Add option to admin area to sign in as a specific user

Closes #2291

app/controllers/admin/users_controller.rb

@@ -63,6 +63,12 @@ class Admin::UsersController < Admin::ApplicationController
     end
   end
 
+  def login_as
+    sign_in(user)
+    flash[:alert] = "Logged in as #{user.username}"
+    redirect_to root_path
+  end
+
   def disable_two_factor
     user.disable_two_factor!
     redirect_to admin_user_path(user),

app/views/admin/users/index.html.haml

@@ -72,7 +72,7 @@
           = link_to 'New User', new_admin_user_path, class: "btn btn-new btn-sm"
       %ul.well-list
         - @users.each do |user|
-          %li
+          %li{ class: "user-#{user.id}" }
             .list-item-name
               - if user.blocked?
                 %i.fa.fa-lock.cred
@@ -90,6 +90,7 @@
                
               = link_to 'Edit', edit_admin_user_path(user), id: "edit_#{dom_id(user)}", class: "btn btn-xs"
               - unless user == current_user
+                = link_to 'Log in', login_as_admin_user_path(user), method: :put, class: "btn btn-xs btn-primary"
                 - if user.blocked?
                   = link_to 'Unblock', unblock_admin_user_path(user), method: :put, class: "btn btn-xs btn-success"
                 - else

config/routes.rb

@@ -262,6 +262,7 @@ Gitlab::Application.routes.draw do
         put :unblock
         put :unlock
         put :confirm
+        put :login_as
         patch :disable_two_factor
         delete 'remove/:email_id', action: 'remove_email', as: 'remove_email'
       end

spec/controllers/admin/users_controller_spec.rb

@@ -7,6 +7,21 @@ describe Admin::UsersController do
     sign_in(admin)
   end
 
+  describe 'PUT login_as' do
+    let(:user) { create(:user) }
+
+    it 'logs admin as another user' do
+      expect(warden.authenticate(scope: :user)).not_to eq(user)
+      put :login_as, id: user.username
+      expect(warden.authenticate(scope: :user)).to eq(user)
+    end
+
+    it 'redirects user to homepage' do
+      put :login_as, id: user.username
+      expect(response).to redirect_to(root_path)
+    end
+  end
+
   describe 'DELETE #user with projects' do
     let(:user) { create(:user) }
     let(:project) { create(:project, namespace: user.namespace) }

spec/features/admin/admin_users_spec.rb

@@ -166,4 +166,20 @@ describe "Admin::Users", feature: true  do
       end
     end
   end
+
+  it 'should be able to log in as another user' do
+    another_user = create(:user)
+
+    visit admin_users_path
+
+    page.within ".user-#{another_user.id}" do
+      click_link 'Log in'
+    end
+
+    expect(page).to have_content("Logged in as #{another_user.username}")
+
+    page.within '.sidebar-user .username' do
+      expect(page).to have_content(another_user.username)
+    end
+  end
 end