Re: [Qemu-devel] [PATCH v1 1/1] io/channel-watch.c: Only select on what we are actually waiting for

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Jul 13, 2017 at 03:15:49AM -0700, Alistair Francis wrote:
> When calling WAEventSelect() only wait on events as specified by the
> condition variable. This requires that the condition variable is set
> correctly for the specific events that we need to wait for.

Can you describe the actual problem / buggy behaviour you were seeing
with the current code.

> Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
> ---
> 
>  io/channel-watch.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/io/channel-watch.c b/io/channel-watch.c
> index 8640d1c464..d80722f496 100644
> --- a/io/channel-watch.c
> +++ b/io/channel-watch.c
> @@ -286,9 +286,21 @@ GSource *qio_channel_create_socket_watch(QIOChannel *ioc,
>      QIOChannelSocketSource *ssource;
>  
>  #ifdef WIN32
> -    WSAEventSelect(socket, ioc->event,
> -                   FD_READ | FD_ACCEPT | FD_CLOSE |
> -                   FD_CONNECT | FD_WRITE | FD_OOB);
> +    long bitmask = 0;
> +
> +    if (condition & (G_IO_IN | G_IO_PRI)) {
> +        bitmask |= FD_READ | FD_ACCEPT;
> +    }
> +
> +    if (condition & G_IO_HUP) {
> +        bitmask |= FD_CLOSE;
> +    }
> +
> +    if (condition & G_IO_OUT) {
> +        bitmask |= FD_WRITE | FD_CONNECT;
> +    }
> +
> +    WSAEventSelect(socket, ioc->event, bitmask);
>  #endif

I think the problem with doing this is that WSAEventSelect is a global
setting that applies to the socket handle. If you use qio_channel_create_watch
twice on the same socket with different events, the second watch will break
the first watch by potentially discarding events that it requested.


Regards,
Daniel
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