bigfile/zodb: Do not hold reference to ZBigFileH indefinitely in Connection.onOpenCallback

If we do - ZBigFileH objects just don't get garbage collected, and
sooner or later this way it leaks enough filedescriptors so that main
zope loop breaks:

    Traceback (most recent call last):
      File ".../bin/runzope", line 194, in <module>
        sys.exit(Zope2.Startup.run.run())
      File ".../eggs/Zope2-2.13.22-py2.7.egg/Zope2/Startup/run.py", line 26, in run
        starter.run()
      File ".../eggs/Zope2-2.13.22-py2.7.egg/Zope2/Startup/__init__.py", line 105, in run
        Lifetime.loop()
      File ".../eggs/Zope2-2.13.22-py2.7.egg/Lifetime/__init__.py", line 43, in loop
        lifetime_loop()
      File ".../eggs/Zope2-2.13.22-py2.7.egg/Lifetime/__init__.py", line 53, in lifetime_loop
        asyncore.poll(timeout, map)
      File ".../parts/python2.7/lib/python2.7/asyncore.py", line 145, in poll
        r, w, e = select.select(r, w, e, timeout)
    ValueError: filedescriptor out of range in select()

    $ lsof -p <runzope-pid> |grep ramh | wc -l
    950

So continuing 64d1f40b (bigfile/zodb: Monkey-patch for ZODB.Connection
to support callback on .open()) let's change the implementation to use
WeakSet for callbacks list.

Yes, because weakref to bound methods release immediately, we give up
flexibility to subscribe to arbitrary callbacks. If it become an issue,
something like WeakMethod from py3 or recipes from the net how to do it
are there.

bigfile/file_zodb.py

@@ -260,7 +260,7 @@ 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 = WeakSet()
     self._onOpenCallbacks.add(f)
 
 assert not hasattr(Connection, 'onOpenCallback')
@@ -270,9 +270,12 @@ orig_Connection_open = Connection.open
 def Connection_open(self, transaction_manager=None, delegate=True):
     orig_Connection_open(self, transaction_manager, delegate)
 
+    # FIXME method name hardcoded. Better not do it and allow f to be general
+    # callable, but that does not work with bound method - see above.
+    # ( Something like WeakMethod from py3 could help )
     if self._onOpenCallbacks:
         for f in self._onOpenCallbacks:
-            f()
+            f.on_connection_open()
 
 Connection.open = Connection_open
 # ------------
@@ -332,7 +335,7 @@ class _ZBigFileH(object):
         self.transaction_manager = zfile._p_jar.transaction_manager
 
         # when connection will be reopened -> txn_manager.registerSynch(self)
-        zfile._p_jar.onOpenCallback(self.on_connection_open)
+        zfile._p_jar.onOpenCallback(self)   # -> self.on_connection_open()
 
         # when we are just initially created, the connection is already opened,
         # so manually compensate for it.

bigfile/tests/test_filezodb.py

@@ -25,6 +25,8 @@ from transaction import TransactionManager
 from numpy import ndarray, array_equal, uint8, zeros
 from threading import Thread
 from six.moves import _thread
+import weakref
+import gc
 
 from pytest import raises
 from six.moves import range as xrange
@@ -539,3 +541,34 @@ def test_bigfile_filezodb_vs_cache_invalidation():
     conn2.close()
     del conn2, root2
     dbclose(root1)
+
+
+# verify that fileh are garbage-collected after user free them
+def test_bigfile_filezodb_fileh_gc():
+    root1= dbopen()
+    conn1= root1._p_jar
+    db   = conn1.db()
+    root1['zfile4'] = f1 = ZBigFile(blksize)
+    transaction.commit()
+
+    fh1  = f1.fileh_open()
+    vma1 = fh1.mmap(0, 1)
+    wfh1 = weakref.ref(fh1)
+    assert wfh1() is fh1
+
+    conn1.close()
+    del vma1, fh1, f1, root1
+
+
+    conn2 = db.open()
+    root2 = conn2.root()
+    f2 = root2['zfile4']
+
+    fh2  = f2.fileh_open()
+    vma2 = fh2.mmap(0, 1)
+
+    gc.collect()
+    assert wfh1() is None   # fh1 should be gone
+
+    del vma2, fh2, f2
+    dbclose(root2)