clear ContentType on each schema change (fixes #9891)

Also replace the cache dictionnary by a thread local variable to make it safe to
use multitenancy with threads.

tenant_schemas/postgresql_backend/base.py

@@ -1,11 +1,13 @@
 import re
 import warnings
 import psycopg2
+import threading
 
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured, ValidationError
 import django.db.utils
 
+from django.contrib.auth.models import ContentType, ContentTypeManager
 from tenant_schemas.utils import get_public_schema_name, get_limit_set_calls
 from tenant_schemas.postgresql_backend.introspection import DatabaseSchemaIntrospection
 
@@ -72,6 +74,7 @@ class DatabaseWrapper(original_backend.DatabaseWrapper):
         self.include_public_schema = include_public
         self.set_settings_schema(self.schema_name)
         self.search_path_set = False
+        ContentType.objects.clear_cache()
 
     def set_schema(self, schema_name, include_public=True):
         """
@@ -83,6 +86,7 @@ class DatabaseWrapper(original_backend.DatabaseWrapper):
         self.include_public_schema = include_public
         self.set_settings_schema(schema_name)
         self.search_path_set = False
+        ContentType.objects.clear_cache()
 
     def set_schema_to_public(self):
         """
@@ -92,6 +96,7 @@ class DatabaseWrapper(original_backend.DatabaseWrapper):
         self.schema_name = get_public_schema_name()
         self.set_settings_schema(self.schema_name)
         self.search_path_set = False
+        ContentType.objects.clear_cache()
 
     def set_settings_schema(self, schema_name):
         self.settings_dict['SCHEMA'] = schema_name
@@ -170,3 +175,12 @@ class FakeTenant:
     """
     def __init__(self, schema_name):
         self.schema_name = schema_name
+
+# Make the ContentType cache tenant and thread safe
+ContentTypeManager._thread_local_cache = threading.local()
+class ContentTypeCacheDescriptor(object):
+    def __get__(self, obj):
+        if not hasattr(obj._thread_local_cache, '_cache'):
+            obj._thread_local_cache._cache = {}
+        return obj._thread_local_cache._cache
+ContentTypeManager._cache = ContentTypeCacheDescriptor()