ask clone() to use the SIGCHLD as the termination signal

ask clone() to use the SIGCHLD as the termination signal

Origin: upstream, https://code.qt.io/cgit/qt/qtbase.git/commit/?id=a82032351c921c3d
Last-Update: 2020-11-19

Because of these lines in the Linux kernel (kernel/fork.c, see [1][3]):

    if (clone_flags & CLONE_VFORK)
        trace = PTRACE_EVENT_VFORK;
    else if (args->exit_signal != SIGCHLD)
        trace = PTRACE_EVENT_CLONE;
    else
        trace = PTRACE_EVENT_FORK;

Without CLONE_VFORK (which we can't use), if the exit signal isn't
SIGCHLD, the debugger will get a PTRACE_EVENT_CLONE, which makes it
think the process we're starting is a thread, not a new process. Both
gdb and lldb remain attached to the child and when it later performs an
execve(), they get mightily confused. See gdb bug report[5].

The idea of not having an exit_signal was so that no SIGCHLD would be
delivered to the parent process in the first place. That way, some
misguided SIGCHLD handler (*cough* GLib *cough*) wouldn't reap our
processes. Unfortunately, what I didn't realize was that the kernel
sends SIGCHLD anyway (see [2][4]), so this defensive measure didn't
actually work. Consequently, we can pass SIGCHLD to clone() and get the
debuggers working again.

[1] https://code.woboq.org/linux/linux/kernel/fork.c.html#_do_fork
[2] https://code.woboq.org/linux/linux/kernel/signal.c.html#do_notify_parent
[3] https://elixir.bootlin.com/linux/v5.8/source/kernel/fork.c#L2432
[4] https://elixir.bootlin.com/linux/v5.8/source/kernel/signal.c#L1925
[5] https://sourceware.org/bugzilla/show_bug.cgi?id=26562

Gbp-Pq: Name clone_sigchld.diff