Re: [Qemu-devel] [PATCH 1/5] oslib-posix: add qemu_pipe_non_block

[Alon Levy <alevy@redhat.com>]

> On 06/04/2013 02:23 PM, Alon Levy wrote:
> > Used by the followin patch.
> 
> s/followin/following/

Thanks.

> 
> >  
> > +int qemu_pipe_non_block(int pipefd[2])
> > +{
> > +    int ret;
> > +
> > +    ret = qemu_pipe(pipefd);
> 
> qemu_pipe() already uses pipe2() when available; it seems like it would
> be nicer to use pipe2's O_NONBLOCK option directly in one syscall (where
> supported) instead of having to make additional syscalls after the fact.
>  Would it just be smarter to change the signature of qemu_pipe() to add
> a bool block parameter, and then change the 5 existing callers to pass
> false with your later patch in the series passing true, and do it
> without creating a new wrapper?

Answered below.

> 
> > +    if (ret) {
> > +        return ret;
> > +    }
> > +    if (fcntl(card->pipe[0], F_SETFL, O_NONBLOCK) == -1) {
> > +        return -errno;
> > +    }
> > +    if (fcntl(card->pipe[1], F_SETFL, O_NONBLOCK) == -1) {
> > +        return -errno;
> 
> Leaks fds.  If you're going to report error, then you must close the fds
> already created.

As Peter pointed out, I should not go here, so I'll drop these checks, instead doing naked fcntl calls, so no fd leak possible (no returns).

> 
> > +    }
> > +    if (fcntl(card->pipe[0], F_SETOWN, getpid()) == -1) {
> > +        return -errno;
> 
> Same comment about fd leaks.
> 
> This part seems like a useful change, IF you plan on using SIGIO and
> SIGURG signals; and it is something which pipe2() cannot optimize, so I
> can see why you are adding a new function instead of changing
> qemu_pipe() and adjust all its callers to pass an additional parameter.
>  But are you really planning on using SIGIO/SIGURG?

I don't plan on using those signals, so I'll add a parameter instead.

> 
> Furthermore, this is undefined behavior.  According to POSIX, use of
> F_SETOWN is only portable on sockets, not pipes.  It may work on Linux,
> but you'll need to be aware of what it does on other platforms.
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/fcntl.html
> --
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
> 
>