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

[David Gibson]

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.

> +                 */
> +                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

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