Simple thread safety fix + a more reliable way to check if managed

xact.py

@@ -1,14 +1,14 @@
 """ This code provides a decorator / context manager for transaction management in
     Django on PostgreSQL.  It is intended as a replacement for the existing Django
     commit_on_success() function, and provides some nice features:
-    
+
     * Nested transactions: The top-level transaction will be a BEGIN/COMMIT/ROLLBACK
       block; inner "transactions" are implemented as savepoints.
     * Commits even if is_dirty is False, eliminating the mistake of forgetting to set
       the dirty flag when doing database-modifying raw SQL.
     * Better interaction with pgPool II, if you're using it.
     * A workaround for a subtle but nasty bug in Django's transaction management.
-    
+
     As currently implemented, it is NOT thread-safe as a decorator (it IS thread-safe
     as a context manager).  Fix coming.
 
@@ -17,27 +17,30 @@
 
 from functools import wraps
 
+from threading import local
+
 from django.db import transaction, DEFAULT_DB_ALIAS, connections
 
 import psycopg2.extensions
 
 class _Transaction(object):
     def __init__(self, using):
         self.using = using
-        self.sid = None
-    
+        self._local = local()
+
     def __enter__(self):
-        if connections[self.using].features.uses_savepoints:
+        if transaction.is_managed(self.using):
             # We're already in a transaction; create a savepoint.
-            self.sid = transaction.savepoint(self.using)
+            self._local.sid = transaction.savepoint(self.using)
         else:
+            self._local.sid = None
             transaction.enter_transaction_management(using=self.using)
             transaction.managed(True, using=self.using)
-   
+
     def __exit__(self, exc_type, exc_value, traceback):
         if exc_value is None:
             # commit operation
-            if self.sid is None:
+            if self._local.sid is None:
                 # Outer transaction
                 try:
                     transaction.commit(self.using)
@@ -49,20 +52,20 @@ def __exit__(self, exc_type, exc_value, traceback):
             else:
                 # Inner savepoint
                 try:
-                    transaction.savepoint_commit(self.sid, self.using)
+                    transaction.savepoint_commit(self._local.sid, self.using)
                 except:
-                    transaction.savepoint_rollback(self.sid, self.using)
+                    transaction.savepoint_rollback(self._local.sid, self.using)
                     raise
         else:
             # rollback operation
-            if self.sid is None:
+            if self._local.sid is None:
                 # Outer transaction
                 transaction.rollback(self.using)
                 self._leave_transaction_management()
             else:
                 # Inner savepoint
-                transaction.savepoint_rollback(self.sid, self.using)
-        
+                transaction.savepoint_rollback(self._local.sid, self.using)
+
         return False
 
     def _leave_transaction_management(self):
@@ -71,10 +74,10 @@ def _leave_transaction_management(self):
             connections[self.using]._set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT)
             # Patch for bug in Django's psycopg2 backend; see:
             # https://code.djangoproject.com/ticket/16047
-    
+
     # This is a great recipe for allowing a single object to handle both @ decorators
     # and with contexts.
-    
+
     def __call__(self, func):
         @wraps(func)
         def inner(*args, **kwargs):