"Suggests" API for uniquifying usernames

app/controllers/users_controller.rb

@@ -16,7 +16,7 @@ class UsersController < ApplicationController
 
   skip_before_action :authenticate_user!
   prepend_before_action(only: [:show]) { authenticate_sessionless_user!(:rss) }
-  before_action :user, except: [:exists]
+  before_action :user, except: [:exists, :suggests]
   before_action :authorize_read_user_profile!,
                 only: [:calendar, :calendar_activities, :groups, :projects, :contributed_projects, :starred_projects, :snippets]
 
@@ -114,6 +114,14 @@ class UsersController < ApplicationController
     render json: { exists: !!Namespace.find_by_path_or_name(params[:username]) }
   end
 
+  def suggests
+    namespace_path = params[:username]
+    exists = !!Namespace.find_by_path_or_name(namespace_path)
+    suggestions = exists ? [Namespace.clean_path(namespace_path)] : []
+
+    render json: { exists: exists, suggests: suggestions }
+  end
+
   private
 
   def user

config/routes/user.rb

@@ -55,6 +55,7 @@ scope(constraints: { username: Gitlab::PathRegex.root_namespace_route_regex }) d
     get :starred, as: :starred_projects
     get :snippets
     get :exists
+    get :suggests
     get :activity
     get '/', to: redirect('%{username}'), as: nil
   end

spec/controllers/users_controller_spec.rb

@@ -348,6 +348,48 @@ describe UsersController do
     end
   end
 
+  describe 'GET #suggests' do
+    context 'when user exists' do
+      it 'returns JSON indicating the user exists and a suggestion' do
+        get :suggests, params: { username: user.username }
+
+        expected_json = { exists: true, suggests: ["#{user.username}1"] }.to_json
+        expect(response.body).to eq(expected_json)
+      end
+
+      context 'when the casing is different' do
+        let(:user) { create(:user, username: 'CamelCaseUser') }
+
+        it 'returns JSON indicating the user exists and a suggestion' do
+          get :suggests, params: { username: user.username.downcase }
+
+          expected_json = { exists: true, suggests: ["#{user.username.downcase}1"] }.to_json
+          expect(response.body).to eq(expected_json)
+        end
+      end
+    end
+
+    context 'when the user does not exist' do
+      it 'returns JSON indicating the user does not exist' do
+        get :suggests, params: { username: 'foo' }
+
+        expected_json = { exists: false, suggests: [] }.to_json
+        expect(response.body).to eq(expected_json)
+      end
+
+      context 'when a user changed their username' do
+        let(:redirect_route) { user.namespace.redirect_routes.create(path: 'old-username') }
+
+        it 'returns JSON indicating a user by that username does not exist' do
+          get :suggests, params: { username: 'old-username' }
+
+          expected_json = { exists: false, suggests: [] }.to_json
+          expect(response.body).to eq(expected_json)
+        end
+      end
+    end
+  end
+
   describe '#ensure_canonical_path' do
     before do
       sign_in(user)