Snapshot Isolation provided by MVCC allows for `write skew` effects

The isolation level of ZODB only implements a relaxed version of the `Isolation` property of ACID.

Here is a sample where the serializability fails:

class X(object):
    set = None

    def add(self, x):
        if self.set is None:
            self.set = OOSet()
        set.add(x)

    def remove(self, x):
        if self.set is None:
            return
        if x in self.set:
            if len(self.set) == 1:
                self.set = None
            else:
                self.set.remove(x)

Now, consider the two transactions T1, T2, and T3, first in parallel:

T1: begin()
T1: root['x'] = X()
T1: root['x'].add(1)
T1: commit()

T2: begin()
T3: begin()
T2: root['x'].remove(1)
T3: root['x'].add(2)
T2: commit()
T3: commit()

All transactions will commit successfully and the value of root['x'].set will be None.

If we look at the transactions if they ran serialized:

(Same for T1)

T2: begin()
T2: root['x'].remove(1)
T2: commit()

T3: begin()
T3: root['x'].add(2)
T3: commit()

Now, the result would be root['x'].set == set([2]).

Note: The initial discussion for this bug was spawned by this mailing list thread (in German, sorry): https://mail.dzug.org/pipermail/zope/2007-June/003937.html