[PATCH] Improve LinuxThreads compatibility in rfork()

[Petr Salinger]

>>> Can you, please, describe the reasoning behind the
>>>> +	        if (sig == SIGCHLD) sig = 0;
>>> line ?
>>
>> The main reason is backward compatibility.
>> The original FreeBSD code allows only to select between
>> SIGUSR1 or SIGCHLD signals.
>>
>> The our extension changes meaning of RFLINUXTHPN to select signal based on
>> bits 24-30 of passed flags instead of using SIGUSR1 every time.
>>
>> When the passed "signal" number is zero, it should behave identically
>> on plain FreeBSD and in our environment, therefore SIGUSR1 is selected.
>> The assumption is (have been) that (yet) undefined bits are zero.
>> That way we are backward compatible with original FreeBSD.
>>
>> We still need an alternative way to select "none signal is sent"
>> after child exit (under linux #0 is used).
>>
>> The SIGCHLD can be "selected" (also on original FreeBSD) by not specifying
>> RFLINUXTHPN, therefore combination of RFLINUXTHPN and passed "signal"
>> number SIGCHLD is (have been) used for "none signal".
>>
>> BTW, the opposite side is in
>>
>> http://anonscm.debian.org/viewvc/glibc-bsd/trunk/glibc-ports/kfreebsd/clone.c?view=markup
>
> I shall state that the sig == SIGCHLD case is ugly. Having the separate
> flag "do not send signal to the parent" would be much less clumsy.
> What are the requirements for the ABI stability for Debian/kFreeBSD ?
> Can this be fixed now, or is it too late ?

It should be backward compatible with one previous version.

What about in long term this:

RFLINUXTHPN bit will be renamed and will have meaning
"select signal based on bits 24-30 of passed flags"

- zero would mean "no signal"
- SIGCHLD would mean undefined
- SIGUSR1 would mean SIGUSR1

It is ABI/API breakage under original FreeBSD.
The question is how frequently RFLINUXTHPN is used under native FreeBSD
and its port collection.

And under "Debian GNU/kFreeBSD COMPAT" or 8-COMPAT
- SIGCHLD would mean "no signal"

We do not use SIGUSR1 currently, the eglibc side can detect whether
it runs under new-enough kernel and decide whether use 0 or SIGCHLD
for "no signal".

The kernel side would be something like:

     if (flags & RFLINUXTHPN)
     {
         p2->p_sigparent = RFTHPNSIGNUM(flags);
#if COMPAT8
         if (p2->p_sigparent == SIGCHLD)
            p2->p_sigparent = 0;
#endif
     }


Petr