Re: [PATCH v2] qemu-nbd: Use SOMAXCONN for socket listen() backlog

[Daniel P . Berrangé]

On Fri, Feb 05, 2021 at 12:57:05PM -0600, Eric Blake wrote:
> Our default of a backlog of 1 connection is rather puny, particularly
> for scenarios where we expect multiple listeners to connect (such as
> qemu-nbd -e X).  This is especially important for Unix sockets, as a
> definite benefit to clients: at least on Linux, a client trying to
> connect to a Unix socket with a backlog gets an EAGAIN failure with no
> way to poll() for when the backlog is no longer present short of
> sleeping an arbitrary amount of time before retrying.
> 
> See https://bugzilla.redhat.com/1925045 for a demonstration of where
> our low backlog prevents libnbd from connecting as many parallel
> clients as it wants.
> 
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
> 
> v2: target the correct API used by qemu-nbd, rather than an unrelated
> legacy wrapper [Dan]
> 
>  qemu-nbd.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/qemu-nbd.c b/qemu-nbd.c
> index 608c63e82a25..cd20ee73be19 100644
> --- a/qemu-nbd.c
> +++ b/qemu-nbd.c
> @@ -965,7 +965,8 @@ int main(int argc, char **argv)
>      server = qio_net_listener_new();
>      if (socket_activation == 0) {
>          saddr = nbd_build_socket_address(sockpath, bindto, port);
> -        if (qio_net_listener_open_sync(server, saddr, 1, &local_err) < 0) {
> +        if (qio_net_listener_open_sync(server, saddr, SOMAXCONN,
> +                                       &local_err) < 0) {

This addresses qemu-nbd, but surely we want to be consistent with the
QMP  nbd-server-start impl too, in blockdev-nbd.c

>              object_unref(OBJECT(server));
>              error_report_err(local_err);
>              exit(EXIT_FAILURE);

Regards,
Daniel
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