move the oauth2 last_used update to the bottom of the request

this avoids acquiring a row lock (and holding it for the duration of
transactions that include a very slow query)

see: ansible#4694

awx/api/authentication.py

@@ -53,4 +53,6 @@ def authenticate(self, request):
                 )
             ))
             setattr(user, 'oauth_scopes', [x for x in token.scope.split() if x])
+            setattr(user, 'oauth_token', token)
+
         return ret

awx/main/middleware.py

@@ -16,6 +16,7 @@
 from django.db.migrations.executor import MigrationExecutor
 from django.db import IntegrityError, connection
 from django.utils.functional import curry
+from django.utils.timezone import now
 from django.shortcuts import get_object_or_404, redirect
 from django.apps import apps
 from django.utils.deprecation import MiddlewareMixin
@@ -52,6 +53,10 @@ def process_response(self, request, response):
             cprofile_file = self.save_profile_file(request)
             response['cprofile_file'] = cprofile_file
         perf_logger.info('api response times', extra=dict(python_objects=dict(request=request, response=response)))
+        token = getattr(request.user, 'oauth_token', None)
+        if token:
+            token.last_used = now()
+            token.save(update_fields=['last_used'])
         return response
 
     def save_profile_file(self, request):

awx/main/models/oauth.py

@@ -4,7 +4,6 @@
 # Django
 from django.core.validators import RegexValidator
 from django.db import models
-from django.utils.timezone import now
 from django.utils.translation import ugettext_lazy as _
 from django.conf import settings
 
@@ -119,9 +118,6 @@ class Meta:
 
     def is_valid(self, scopes=None):
         valid = super(OAuth2AccessToken, self).is_valid(scopes)
-        if valid:
-            self.last_used = now()
-            self.save(update_fields=['last_used'])
         return valid
 
     def save(self, *args, **kwargs):