Close a thread race in Connection.close(), diagnosed by Marius Gedminas.

DB._closeConnection() sets the connection's _db member to None now, under
protection of the lock it holds anyway.  At a deeper level, it's unclear
why _db keeps getting set and unset to begin with, but no time for that
now (and there are already XXX comments about it in the code).

Alas, I wasn't able to write a test that provoked the original bug in
finite time (it's a tiny timing hole), except via calling sleep() between
two lines that don't exist anymore.  Good enough.

src/ZODB/Connection.py

@@ -13,7 +13,7 @@
 ##############################################################################
 """Database connection support
 
-$Id: Connection.py,v 1.144 2004/04/06 21:47:12 tim_one Exp $"""
+$Id: Connection.py,v 1.145 2004/04/08 18:12:25 tim_one Exp $"""
 
 import logging
 import sys
@@ -466,7 +466,9 @@ class Connection(ExportImport, object):
         # Return the connection to the pool.
         if self._db is not None:
             self._db._closeConnection(self)
-            self._db = None
+            # _closeConnection() set self._db to None.  However, we can't
+            # assert that here, because self may have been reused (by
+            # another thread) by the time we get back here.
 
     def commit(self, obj, transaction):
         if obj is self:

src/ZODB/DB.py

@@ -13,7 +13,7 @@
 ##############################################################################
 """Database objects
 
-$Id: DB.py,v 1.72 2004/04/01 03:56:58 jeremy Exp $"""
+$Id: DB.py,v 1.73 2004/04/08 18:12:25 tim_one Exp $"""
 
 import cPickle, cStringIO, sys
 from thread import allocate_lock
@@ -163,14 +163,21 @@ class DB(object):
         return m
 
     def _closeConnection(self, connection):
-        """Return a connection to the pool"""
+        """Return a connection to the pool.
+
+        connection._db must be self on entry.
+        """
+
         self._a()
         try:
+            assert connection._db is self
+            connection._db = None
+
             am = self._activity_monitor
             if am is not None:
                 am.closedConnection(connection)
-            version=connection._version
-            pools,pooll=self._pools
+            version = connection._version
+            pools, pooll = self._pools
             try:
                 pool, allocated, pool_lock = pools[version]
             except KeyError:
@@ -184,7 +191,7 @@ class DB(object):
                 return
 
             pool.append(connection)
-            if len(pool)==1:
+            if len(pool) == 1:
                 # Pool now usable again, unlock it.
                 pool_lock.release()
         finally: