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[PATCH] sockets: Use SOMAXCONN for Unix socket listen()

From: Eric Blake
Subject: [PATCH] sockets: Use SOMAXCONN for Unix socket listen()
Date: Thu, 4 Feb 2021 16:20:18 -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).  For Unix sockets, there's no real harm in supporting
a larger backlog, and a definite benefit to the 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

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>
 util/qemu-sockets.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
index 8af0278f15c6..a7573e9f0fda 100644
--- a/util/qemu-sockets.c
+++ b/util/qemu-sockets.c
@@ -1059,7 +1059,7 @@ int unix_listen(const char *str, Error **errp)

     saddr = g_new0(UnixSocketAddress, 1);
     saddr->path = g_strdup(str);
-    sock = unix_listen_saddr(saddr, 1, errp);
+    sock = unix_listen_saddr(saddr, SOMAXCONN, errp);
     return sock;

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