[PATCH] posix: Do not use WNOHANG in waitpid call for Linux posix_spawn

[Szabolcs Nagy]

On 23/10/17 07:32, Florian Weimer wrote:
> On 10/22/2017 10:51 PM, Adhemerval Zanella wrote:
>> As shown in some buildbot issues on aarch64 and powerpc, calling
>> clone (VFORK) and waitpid (WNOHANG) does not guarantee the child
>> is ready to be collected.  This patch changes the call back to 0
>> as before fe05e1cb6d64 fix.
> 
> I see it on x86-64, too.  It does look like a kernel bug.
> 
>> This change can lead to the scenario 4.3 described in the commit,
>> where the waitpid call can hang undefinitely on the call.  However
>> this is also a very unlikely and also undefinied situation where
>> both the caller is trying to terminate a pid before posix_spawn
>> returns and the race pid reuse is triggered.  I don't see how to
>> correct handle this specific situation within posix_spawn.
> 
> Agreed.  I wish we could do better here, but it seems we can't.
> 

musl writes a close-on-exec pipe in the child on error,
reading it in the parent tells if the child died before
exec or not. (so the waitpid can be made precise)

VFORK is not precise (i think under ptrace parent can
continue before child execs) so using close-on-exec
fd is a better way to sync with exec.