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

[Simon Marchi via Gdb-patches <gdb-patches@sourceware.org>]

On 2021-06-26 9:41 a.m., John Baldwin wrote:
> 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))

SIG -> Sig

>          sigwait (&set, nullptr);
> #endif
>      }
> 
> (Ironically, sigwait() was added to POSIX in the same version of the spec as
> sigtimedwait().)
> 

Thanks.  At least that change fixes that particular build issue, but I
can't tell if it has the correct behavior though (I haven't looked at
what sigtimedwait / sigwait do yet...).

Simon