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

[Konstantin Osipov]

* Cyrill Gorcunov <gorcunov at gmail.com> [19/11/29 18:18]:
> > Cyrill, the difference is popen works with a pipe, not a file.
> > Unix supports non-blocking IO for pipes, and usually it doesn't
> > support it for files. 
> 
> OK, i've got what you mean. Kostya, let me express some more details
> on current implementation, maybe I simply miss something obvious:
> 
> 1) The pipes I use are opened in blocking mode, non-blocking read
>    implemented via explicit call to poll() with timeout option
>    (to be honest I'm a bit worried about signal interruption from
>     timers which libev provides, won't they interrupt poll since
>     they all are living in same thread, I need to understand this
>     moment later).
> 
>    IOW, using pipes in blocking mode and poll with timeout for
>    nonblocking read is correct solution or we shoudl use nonbloking
>    ops from the very beginning?

I suggest using non-blocking IO.

> 2) When I do various ops on popen object (say sending kill, fetching
>    status of a process and etc) I block SIGCHLD of coio thread,

Let's call this *eio* thread, please. coio is co-operative io. eio is
thread-pool-based-io. eio API was in coeio namespace first, later I moved it
to coio namespace.

>    otherwise there is a race with external users which could simply
>    kill the "command" process we're running and popen->pid no longer
>    valid, what is worse someone else could be take this pid already.

We discussed this and pid reuse is impossible unless you collect
the status of a child. You can easily mark the handle as dead as
soon as you get sigchild and collect it. I don't see any issue
here. 
> 
>    Thus I need to block signals for this sake, and now if I start
>    calling the popen helpers without entering coio thread (ie without
>    coio_custom helpers) I wont be able to block signals. If I understand
>    correctly the console is running inside own thread, no?

I don't understand this idea of blocking signals. You can't
control signal masks of all tarantool threads, so what's the point
of blocking a signal in a single thread anyway? It will get
delivered to a different thread in the same process. 

libev handles the signal masks for you already. You should do
nothing about it - just install the child handler and let it work
for you.

Or else I don't understand what you're trying to accomplish.


-- 
Konstantin Osipov, Moscow, Russia