[Martijn Pieters]

Did the conflict resolution code for BTrees.Length ever work? Because as it stands now the code will fail

You haven't seen this fail, you're _deducing_ that it "must" fail, right?

as it assumes that integers are passed in, instead of state dictionaries:

def _p_resolveConflict(self, old, s1, s2): return s1 + s2 - old

Don't overlook this other Length method: def __getstate__(self): return self.value That is, when a Length instance is asked for its state, it returns an integer. Similarly setting its state expects an integer: def __setstate__(self, v): self.value = v See: http://docs.python.org/lib/pickle-inst.html ... If a class defines both __getstate__() and __setstate__(), the state object needn't be a dictionary and these methods can do what they want.

As there are no tests for this that I can see (the BTrees tests are kinda very dense),

Confirmed: there are no Length tests in ZODB.

I am not too keen to go touch this,

Good instincts <wink>.

but I think this should read:

def _p_resolveConflict(self, old, s1, s2): s1['value'] += s2['value'] - old['value'] return s1

I expect that would blow up (TypeError: unsubscriptable object), unless you also removed Length's __getstate__ and __setstate__ methods.