lib/zodb: Add ZODB.Connection.onShutdownCallback

Add patch to ZODB.Connection to support callback on after database is
closed. ZBigFile + WCFS client integration will use this callback to
close WCFS connection when corresponding ZODB.DB is closed.

Preliminary history:

a26d9659    X lib/zodb: Connection += onShutdownCallback

lib/tests/test_zodb.py

@@ -355,6 +355,40 @@ def test_zodb_onresync():
     conn.close()
 
 
+# verify that ZODB.Connection.onShutdownCallback works
+@func
+def test_zodb_onshutdown():
+    stor = testdb.getZODBStorage()
+    defer(stor.close)
+    db  = DB(stor)
+
+    class T:
+        def __init__(t):
+            t.nshutdown = 0
+        def on_connection_shutdown(t):
+            t.nshutdown += 1
+
+    t1 = T()
+    t2 = T()
+
+    # conn1 stays alive outside of db.pool
+    conn1 = db.open()
+    conn1.onShutdownCallback(t1)
+
+    # conn2 stays alive inside db.pool
+    conn2 = db.open()
+    conn2.onShutdownCallback(t2)
+    conn2.close()
+
+    assert t1.nshutdown == 0
+    assert t2.nshutdown == 0
+
+    # db.close triggers conn1 and conn2 shutdown
+    db.close()
+    assert t1.nshutdown == 1
+    assert t2.nshutdown == 1
+
+
 # test that zurl does not change from one open to another storage open.
 def test_zurlstable():
     if not isinstance(testdb, (testing.TestDB_FileStorage, testing.TestDB_ZEO, testing.TestDB_NEO)):

lib/zodb.py

@@ -296,6 +296,30 @@ else:
     raise AssertionError("ZODB3 is not supported anymore")
 
 
+# patch for ZODB.Connection to support callback on after database is closed
+ZODB.Connection.Connection._onShutdownCallbacks = None
+def Connection_onShutdownCallback(self, f):
+    if self._onShutdownCallbacks is None:
+        # NOTE WeakSet does not work for bound methods - they are always created
+        # anew for each obj.method access, and thus will go away almost immediately
+        self._onShutdownCallbacks = WeakSet()
+    self._onShutdownCallbacks.add(f)
+
+assert not hasattr(ZODB.Connection.Connection, 'onShutdownCallback')
+ZODB.Connection.Connection.onShutdownCallback = Connection_onShutdownCallback
+
+_orig_DB_close = ZODB.DB.close
+def _ZDB_close(self):
+    # the same code for ZODB3/4/5
+    @self._connectionMap
+    def _(conn):
+        if conn._onShutdownCallbacks:
+            for f in conn._onShutdownCallbacks:
+                f.on_connection_shutdown()
+    _orig_DB_close(self)
+
+ZODB.DB.close = _ZDB_close
+
 
 # zstor_2zurl converts a ZODB storage to URL to access it.
 def zstor_2zurl(zstor):