[Python-ideas] New PyThread_tss_ C-API for CPython

[Erik Bray]

On Mon, Dec 19, 2016 at 1:11 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 17 December 2016 at 03:51, Antoine Pitrou <solipsis at pitrou.net> wrote:
>>
>> On Fri, 16 Dec 2016 13:07:46 +0100
>> Erik Bray <erik.m.bray at gmail.com> wrote:
>> > Greetings all,
>> >
>> > I wanted to bring attention to an issue that's been languishing on the
>> > bug tracker since last year, which I think would best be addressed by
>> > changes to CPython's C-API.  The original issue is at
>> > http://bugs.python.org/issue25658, but I have made an effort below in
>> > a sort of proto-PEP to summarize the problem and the proposed
>> > solution.
>> >
>> > I haven't written this up in the proper PEP format because I want to
>> > see if the idea has some broader support first, and it's also not
>> > clear to me whether C-API changes (especially to undocumented APIs)
>> > even require their own PEP.
>>
>> This is a nice detailed write-up and I'm in favour of the proposal.
>
>
> Likewise - we know the status quo isn't right, and the proposed change
> addresses that. In reviewing the patch on the tracker, the one downside I've
> found is that due to "pthread_key_t" being an opaque type with no defined
> sentinel, the consuming code in _tracemalloc.c and pystate.c needed to add
> separate boolean flag variables to track whether or not the key had been
> created. (The pthread examples at
> http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_key_create.html
> use pthread_once for a similar effect)
>
> I don't see any obvious way around that either, as even using a small struct
> for native pthread TLS keys would still face the problem of how to
> initialise the pthread_key_t field.

Hmm...fair point that it's not pretty.  One way around it, albeit
requiring more work/complexity, would be to extend this proposal to
add a new function analogous to pthread_once--say--PyThread_call_once,
and an associated Py_once_flag_t