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

From: Eric Blake
Subject: [PATCH v2] qemu-nbd: Use SOMAXCONN for socket listen() backlog
Date: Fri, 5 Feb 2021 12:57:05 -0600

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) {

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