Issue2326

Currently, WeakValueDictionary.valuerefs looks like this:

 return [ref(value) for value in self.itervalues()]

But, as the CPython documentation says:

   Caution: Because a WeakKeyDictionary is built on top of a Python dictionary, it must not change size when iterating over it. This can be difficult to ensure for a WeakKeyDictionary because actions performed by the program during iteration may cause items in the dictionary to vanish “by magic” (as a side effect of garbage collection).

This is enforced by the Jython dictionary implementation, and together with of having Java's truly non-deterministic GC means that using itervalues is extremely dangerous. Practically speaking, this makes `valuerefs` impossible to use.

This came up in porting the widely used `transaction` package to work on Jython[1]. When the system is put under any strain, a RuntimeError results from this method. Here's an example from running the ZODB test suite; changing `itervalues` to `values` solves the problem:

      File "<doctest ZODB.tests.testblob.loadblob_tmpstore[12]>", line 1, in <module>
        database.close()
      File "//ZODB/src/ZODB/DB.py", line 629, in close
        @self._connectionMap
      File "//ZODB/src/ZODB/DB.py", line 505, in _connectionMap
        self.pool.map(f)
      File "//ZODB/src/ZODB/DB.py", line 205, in map
        self.all.map(f)
      File "//transaction-1.4.4.dev0-py2.7.egg/transaction/weakset.py", line 63, in map
        f(elt)
      File "//ZODB/src/ZODB/DB.py", line 631, in _
        c.transaction_manager.abort()
      File "//transaction-1.4.4.dev0-py2.7.egg/transaction/_manager.py", line 116, in abort
        return self.get().abort()
      File "//transaction-1.4.4.dev0-py2.7.egg/transaction/_transaction.py", line 443, in abort
        self._synchronizers.map(lambda s: s.beforeCompletion(self))
      File "//transaction-1.4.4.dev0-py2.7.egg/transaction/weakset.py", line 60, in map
        for wr in self.as_weakref_list():
      File "//transaction-1.4.4.dev0-py2.7.egg/transaction/weakset.py", line 87, in as_weakref_list
        refs = self.data.valuerefs()
      File "//jython2.7rc2/Lib/weakref.py", line 69, in valuerefs
        return [ref(value) for value in self.itervalues()]
    RuntimeError: dictionary changed size during iteration

The same reasoning for `WeakKeyDictionary.keyrefs`

[1] https://github.com/zopefoundation/transaction/pull/8

Ack, we should relax this check in PyDictionary, this is unnecessary in general given concurrent modification support in ConcurrentHashMap. Something similar was already changed in PySet.

The other unfortunate thing here is that this test

            if (getMap().size() != size) {
                throw Py.RuntimeError("dictionary changed size during iteration");
            }

is not even correct on weak maps in Guava, given eventual consistency.

Should be fixed as of https://hg.python.org/jython/rev/5064a5c5b1a3, so please verify with your testing

With that commit, the ZODB tests run clean. Seems fixed to me.