Re: [PATCH 3/4] scoped_ignore_signal: Use sigprocmask+sigtimedwait instead of signal

[John Baldwin <jhb@FreeBSD.org>]

On 6/26/21 5:35 AM, Simon Marchi via Gdb-patches wrote:
> On 2021-06-15 7:14 a.m., Pedro Alves wrote:
>> The problem with using signal(...) to temporarily ignore a signal, is
>> that that changes the the signal disposition for the whole process.
>> If multiple threads do it at the same time, you have a race.
>>
>> Fix this by using sigprocmask + sigtimedwait to implement the ignoring
>> instead, if available, which I think probably means everywhere except
>> Windows nowadays.  This way, we only change the signal mask for the
>> current thread, so there's no race.
> 
> I tried to build on macOS today and got:
> 
> 
>        CXX    compile/compile.o
>      In file included from /Users/smarchi/src/binutils-gdb/gdb/compile/compile.c:46:
>      /Users/smarchi/src/binutils-gdb/gdb/../gdbsupport/scoped_ignore_signal.h:69:4: error: use of undeclared identifier 'sigtimedwait'
>                sigtimedwait (&set, nullptr, &zero_timeout);
>                ^
> 
> I didn't have time to dig yet.

It looks like macOS doesn't implement either sigtimedwait() or sigwaitinfo()
which are part of POSIX from 1996 (*sigh*); however, macOS does provide
sigpending() and sigwait(), so perhaps we can do something like:

    if (ConsumePending)
      {
#ifdef HAVE_SIGTIMEDWAIT
        const timespec zero_timeout = {};

        sigtimedwait (&set, nullptr, &zero_timeout);
#else
        sigset_t pending;

        sigpending (&pending);
        if (sigismember (&pending, SIG))
          sigwait (&set, nullptr);
#endif
      }

(Ironically, sigwait() was added to POSIX in the same version of the spec as
sigtimedwait().)

-- 
John Baldwin