Re: [Qemu-devel] [PATCH v1 2/7] io: simplify websocket ping reply handling

[Eric Blake]

On 10/10/2017 10:43 AM, Daniel P. Berrange wrote:
> We must ensure we don't get flooded with ping replies if the outbound
> channel is slow. Currently we do this by keeping the ping reply in a
> separate temporary buffer and only writing it if the encoutput buffer
> is completely empty. This is overly pessimistic, as it is reasonable
> to add a ping reply to the encoutput buffer even if it has previous
> data in it, as long as that previous data doesn't include a ping
> reply.
> 
> To track this better, put the ping reply directly into the encoutput
> buffer, and then record the size of encoutput at this time in
> ping_remain. As we write encoutput to the underlying channel, we
> can decrement the ping_remain counter. Once it hits zero, we can
> accept further ping replies for transmission.
> 
> Signed-off-by: Daniel P. Berrange <address@hidden>
> ---
>  include/io/channel-websock.h |  2 +-
>  io/channel-websock.c         | 28 +++++++++++++++-------------
>  2 files changed, 16 insertions(+), 14 deletions(-)
> 

> +++ b/io/channel-websock.c
> @@ -825,11 +825,14 @@ static int 
> qio_channel_websock_decode_payload(QIOChannelWebsock *ioc,
>          }
>          return -1;
>      } else if (ioc->opcode == QIO_CHANNEL_WEBSOCK_OPCODE_PING) {
> -        /* ping frames produce an immediate reply */
> -        buffer_reset(&ioc->ping_reply);
> -        qio_channel_websock_encode_buffer(
> -            ioc, &ioc->ping_reply, QIO_CHANNEL_WEBSOCK_OPCODE_PONG,
> -            &ioc->encinput);
> +        /* ping frames produce an immediate reply, as long as we've not still
> +         * got a previous ping queued, in which case we drop the new pong */

Wouldn't that be a 'previous pong queued'?

> +        if (ioc->ping_remain == 0) {
> +            qio_channel_websock_encode_buffer(
> +                ioc, &ioc->encoutput, QIO_CHANNEL_WEBSOCK_OPCODE_PONG,
> +                &ioc->encinput);
> +            ioc->ping_remain = ioc->encoutput.offset;
> +        }

But if you change the comment, then naming the variable pong_remain may
make more sense.

But naming is a bikeshed issue, so either way,

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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