loop/010: fix the flood udev events during the test

There still have many udev events on some special server like [1], let's
try to stop systemd-udevd before the test, and start systemd-udevd after
the test, you can find more details on the link.

[1]
$ uname -r
6.17.0-0.rc2.250821g068a56e56fa8.27.fc44.aarch64
$ free -g
               total        used        free      shared  buff/cache   available
Mem:              68           0          65           0           2          67
Swap:              7           0           7
$ nproc
8
$ rpm -qa systemd
systemd-257.7-1.fc43.aarch64
$ ./check loop/010; time udevadm settle --timeout 1200
loop/010 (check stale loop partition)                        [passed]
    runtime  91.214s  ...  91.224s

real	19m37.788s
user	0m0.002s
sys	0m0.006s

Link: #181
Signed-off-by: Yi Zhang <[email protected]>

Author: yizhanglinux

common/rc

@@ -544,6 +544,37 @@ _systemctl_stop() {
 	done
 }
 
+_systemctl_unit_is_active() {
+	local unit="$1"
+
+	if command -v systemctl &>/dev/null &&
+			systemctl is-active --quiet "$unit"; then
+		return 0
+	fi
+	return 1
+}
+
+_systemd_stop_udevd() {
+	if _systemctl_unit_is_active "systemd-udevd"; then
+		systemctl --quiet stop systemd-udevd
+		systemctl --quiet mask systemd-udevd
+	else
+		echo "WARNING: systemd-udevd is not active"
+	fi
+}
+
+_systemd_start_udevd() {
+	if ! _systemctl_unit_is_active "systemd-udevd"; then
+		systemctl --quiet unmask systemd-udevd
+		systemctl --quiet start systemd-udevd
+	fi
+	if ! _systemctl_unit_is_active "systemd-udevd"; then
+		echo "WARNING: systemd-udevd not started"
+		return 1
+	fi
+	return 0
+}
+
 # Run the given command as NORMAL_USER
 _run_user() {
 	su "$NORMAL_USER" -c "$1"

tests/loop/010

@@ -60,6 +60,11 @@ test() {
 	mkfs.xfs --force "${loop_device}p1" >& /dev/null
 	losetup --detach "${loop_device}" >&  /dev/null
 
+	# This test generates udev events faster than the rate at which udevd
+	# can process events. Stop systemd-udevd to prevent future test cases
+	# fail.
+	_systemd_stop_udevd
+
 	create_loop "${loop_device}" &
 	create_pid=$!
 	detach_loop "${loop_device}" &
@@ -78,5 +83,8 @@ test() {
 	if _dmesg_since_test_start | grep --quiet "$grep_str"; then
 		echo "Fail"
 	fi
+
+	_systemd_start_udevd
+
 	echo "Test complete"
 }