[Monorail] Implement "Switch accounts" via google.com/a/SelectSession.

appengine/monorail/framework/servlet.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is govered by a BSD-style
 # license that can be found in the LICENSE file or at
 # https://developers.google.com/open-source/licenses/bsd
 
 """Base classes for Monorail servlets.
 
 This base class provides HTTP get() and post() methods that
 conveniently drive the process of parsing the request, checking base
 permissions, gathering common page information, gathering
@@ skipping 845 matching lines @@
   else:
     after_login_url = framework_helpers.FormatAbsoluteURL(
         mr, urls.ISSUE_ENTRY_AFTER_LOGIN)
     return _SafeCreateLoginURL(mr, after_login_url)
 
 
 def _SafeCreateLoginURL(mr, continue_url=None):
   """Make a login URL w/ a detailed continue URL, otherwise use a short one."""
   continue_url = continue_url or mr.current_page_url
   try:
-    return users.create_login_url(continue_url)
+    url = users.create_login_url(continue_url)
   except users.RedirectTooLongError:
     if mr.project_name:
-      return users.create_login_url('/p/%s' % mr.project_name)
+      url = users.create_login_url('/p/%s' % mr.project_name)
     else:
-      return users.create_login_url('/')
+      url = users.create_login_url('/')
+
+  # Give the user a choice of existing accounts in their session
+  # or the option to add an account, even if they are currently
+  # signed in to exactly one account.
+  url = url.replace('/accounts/ServiceLogin', '/a/SelectSession', 1)
+  return url
 
 
 def _SafeCreateLogoutURL(mr):
   """Make a logout URL w/ a detailed continue URL, otherwise use a short one."""
   try:
     return users.create_logout_url(mr.current_page_url)
   except users.RedirectTooLongError:
     if mr.project_name:
       return users.create_logout_url('/p/%s' % mr.project_name)
     else:
       return users.create_logout_url('/')
 
 
 class Error(Exception):
   """Base class for errors from this module."""
   pass
 
 
 class AlreadySentResponseException(Error):
   """The servlet already responded, no need to render a page template."""
   pass