Re: [Qemu-devel] [PATCH 2/5] Add SCM_RIGHTS support to unix socket character devices

[Mark McLoughlin <markmc@redhat.com>]

On Thu, 2009-08-13 at 10:20 -0600, Cam Macdonell wrote:
> > @@ -1957,6 +2004,10 @@ static void tcp_chr_read(void *opaque)
> >              tcp_chr_process_IAC_bytes(chr, s, buf, &size);
> >          if (size > 0)
> >              qemu_chr_read(chr, buf, size);
> > +        if (s->msgfd != -1) {
> > +            close(s->msgfd);
> > +            s->msgfd = -1;
> > +        }
> 
> Hi Mark,
> 
> I'm trying to understand the intended behaviour here so I can use your 
> patch.  In your comments for the patch it reads "store the file 
> descriptor for use in the chr_read() handler.  Close the file descriptor 
> if it was not used."  It seems that upon returning from the chr_read() 
> handler the descriptor is closed if s->msgfd is not set to -1.  Does 
> this mean the handler should set the fd to -1 after retrieving it if the 
> process wants to keep that fd open, otherwise it will be closed when 
> chr_read() returns?  Should tcp_get_msgfd() perhaps set  s->msgfd to -1 
> after it is retrieved?  Sort of a "read and clear" behaviour?

Yes, that would work and TBH I can't recall why I chose not to do that.
It'd be like popping the fd from a one item queue.

The current caller of get_msgfd() dup()s the fd which achieves the same
thing.

Cheers,
Mark.