[Python-Dev] What do we do about bad slicing and possible crashes (issue 27867)

[Maciej Fijalkowski]

On Tue, Aug 30, 2016 at 2:31 PM, Dima Tisnek <dimaqq at gmail.com> wrote:
> On 30 August 2016 at 14:13, Serhiy Storchaka <storchaka at gmail.com> wrote:
>>> 1. Detect length change and raise.
>>
>>
>> It would be simpler solution. But I afraid that this can break third-party
>> code that "just works" now. For example slicing a list "just works" if step
>> is 1. It can return not what the author expected if a list grows, but it
>> never crashes, and existing code can depends on current behavior. This
>> solution is not applicable in maintained versions.
>
> Serhiy,
>
> If dictionary is iterated in thread1 while thread2 changes the
> dictionary, thread1 currently raises RuntimeError.
>
> Would cloning current dict behaviour to slice with overridden
> __index__ make sense?
>
>
> I'd argue 3rd party code depends on slicing not to raise an exception,
> is same as 3rd party code depending on dict iteration not to raise and
> exception; If same container may be concurrently used in another
> thread, then 3rd party code is actually buggy. It's OK to break such
> code.
>
>
> Just my 2c.
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I'm with Dima here.

It's more complicated - if the third party rely on the code working
when one thread slices while the other thread modifies that gives
implicit atomicity requirements. Those specific requirements are very
hard to maintain across the python versions and python
implementations. Replicating the exact CPython behavior (for each
CPython version too!) is a major nightmare for such specific
scenarios.

I propose the following:

* we raise an error if detected

-or-

* we define the exact behavior what it means to modify the collection
in one thread while the other is slicing it (what do you get? what are
the guarantees? does it also apply if the list is resized?)