[Tarantool-patches] [PATCH 1/5] popen: Introduce a backend engine

[Konstantin Osipov]

* Cyrill Gorcunov <gorcunov at gmail.com> [19/11/30 07:03]:
> > but before we reach kill() this process exited by self or killed
> > by a user on the node. The signal handler sets pid = -1 and we
> > call kill(-1). Which is wrong of course.
> 
> You know, without signal blocking I fear we simply won't be able
> to tack children properly. Look here is an example

First of all, thanks for the explanation - now I understand the
problem at least.

> 	pid = vfork();
> 	if (pid == 0) {
> 		_exit(1);
> 
> 		here sigchld already delivered to the libev
> 		and it reaped it, vanishing from the system
> 		so that anyother application can reuse it

No, this is not how it works. Yes, the signal itself *may* 
be delivered asynchronously (*sometimes*), but the callback is called
synchronously. In other words, when you do a vfork()
and check the non-zero pid return value you're executing an
existing libev callback. No other callback will get invoked until
you finish with your callback.

Even though it's an implementation detail, the signal is *also*
delivered synchronously on Linux. It is sent to signal_fd, and 
will be processed only upon the next iteration of the event loop,
when all activated callbacks are finished. 

So you can safely assume that your pid will not get lost.
And you should not assume - you should test it.

> 	} else {
> 		ev_child_init(&handle->ev_sigchld, ev_sigchld_cb, pid, 0);
> 		ev_child_start(EV_DEFAULT_ &handle->ev_sigchld);
> 
> 		but pid already dead and reused by someone else,
> 		as I said wait() called under the hood, we simply
> 		don't control it
> 	}
> 
> and without signal blocking we can't order pid livetime anyhow.
> You know in my first versions I setup ev_child_init with pid=0
> and been expecting the sgnal handler catches _any_ sigchld but
> it didn't work :/ Thus early _exit(1) in child process was
> simply invisible to the rest of tarantool code.


-- 
Konstantin Osipov, Moscow, Russia