Clean DB properties on close (#123)

* clean up references to DBs and Connections in pools
* Implement an explicit clear() method for clearing all the items in the pool.

src/ZODB/DB.py

@@ -107,6 +107,10 @@ def getTimeout(self):
 
     size = property(getSize, lambda self, v: self.setSize(v))
 
+    def clear(self):
+        pass
+
+
 class ConnectionPool(AbstractConnectionPool):
 
     def __init__(self, size, timeout=1<<31):
@@ -230,6 +234,11 @@ def availableGC(self):
             self.available[:] = [i for i in self.available
                                  if i[1] not in to_remove]
 
+    def clear(self):
+        while self.pop():
+            pass
+
+
 class KeyedConnectionPool(AbstractConnectionPool):
     # this pool keeps track of keyed connections all together.  It makes
     # it possible to make assertions about total numbers of keyed connections.
@@ -285,6 +294,11 @@ def availableGC(self):
             if not pool.all:
                 del self.pools[key]
 
+    def clear(self):
+        for pool in self.pools.values():
+            pool.clear()
+        self.pools.clear()
+
     @property
     def test_all(self):
         result = set()
@@ -645,6 +659,11 @@ def _(c):
         self._mvcc_storage.close()
         del self.storage
         del self._mvcc_storage
+        # clean up references to other DBs
+        self.databases = {}
+        # clean up the connection pool
+        self.pool.clear()
+        self.historical_pool.clear()
 
     def getCacheSize(self):
         """Get the configured cache size (objects).

src/ZODB/tests/multidb.txt

@@ -137,7 +137,7 @@ an exception:
 
 Clean up:
 
-    >>> for a_db in dbmap.values():
+    >>> for a_db in list(dbmap.values()):
     ...     a_db.close()
 
 

src/ZODB/tests/testDB.py

@@ -397,6 +397,31 @@ def minimally_test_connection_timeout():
 
     """
 
+def cleanup_on_close():
+    """Verify that various references are cleared on close
+
+    >>> db = ZODB.DB(None)
+
+    >>> conn = db.open()
+    >>> conn.root.x = 'x'
+    >>> transaction.commit()
+    >>> conn.close()
+
+    >>> historical_conn = db.open(at=db.lastTransaction())
+    >>> historical_conn.close()
+
+    >>> db.close()
+
+    >>> db.databases
+    {}
+
+    >>> db.pool.pop() is None
+    True
+
+    >>> [pool is None for pool in db.historical_pool.pools.values()]
+    []
+"""
+
 def test_suite():
     s = unittest.makeSuite(DBTests)
     s.addTest(doctest.DocTestSuite(