pthread_cancel async-signal-safe

[Adhemerval Zanella]

On 04/12/2020 15:29, Florian Weimer wrote:
> * Adhemerval Zanella via Libc-alpha:
> 
>> Florian, I recall that you had investigate some issue regarding C++
>> exceptions and signal handler that you presented on previous GNU 
>> Cauldron, so I wonder if you might have an idea how we can fix it.
> 
> I didn't realize that pthread_cancel was required to be
> async-signal-safe.  We don't have an async-signal-safe dlopen and we
> can't block signals during dlopen because that breaks the web browser
> sandboxes.
> 
> Are sure that pthread_cancel is on the list?  I don't see it in
> signal-safety(7).
> 

It is on the 2.9.5 Thread Cancellation at 'Async-Cancel Safety' entry
in POSIX manual [1]:

  "The pthread_cancel(), pthread_setcancelstate(), and pthread_setcanceltype() 
   functions are defined to be async-cancel safe."

Also, neither pthread_setcancelstate nor pthread_setcanceltype are 
async-signal-safe due __do_cancel usage (which calls _Unwind_ForcedUnwind
which in turn might call pthread_cancel_init).

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html