Re: [Qemu-devel] [PATCH v4 11/47] Return path: socket_writev_buffer: Block even on non-blocking fd's

[Dr. David Alan Gilbert]

* David Gibson (address@hidden) wrote:
> On Fri, Oct 03, 2014 at 06:47:17PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> > 
> > The return path uses a non-blocking fd so as not to block waiting
> > for the (possibly broken) destination to finish returning a message,
> > however we still want outbound data to behave in the same way and block.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > ---
> >  qemu-file.c | 39 +++++++++++++++++++++++++++++++++++----
> >  1 file changed, 35 insertions(+), 4 deletions(-)
> > 
> > diff --git a/qemu-file.c b/qemu-file.c
> > index 7393415..57eabd8 100644
> > --- a/qemu-file.c
> > +++ b/qemu-file.c
> > @@ -85,12 +85,43 @@ static ssize_t socket_writev_buffer(void *opaque, 
> > struct iovec *iov, int iovcnt,
> >      QEMUFileSocket *s = opaque;
> >      ssize_t len;
> >      ssize_t size = iov_size(iov, iovcnt);
> > +    ssize_t offset = 0;
> > +    int     err;
> >  
> > -    len = iov_send(s->fd, iov, iovcnt, 0, size);
> > -    if (len < size) {
> > -        len = -socket_error();
> > +    while (size > 0) {
> > +        len = iov_send(s->fd, iov, iovcnt, offset, size);
> > +
> > +        if (len > 0) {
> > +            size -= len;
> > +            offset += len;
> > +        }
> > +
> > +        if (size > 0) {
> > +            err = socket_error();
> > +
> > +            if (err != EAGAIN) {
> > +                error_report("socket_writev_buffer: Got err=%d for 
> > (%zd/%zd)",
> > +                             err, size, len);
> > +                /*
> > +                 * If I've already sent some but only just got the error, I
> > +                 * could return the amount validly sent so far and wait 
> > for the
> > +                 * next call to report the error, but I'd rather flag the 
> > error
> > +                 * immediately.
> 
> Is that safe?  This gives the caller no means to detect a partially
> completed send.

Well I'm returning the -err, so the caller knows something has gone wrong - it 
just
doesn't know whether it managed to send some part of the data before the 
failure.

Dave

> 
> > +                 */
> > +                return -err;

> > +            }
> > +
> > +            /* Emulate blocking */
> > +            GPollFD pfd;
> > +
> > +            pfd.fd = s->fd;
> > +            pfd.events = G_IO_OUT | G_IO_ERR;
> > +            pfd.revents = 0;
> > +            g_poll(&pfd, 1 /* 1 fd */, -1 /* no timeout */);
> > +        }
> >      }
> > -    return len;
> > +
> > +    return offset;
> >  }
> >  
> >  static int socket_get_fd(void *opaque)
> 
> -- 
> David Gibson                  | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au        | minimalist, thank you.  NOT _the_ 
> _other_
>                               | _way_ _around_!
> http://www.ozlabs.org/~dgibson


--
Dr. David Alan Gilbert / address@hidden / Manchester, UK