Re: SIGCHLD traps shouldn't recurse

[Oğuz]

5 Nisan 2021 Pazartesi tarihinde L A Walsh <bash@tlinx.org> yazdı:

> 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?


That's a good question but I don't know the answer. What do you think
should happen? Plus, should SIGCHLD, when there is a trap set for it,
interrupt `wait' as other signals do?


>
> 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.
>

`uname' is called once for each `uname -sr' there. With that stupid change
I suggested, a SIGCHLD from an asynchronous job is lost if it arrives while
the trap handler for SIGCHLD is running, so it's useless anyway. Man, what
a mess


-- 
Oğuz