[newlib-cygwin] kill(pid, sig) before waitpid() returns -1 for sig != 0

[Corinna Vinschen <corinna at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=86f79af827729f3968d8b3b8f860ac29d200da0d

commit 86f79af827729f3968d8b3b8f860ac29d200da0d
Author: Erik Bray <erik.m.bray@gmail.com>
Date:   Thu Aug 11 15:57:53 2016 +0200

    kill(pid, sig) before waitpid() returns -1 for sig != 0
    
    This is a followup to a report back in 2011 about essentially the same issue:
    
    https://cygwin.com/ml/cygwin/2011-04/msg00031.html
    
    The same test program in that report demonstrates the issue, but with
    kill sending any non-zero signal.  To reiterate, the problem here is
    POSIX compliance with respect to sending signals to zombie processes.
    
    http://pubs.opengroup.org/onlinepubs/9699919799/functions/kill.html
    claims:
    
      Existing implementations vary on the result of a kill() with pid
      indicating an inactive process (a terminated process that has not been
      waited for by its parent). Some indicate success on such a call
      (subject to permission checking), while others give an error of
      [ESRCH].  Since the definition of process lifetime in this volume of
      POSIX.1-2008 covers inactive processes, the [ESRCH] error as described
      is inappropriate in this case. In particular, this means that an
      application cannot have a parent process check for termination of a
      particular child with kill().  (Usually this is done with the null
      signal; this can be done reliably with waitpid().)
    
    In response to the originally issue, this was fixed *specifically* for
    the case of kill(pid, 0).  But my reading of the above is that kill()
    should return 0 in this case regardless of the signal (modulo
    permissions, etc.).  On Linux, for example, when calling kill with pid
    of a zombie process the kernel will happily deliver the signal to the
    relevant task_struct; it will just never be acted on since the task
    will never run again.
    
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/signal.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/winsup/cygwin/signal.cc b/winsup/cygwin/signal.cc
index ff101e3..d819e77 100644
--- a/winsup/cygwin/signal.cc
+++ b/winsup/cygwin/signal.cc
@@ -260,7 +260,7 @@ _pinfo::kill (siginfo_t& si)
 	}
       this_pid = pid;
     }
-  else if (si.si_signo == 0 && this && process_state == PID_EXITED)
+  else if (this && process_state == PID_EXITED)
     {
       this_process_state = process_state;
       this_pid = pid;