Re: [PATCH 5/7] linux: Use waitid on wait4 if __NR_wait4 is not defined

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 14/11/2019 11:47, Adhemerval Zanella wrote:
> +pid_t
> +__wait4 (pid_t pid, int *stat_loc, int options, struct rusage *usage)
> +{
> +#if __NR_wait4
> +   return SYSCALL_CANCEL (wait4, pid, stat_loc, options, usage);
> +#elif defined (__ASSUME_WAITID_PID0_P_PGID)
[...]
> +# else
> +/* Linux waitid prior kernel 5.4 does not support waiting for the current
> +   process.  It would be possible to emulate it by calling getpgid for pid 0,
> +   however, it would require an additional syscall and it is inherent racy:
> +   after the current process group is received and before it is passed
> +   to waitid a signal could arrive causing the current process group to
> +   change.  */
> +# error "The kernel ABI does not provide a way to implement wait4"
> +#endif

So the only design here that I am not sure is if the best one is to trigger
a build error to avoid an architecture to not define __NR_wait4 and also 
support kernels older than 5.4 (which would not define 
__ASSUME_WAITID_PID0_P_PGID), or if it should do as generic implementation
and return ENOSYS along with a stub.

Thoughts?