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

[Eric Blake <eblake@redhat.com>]

On 2/8/21 4:00 AM, Daniel P. Berrangé wrote:
> 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
> 

Good point, I'll send v3.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org