spa: v4l2: Use systemd-logind to listen for access changes

There is a race between logind applying ACLs to allow the active session
of a locally present user access to devices with a udev uaccess tag
(like /dev/video# nodes) getting applied vs wireplumber/pipewire starting.

Wireplumber/pipewire are part of the (user) default.target, which gets
started as soon as a user logs in and systemd --user is started for that
user, where as logind only starts applying the ACLs after the gnome-shell
associated logind session has been created.

This race may cause pipewire to not see v4l2 video sources at login,
this can be reproduced with these steps:

1. sudo setfacl --remove-all /dev/video*
2. systemctl --user restart pipewire
3. Now wp-ctl status will not show any video devices
   (like if pipewire was started before the udev uaccess ACLs got applied)
4. Do a switch to another (test) user without logging out, e.g. in GNOME
   go to the top right system menu press the power on/off icon and select
   "Switch User..."
5. Switch back to your normal user. Run getfacl /dev/video0 this will show
   your user has access now.
6. wp-ctl status should show the camera now, but it does not.

Fix pipewire not seeing v4l2 sources in this case by making v4l2-udev
monitor systemd-logind session changes and redoing the access() checks
on /dev/video# nodes when the session changes.

Closes: #3539
Closes: #3960
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
(cherry picked from commit 2a6ba61264)

spa/plugins/v4l2/v4l2-udev.c

@@ -24,8 +24,13 @@
 #include <spa/monitor/device.h>
 #include <spa/monitor/utils.h>
 
+#include "config.h"
 #include "v4l2.h"
 
+#ifdef HAVE_SYSTEMD
+#include <systemd/sd-login.h>
+#endif
+
 #define MAX_DEVICES	64
 
 enum action {
@@ -61,6 +66,10 @@ struct impl {
 
 	struct spa_source source;
 	struct spa_source notify;
+#ifdef HAVE_SYSTEMD
+	struct spa_source logind;
+	sd_login_monitor *logind_monitor;
+#endif
 };
 
 static int impl_udev_open(struct impl *this)
@@ -463,6 +472,12 @@ static int start_inotify(struct impl *this)
 {
 	int notify_fd;
 
+#ifdef HAVE_SYSTEMD
+	/* Do not use inotify when using logind session monitoring */
+	if (this->logind_monitor)
+		return 0;
+#endif
+
 	if (this->notify.fd != -1)
 		return 0;
 
@@ -480,6 +495,65 @@ static int start_inotify(struct impl *this)
 	return 0;
 }
 
+#ifdef HAVE_SYSTEMD
+static void impl_on_logind_events(struct spa_source *source)
+{
+	struct impl *this = source->data;
+
+	/* Recheck access on all v4l2 devices on logind session changes */
+	for (size_t i = 0; i < this->n_devices; i++)
+		process_device(this, ACTION_CHANGE, &this->devices[i]);
+
+	sd_login_monitor_flush(this->logind_monitor);
+}
+
+static int start_logind(struct impl *this)
+{
+	int res;
+
+	if (this->logind_monitor)
+		return 0;
+
+	/* If we are not actually running logind become a NOP */
+	if (access("/run/systemd/seats/", F_OK) < 0)
+		return 0;
+
+	res = sd_login_monitor_new("session", &this->logind_monitor);
+	if (res < 0)
+		return res;
+
+	spa_log_info(this->log, "start logind monitoring");
+
+	this->logind.func = impl_on_logind_events;
+	this->logind.data = this;
+	this->logind.fd = sd_login_monitor_get_fd(this->logind_monitor);
+	this->logind.mask = SPA_IO_IN | SPA_IO_ERR;
+
+	spa_loop_add_source(this->main_loop, &this->logind);
+
+	return 0;
+}
+
+static void stop_logind(struct impl *this)
+{
+	if (this->logind_monitor) {
+		spa_loop_remove_source(this->main_loop, &this->logind);
+		sd_login_monitor_unref(this->logind_monitor);
+		this->logind_monitor = NULL;
+	}
+}
+#else
+/* Stubs to avoid more ifdefs below */
+static int start_logind(struct impl *this)
+{
+	return 0;
+}
+
+static void stop_logind(struct impl *this)
+{
+}
+#endif
+
 static void impl_on_fd_events(struct spa_source *source)
 {
 	struct impl *this = source->data;
@@ -527,6 +601,9 @@ static int start_monitor(struct impl *this)
 	spa_log_debug(this->log, "monitor %p", this->umonitor);
 	spa_loop_add_source(this->main_loop, &this->source);
 
+	if ((res = start_logind(this)) < 0)
+		return res;
+
 	if ((res = start_inotify(this)) < 0)
 		return res;
 
@@ -545,6 +622,7 @@ static int stop_monitor(struct impl *this)
 	this->umonitor = NULL;
 
 	stop_inotify(this);
+	stop_logind(this);
 
 	return 0;
 }
@@ -691,6 +769,9 @@ impl_init(const struct spa_handle_factory *factory,
 
 	this = (struct impl *) handle;
 	this->notify.fd = -1;
+#ifdef HAVE_SYSTEMD
+	this->logind_monitor = NULL;
+#endif
 
 	this->log = spa_support_find(support, n_support, SPA_TYPE_INTERFACE_Log);
 	this->main_loop = spa_support_find(support, n_support, SPA_TYPE_INTERFACE_Loop);