Config do
     it 'returns data for the frontend component' do
       is_expected.to eq(
         branch: 'master',
-        commit: repository.commit.id,
+        commit_id: repository.commit.id,
         namespace: 'namespace',
         path: 'README.md',
         project: 'project',
         project_id: project.id,
-        return_url: 'http://example.com'
+        return_url: 'http://example.com',
+        is_supported_content: true
       )
     end
+
+    context 'when branch is not master' do
+      let(:ref) { 'my-branch' }
+
+      it { is_expected.to include(is_supported_content: false) }
+    end
+
+    context 'when file does not have a markdown extension' do
+      let(:file_path) { 'README.txt' }
+
+      it { is_expected.to include(is_supported_content: false) }
+    end
+
+    context 'when file does not have an extension' do
+      let(:file_path) { 'README' }
+
+      it { is_expected.to include(is_supported_content: false) }
+    end
+
+    context 'when file does not exist' do
+      let(:file_path) { 'UNKNOWN.md' }
+
+      it { is_expected.to include(is_supported_content: false) }
+    end
+
+    context 'when repository is empty' do
+      let(:project) { create(:project_empty_repo) }
+
+      it { is_expected.to include(is_supported_content: false) }
+    end
   end
 end