Clean DB properties on close (#123)

( This is ZODB4 backport of upstream commit e9c7492a )

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

--------

Kirr: with wendelin.core 2 this release of resources becomes important
because if DB.close does not clear connection pools, and something
continues to reference DB object, the connections - that stay referenced
from DB.pool - will be kept alive, and so will be WCFS client connection
associated with those ZODB Connections. As the result, even after proper
zconn.close() and DB.close(), WCFS server stays referenced via files
opened by WCFS client connections, and so cannot be unmounted cleanly.

src/ZODB/DB.py

@@ -104,6 +104,10 @@ class AbstractConnectionPool(object):
 
     size = property(getSize, lambda self, v: self.setSize(v))
 
+    def clear(self):
+        pass
+
+
 class ConnectionPool(AbstractConnectionPool):
 
     def __init__(self, size, timeout=1<<31):
@@ -224,6 +228,11 @@ class ConnectionPool(AbstractConnectionPool):
             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.
@@ -279,6 +288,11 @@ class KeyedConnectionPool(AbstractConnectionPool):
             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()
@@ -648,6 +662,11 @@ class DB(object):
 
         self.storage.close()
         del self.storage
+        # clean up references to other DBs
+        self.databases = {}
+        # clean up the connection pool
+        self.pool.clear()
+        self.historical_pool.clear()
 
     def getCacheSize(self):
         return self._cache_size

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

@@ -362,6 +362,31 @@ 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(