Re: SIGCHLD traps shouldn't recurse

[L A Walsh]

On 2021/04/03 00:41, Oğuz wrote:
>  but I
> don't think it's useful at all because the number of pending traps keeps
> piling up, and there is no way to reset that number. If there is no real
> use case for recursive SIGCHLD traps (which I can't think of any), I think
> this should change; no SIGCHLD trap should be queued while a SIGCHLD trap
> is already in progress.
>   
----  
   So what happens if a child dies while you are servicing the previous
child?

From the perl ipc page:

        sub REAPER {
              my $child;
              while (($waitedpid = waitpid(-1, WNOHANG)) > 0) {
                  logmsg "reaped $waitedpid" . ($? ? " with exit $?" : "");
              }
              $SIG{CHLD} = \&REAPER; }
---
   The while loop grabs finished task stats before returning.

> With this change:
>
> diff --git a/trap.c b/trap.c
> index dd1c9a56..5ce6ab4f 100644
> --- a/trap.c
> +++ b/trap.c
> @@ -643,6 +643,8 @@ void
>  queue_sigchld_trap (nchild)
>       int nchild;
>  {
> +  if (sigmodes[SIGCHLD] & SIG_INPROGRESS)
> +    return;
>    if (nchild > 0)
>      {
>        catch_flag = 1;
>
> bash behaves this way:
>
>     $ trap uname chld
>     $ uname -sr
>     Linux 5.4.0-66-generic
>     Linux
>     $ uname -sr
>     Linux 5.4.0-66-generic
>     Linux
>     $
>
> and I think this is what average user would expect. Whether there's a
> better fix is beyond me though.
>   
----
   Looks like your uname is called twice, whereas some langs (perl)
tried to auto-cleanup such things.