Re: wait4(2) do not fail with WNOHANG if there is no child

[Robert Elz <kre%munnari.OZ.AU@localhost>]

    Date:        Sun, 6 Nov 2016 10:39:22 +0100
    From:        Nicolas Joly <njoly%pasteur.fr@localhost>
    Message-ID:  <20161106093922.GA10433%issan.sis.pasteur.fr@localhost>

  | Just tested it on 7.0.2 NetBSD/amd64 under qemu, and it works as
  | expected. Looks like a -current regression.

In -current, the primary wait*() interface is wait6() - which is
posix waitid() (also available) plus the extra args needed to allow
it to also implement wait() wait3() waitpid() and wait4().

Posix is less clear on whether waitid() is supposed to return 0 or
ECHILD when WNOHANG is set, and there are no (applicable) children
(none that match the pid/flags specified - which includes no children
at all of course.)

	If WNOHANG was specified and status is not available for any
	process specified by idtype and id, 0 shall be returned.

but also ...

	[ECHILD]
  	  The calling process has no existing unwaited-for child processes.

but generally, the error values only apply when the sys call is stated
to return an error, which waitid() does not specify when WNOHANG is
set (unlike it does for waitpid()).

We haven't yet switched to implementing wait4() (and the other older calls
all of which use the wait4() interface) via wait6(), so the question now
is, does this get changed just for the wait4() interface, or for wait6()
(and hence waitid()) as well.

kre