[issue14205] Raise an error if a dict is modified during a lookup

[Guido van Rossum]

Guido van Rossum <guido at python.org> added the comment:

On Thu, Mar 8, 2012 at 4:56 PM, STINNER Victor <report at bugs.python.org> wrote:
>
> STINNER Victor <victor.stinner at gmail.com> added the comment:
>
>> Can you implement the counter without adding an extra field to the dict object?
>
> Add a counter requires to change the prototype of the C lookup function:
> PyDictEntry *(*ma_lookup)(PyDictObject *mp, PyObject *key, Py_hash_t hash);
>
> I don't know if it is a problem for ABI compatibility or extension
> modules. I suppose that it is safer and simpler to not change it :-)

Agreed.

>> I worry that we'll get the same objection we had when MAL proposed collision counting as a solution to the hash DoS attack.
>>
>> The numbers 0, 1 and infinity are special; all others are to be treated with skepticism.
>
> Is it really useful to only retry once? Retrying once means that it
> would work in most cases, but not in some corner cases. For example,
> retrying once may hide the problem if you have only 1 client (1
> thread), but you get the exception when you have more clients (more
> threads).
>
> Or do I misunderstand the problem?

Sorry, I was joking; anyway, 1 in this context would mean do the
lookup exactly once (so 0 would mean not to do it at all). One retry
would effectively mean try it twice, which would have to be treated
skeptically. :-)

Given all this I think we should keep it as you have committed and add
it to the docs and whatsnew.

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<http://bugs.python.org/issue14205>
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