Clarification of pthread_cleanup_push() needed

[Florian Weimer]

* Richard Weinberger:

> On Tue, Apr 21, 2020 at 11:41 PM Florian Weimer <fw@deneb.enyo.de> wrote:
>>
>> * Richard Weinberger via Libc-help:
>>
>> > But NPTL implements thread cancellation with signals, if I'm not
>> > completely mistaken the cleanup routine will run in signal context
>> > then.
>>
>> Deferred cancellation only occurs in signal context if the
>> cancellation is acted upon from within a signal handler.  For the
>> signal handler case, whether cancellation handlers are restricted to
>> async-signal-safe function calls depends on the type of signal (some
>> are synchronous, not asynchronous) and what is interrupted by the
>> signal (for asynchronous signals).
>>
>> Asynchronous cancellation has even more constraints than asynchronous
>> signal safety, but it is rarely used.
>
> I should have noted that I'm using asynchronous cancellation.
> Which constraints are these?

See pthread_setcanceltype(3):

  Functions that can be safely asynchronously canceled are called
  async-cancel-safe functions.  POSIX.1-2001 and POSIX.1-2008 require
  only that pthread_cancel(3), pthread_setcancelstate(), and
  pthread_setcanceltype() be async-cancel-safe.  In general, other
  library functions can't be safely called from an asynchronously
  cancelable thread.

The manual pages and the glibc manual also contain information about
AC-safety, but you cannot rely on them.  They depend on implementation
details which may change within release branches.