Not doing so before calling sigaction(3) is "undefined" according to
POSIX[1]:
> Applications shall call either sigemptyset() or sigfillset() at least
> once for each object of type sigset_t prior to any other use of that
> object. If such an object is not initialized in this way, but is
> nonetheless supplied as an argument to any of pthread_sigmask(),
> sigaction(), sigaddset(), sigdelset(), sigismember(), sigpending(),
> sigprocmask(), sigsuspend(), sigtimedwait(), sigwait(), or
> sigwaitinfo(), the results are undefined.
The use of designated initializers means that sa_mask members were
still being initialized, but sigset_t is an opaque type and implicit
initialization doesn't necessarily produce the same results as using
sigemptyset(3) (although it typically does on most implementations).
[1]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigaddset.html
Allow the caller to pass the same FD (for example, a single /dev/null
FD) to spawn().
All we need to do to handle this correctly is ensure we don’t try to
close the same FD multiple times.
slave: no need to restore signal handlers; they are automatically
restored as long as they are not SIG_IGN (which they never are in
foot).
spawn(): restore signal mask after fork. This fixes an issue where a
terminal spawned with ctrl+shift+n did not terminate when its shell
exited.
Closes#366
When calling ‘reaper_add()’, the caller can provide a callback. If
non-NULL, the reaper will call the callback to handle the actual
reaping.
If the callback is NULL, or if it returns false, the reaper reaps the
child process.
