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

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 23/10/17 11:40, Florian Weimer wrote:
> On 10/23/2017 12:38 PM, Szabolcs Nagy wrote:
>> 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)
> 
> How so?  A wildcard wait can still collect the PID, which is the problem we were trying to defend against with
> the WNOHANG flag.
> 

ah i misunderstood the problem, i have no solution to that.
(that just tells us that doing wild card wait is a bug,
may be we should remove that from the test case?)