From 1107799dade18adccfca5097341b6dfb4977e69e Mon Sep 17 00:00:00 2001
From: Eric Blake <eblake@redhat.com>
Date: Mon, 22 Feb 2021 21:34:55 -0500
Subject: [PATCH 19/54] qemu-nbd: Use SOMAXCONN for socket listen() backlog
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

RH-Author: Eric Blake <eblake@redhat.com>
Message-id: <20210222213455.320104-2-eblake@redhat.com>
Patchwork-id: 101192
O-Subject: [RHEL-AV-8.4.0 qemu-kvm PATCH 1/1] qemu-nbd: Use SOMAXCONN for socket listen() backlog
Bugzilla: 1925345
RH-Acked-by: Richard Jones <rjones@redhat.com>
RH-Acked-by: Sergio Lopez Pascual <slp@redhat.com>
RH-Acked-by: Daniel P. Berrange <berrange@redhat.com>

Our default of a backlog of 1 connection is rather puny; it gets in
the way when we are explicitly allowing multiple clients (such as
qemu-nbd -e N [--shared], or nbd-server-start with its default
"max-connections":0 for unlimited), but is even a problem when we
stick to qemu-nbd's default of only 1 active client but use -t
[--persistent] where a second client can start using the server once
the first finishes.  While the effects are less noticeable on TCP
sockets (since the client can poll() to learn when the server is ready
again), it is definitely observable on Unix sockets, where on Linux, a
client will fail with EAGAIN and no recourse but to sleep an arbitrary
amount of time before retrying if the server backlog is already full.

Since QMP nbd-server-start is always persistent, it now always
requests a backlog of SOMAXCONN; meanwhile, qemu-nbd will request
SOMAXCONN if persistent, otherwise its backlog should be based on the
expected number of clients.

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>
CC: qemu-stable@nongnu.org
Message-Id: <20210209152759.209074-2-eblake@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 582d4210eb2f2ab5baac328fe4b479cd86da1647)
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
---
 blockdev-nbd.c |  7 ++++++-
 qemu-nbd.c     | 10 +++++++++-
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/blockdev-nbd.c b/blockdev-nbd.c
index d8443d235b..b264620b98 100644
--- a/blockdev-nbd.c
+++ b/blockdev-nbd.c
@@ -134,7 +134,12 @@ void nbd_server_start(SocketAddress *addr, const char *tls_creds,
     qio_net_listener_set_name(nbd_server->listener,
                               "nbd-listener");
 
-    if (qio_net_listener_open_sync(nbd_server->listener, addr, 1, errp) < 0) {
+    /*
+     * Because this server is persistent, a backlog of SOMAXCONN is
+     * better than trying to size it to max_connections.
+     */
+    if (qio_net_listener_open_sync(nbd_server->listener, addr, SOMAXCONN,
+                                   errp) < 0) {
         goto error;
     }
 
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 1d337b7504..ce1dc43d69 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -970,8 +970,16 @@ int main(int argc, char **argv)
 
     server = qio_net_listener_new();
     if (socket_activation == 0) {
+        int backlog;
+
+        if (persistent) {
+            backlog = SOMAXCONN;
+        } else {
+            backlog = MIN(shared, SOMAXCONN);
+        }
         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, backlog,
+                                       &local_err) < 0) {
             object_unref(OBJECT(server));
             error_report_err(local_err);
             exit(EXIT_FAILURE);
-- 
2.27.0