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From: Erik Bray <erik DOT m DOT bray AT gmail DOT com>
Date: Thu, 11 Aug 2016 11:51:39 +0200
Message-ID: <CAOTD34Yv+namnsv+QNL2F-_c+W1-3ZdxZsgPQP8nxBEPNndcgw@mail.gmail.com>
Subject: 2.5.1: kill(pid, sig) before waitpid() returns -1 for sig != 0
To: cygwin AT cygwin DOT com
Content-Type: text/plain; charset=UTF-8

Hi all,

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:

> 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.

The below (untested) patch demonstrates the change I'm suggesting,
though I don't know what other code, if any, might be involved in
this.

Please CC me on any replies since I'm not subscribed to the list.

Thanks,
Erik

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;

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