From b440e6e9d8574eb08f1ee368d0bcafc73197a555 Mon Sep 17 00:00:00 2001
From: Li-Wen Hsu <lwhsu@FreeBSD.org>
Date: Sat, 11 Jul 2026 02:13:13 +0800
Subject: [PATCH] udev-monitor: make the monitor fd non-blocking

systemd's libudev documents that the monitor socket is non-blocking by
default:

https://github.com/systemd/systemd/blob/v261/src/libudev/libudev-monitor.c#L228-L231

It also creates the monitor socket with SOCK_NONBLOCK:

https://github.com/systemd/systemd/blob/v261/src/libsystemd/sd-device/device-monitor.c#L179-L183

libudev-devd currently uses a blocking pipe. Once all pending events
have been consumed, another call to udev_monitor_receive_device()
blocks instead of returning NULL with EAGAIN.

This was observed in KWin, where display hotplug handling stalled after
disconnecting and reconnecting an external display in a KDE/Wayland
session.

Set O_NONBLOCK on the read end returned by udev_monitor_get_fd(). Keep
the internal write end blocking to preserve the current producer and
backpressure behavior.

Sponsored by:	The FreeBSD Foundation
---
 udev-monitor.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/udev-monitor.c b/udev-monitor.c
index 042e67a..6d295da 100644
--- a/udev-monitor.c
+++ b/udev-monitor.c
@@ -59,6 +59,20 @@ struct udev_monitor {
 	pthread_t thread;
 };
 
+static int
+udev_monitor_set_fd_nonblock(int fd)
+{
+	int flags;
+
+	flags = fcntl(fd, F_GETFL);
+	if (flags == -1)
+		return (-1);
+	if ((flags & O_NONBLOCK) != 0)
+		return (0);
+
+	return (fcntl(fd, F_SETFL, flags | O_NONBLOCK));
+}
+
 LIBUDEV_EXPORT struct udev_device *
 udev_monitor_receive_device(struct udev_monitor *um)
 {
@@ -226,6 +240,13 @@ udev_monitor_new_from_netlink(struct udev *udev, const char *name)
 		free(um);
 		return (NULL);
 	}
+	if (udev_monitor_set_fd_nonblock(um->fds[0]) == -1) {
+		ERR("fcntl failed");
+		close(um->fds[0]);
+		close(um->fds[1]);
+		free(um);
+		return (NULL);
+	}
 
 	um->udev = udev;
 	_udev_ref(udev);