Steps to reproduce on Focal (shutdown-on-runtime)
---

LXD virtual machine

	lxc launch --vm ubuntu:focal lp2059272-focal
	lxc exec lp2059272-focal -- su - ubuntu

Latest Packages and Debug Symbols:

	cat <<EOF | sudo tee /etc/apt/sources.list.d/proposed.list
	deb http://archive.ubuntu.com/ubuntu focal-proposed main universe
	deb http://ddebs.ubuntu.com focal-proposed main restricted
	EOF

	cat <<EOF | sudo tee /etc/apt/preferences.d/proposed
	Package: *
	Pin: release a=focal-proposed
	Pin-Priority: 400
	EOF

	sudo apt install --yes --no-install-recommends gdb qemu-system-x86 ubuntu-dbgsym-keyring
	sudo apt install --yes --no-install-recommends -t focal-proposed libvirt{0,-daemon{,-driver-qemu,-system}}{,-dbgsym} libvirt-clients
 
	$ dpkg -l | awk '$2 ~ /^libvirt/ { print $2, $3 }'
	libvirt-clients 6.0.0-0ubuntu8.17
	libvirt-daemon 6.0.0-0ubuntu8.17
	libvirt-daemon-dbgsym 6.0.0-0ubuntu8.17
	libvirt-daemon-driver-qemu 6.0.0-0ubuntu8.17
	libvirt-daemon-driver-qemu-dbgsym 6.0.0-0ubuntu8.17
	libvirt-daemon-system 6.0.0-0ubuntu8.17
	libvirt-daemon-system-systemd 6.0.0-0ubuntu8.17
	libvirt0:amd64 6.0.0-0ubuntu8.17
	libvirt0-dbgsym:amd64 6.0.0-0ubuntu8.17

Stop libvirt systemd units

	sudo systemctl stop 'libvirtd*'

Start libvirt in GDB

	 sudo gdb \
	   -iex 'set confirm off' \
	   -iex 'set pagination off' \
	   -ex 'set non-stop on' \
	   -ex 'handle SIGTERM nostop noprint pass' \
	   -ex 'add-symbol-file /usr/sbin/libvirtd' \
	   -ex 'add-symbol-file /usr/lib/x86_64-linux-gnu/libvirt.so.0' \
	   -ex 'add-symbol-file /usr/lib/x86_64-linux-gnu/libvirt-qemu.so.0' \
	   -ex 'add-symbol-file /usr/lib/x86_64-linux-gnu/libvirt/connection-driver/libvirt_driver_qemu.so' \
	   /usr/sbin/libvirtd

Add breakpoints for qemu driver cleanup and device deleted event

	b qemuStateCleanup
	b processDeviceDeletedEvent
	run

Start test VM with an USB mouse device

	 cat <<-EOF >test-vm.xml
	 <domain type='qemu'>
	   <name>test-vm</name>
	   <os>
	     <type>hvm</type>
	   </os>
	   <memory unit='MiB'>32</memory>
	   <vcpu>1</vcpu>
	   <devices>
	     <input type='mouse' bus='usb'/>
	   </devices>
	 </domain>
EOF

	virsh define test-vm.xml
	virsh start test-vm

	$ virsh list
	Id   Name      State
	-------------------------
	1    test-vm   running

Delete the USB mouse device

	DEVICE_ID=$(virsh qemu-monitor-command test-vm --hmp 'info qtree' | grep 'dev: usb-mouse' | cut -d'"' -f2)
	virsh qemu-monitor-command test-vm --hmp "device_del $DEVICE_ID"

Back to GDB

	Thread 20 "libvirtd" hit Breakpoint 2, 0x00007f316df78ffe in processDeviceDeletedEvent (devAlias=<optimized out>, vm=0x7f3178016890, driver=0x7f317802edd0) at ../../../src/qemu/qemu_driver.c:4879

Add breakpoint to domain status XML save, and continue the thread above

	b virDomainObjSave
	t 20
	c

	Thread 20 "libvirtd" hit Breakpoint 3, virDomainObjSave (obj=0x7f3178016890, xmlopt=0x7f3178037780, statusDir=0x7f317800aa40 "/run/libvirt/qemu") at ../../../src/conf/domain_conf.c:29157

Check the backtrace of the domain status XML save function, coming from device deleted event

