bigfile/zodb: Monkey-patch for ZODB.Connection to support callback on .open()

ZODB.Connection has support for calling callbacks on .close() but not on
.open() . We'll need to hook into both Connection open/close process in the
next patch (for _ZBigFileH to stay in sync with Connection state).

NOTE

on-open  callbacks are setup once and fire many times on every open,
on-close callbacks are setup once and fire only once on next close.

The reason for this is that on-close callbacks are useful for scheduling
current connection cleanup, after its processing is done. But on-open
callback is for future connection usage, which is generally not related
to current connection.

/cc @jm, @vpelletier

bigfile/file_zodb.py

@@ -35,6 +35,7 @@ from transaction.interfaces import IDataManager, ISynchronizer
 from persistent import Persistent, PickleCache
 from BTrees.LOBTree import LOBTree
 from zope.interface import implementer
+from ZODB.Connection import Connection
 
 # TODO document that first data access must be either after commit or Connection.add
 
@@ -227,6 +228,33 @@ class ZBigFile(LivePersistent):
 
 
 
+# patch for ZODB.Connection to support callback on .open()
+# NOTE on-open  callbacks are setup once and fire many times on every open
+#      on-close callbacks are setup once and fire only once on next close
+Connection._onOpenCallbacks = None
+def Connection_onOpenCallback(self, f):
+    if self._onOpenCallbacks is None:
+        # NOTE WeakSet does not work for bound methods - the are always create
+        # anew for each obj.method access, and thus will go away almost immediately
+        self._onOpenCallbacks = set()
+    self._onOpenCallbacks.add(f)
+
+assert not hasattr(Connection, 'onOpenCallback')
+Connection.onOpenCallback = Connection_onOpenCallback
+
+orig_Connection_open = Connection.open
+def Connection_open(self, transaction_manager=None, delegate=True):
+    orig_Connection_open(self, transaction_manager, delegate)
+
+    if self._onOpenCallbacks:
+        for f in self._onOpenCallbacks:
+            f()
+
+Connection.open = Connection_open
+# ------------
+
+
+
 # BigFileH wrapper that also acts as DataManager proxying changes back to ZODB
 # objects at two-phase-commit (TPC) level.
 #