	(gdb) bt
	#0  virDomainObjSave (obj=0x7f3178016890, xmlopt=0x7f3178037780, statusDir=0x7f317800aa40 "/run/libvirt/qemu") at ../../../src/conf/domain_conf.c:29157
	#1  0x00007f316df790d7 in processDeviceDeletedEvent (devAlias=0x5654c5839310 "input0", vm=0x7f3178016890, driver=0x7f317802edd0) at ../../../src/qemu/qemu_driver.c:4303
	#2  qemuProcessEventHandler (data=0x5654c5879e80, opaque=0x7f317802edd0) at ../../../src/qemu/qemu_driver.c:4879
	#3  0x00007f318b59f1af in virThreadPoolWorker (opaque=opaque@entry=0x5654c5838160) at ../../../src/util/virthreadpool.c:163
	#4  0x00007f318b59e51c in virThreadHelper (data=<optimized out>) at ../../../src/util/virthread.c:196
	#5  0x00007f318b25f609 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
	#6  0x00007f318b184353 in clone () from /lib/x86_64-linux-gnu/libc.so.6

Leave the thread at this point

Let's trigger the shutdown path

	$ sudo kill $(pidof libvirtd)

Thread 1 "libvirtd" hit Breakpoint 1, qemuStateCleanup () at ../../../src/qemu/qemu_driver.c:1118

Check there are 2 stopped threads: cleanup and domain status XML save

	(gdb) i th
	  Id   Target Id                                   Frame
	  1    Thread 0x7f3187745b40 (LWP 4969) "libvirtd" qemuStateCleanup () at ../../../src/qemu/qemu_driver.c:1118
	  2    Thread 0x7f31872af700 (LWP 4973) "libvirtd" (running)
	...
	  18   Thread 0x7f315e7fc700 (LWP 4989) "libvirtd" (running)
	* 20   Thread 0x7f315dffb700 (LWP 5045) "libvirtd" virDomainObjSave (obj=0x7f3178016890, xmlopt=0x7f3178037780, statusDir=0x7f317800aa40 "/run/libvirt/qemu") at ../../../src/conf/domain_conf.c:29157

Confirm the qemu driver's domain xml formatter/options is set/referenced:

	t 20

	(gdb) p xmlopt.privateData.format
	$1 = (virDomainXMLPrivateDataFormatFunc) 0x7f316ded3810 <qemuDomainObjPrivateXMLFormat>

	(gdb) p/x xmlopt.parent
	$3 = {u = {dummy_align1 = 0x1cafe0026, dummy_align2 = 0x1cafe0026, s = {magic = 0xcafe0026, refs = 0x1}}, klass = 0x7f3178037750}

Let the cleanup function and shutdown path finish

	t 1
	c &

Check the formatter/options again; it is *NO* longer referenced:

	t 20
	 
	(gdb) p xmlopt.privateData.format
	$4 = (virDomainXMLPrivateDataFormatFunc) 0x0

	(gdb) p/x xmlopt.parent
	$5 = {u = {dummy_align1 = 0x7f317805d170, dummy_align2 = 0x7f317805d170, s = {magic = 0x7805d170, refs = 0x7f31}}, klass = 0x7f3178000080}

The object data is zeroed in the last unreference in Focal,
and its contents changed, as this is really an use-after-free
(another thread might get/use that memory).

Check the VM status XML *before* the save function finishes:

	$ sudo grep -e '<domstatus' -e '<domain' -e 'monitor path' /run/libvirt/qemu/test-vm.xml
	<domstatus state='running' reason='booted' pid='5031'>
	  <monitor path='/var/lib/libvirt/qemu/domain-1-test-vm/monitor.sock' type='unix'/>
	  <domain type='qemu' id='1'>

Let the save function continue, and libvirt finishes shutting down:

	(gdb) c
	Continuing.
	...
	[Inferior 1 (process 4969) exited normally]

	(gdb) q

Check the VM status XML *after*:

	$ sudo grep -e '<domstatus' -e '<domain' -e 'monitor path' /run/libvirt/qemu/test-vm.xml
	<domstatus state='running' reason='booted' pid='5031'>
	  <domain type='qemu' id='1'>

It no longer has the 'monitor path' tag/field.

Now, the next time libvirtd starts, it fails to parse that XML:

	$ sudo systemctl start libvirtd.service

	$ journalctl -b -u libvirtd.service | grep error

	$ journalctl -b -u libvirtd.service | grep -A1 error
	Mar 30 21:58:37 lp2059272-focal libvirtd[5063]: internal error: no monitor path
	Mar 30 21:58:37 lp2059272-focal libvirtd[5063]: Failed to load config for domain 'test-vm'

And libvirt is not aware of the domain, and cannot manage it:

	$ virsh list
	Id Name State
	--------------------

	$ virsh list --all
	Id Name State
	--------------------------
	- test-vm shut off

Even though it is still running:

	 $ pgrep -af qemu-system-x86_64 | cut -d, -f1
	 5031 /usr/bin/qemu-system-x86_64 -name guest=test-vm

Stop it manually:

	 $ sudo kill $(sudo cat /run/libvirt/qemu/test-vm.pid)
	 $ sudo rm /run/libvirt/qemu/test-vm.{xml,pid}

Start libvirt:

	sudo systemctl start libvirtd.service