Comment 0 for bug 1837869

On Bionic, Qemu complains that it cannot acquire write lock when commiting a snapshot if a read-only backing-store is opened by another qemu process. This behavior does not happen with version 2.12 in Cosmic.

Reproducer
==========
Create two QCOW2 containers sharing the same base file as a backing store :

           base.qcow2
                |
      +---------+---------+
      | |
middle-vm02.img middle-vm02.img
      | |
top-vm01.img top-vm02.img

# cat mkimage
#!/bin/bash
qemu-img create -f qcow2 base.qcow2 10G
qemu-img create -f qcow2 -b base.qcow2 middle-vm01.img 10G
qemu-img create -f qcow2 -b base.qcow2 middle-vm02.img 10G
qemu-img create -f qcow2 -b middle-vm01.img top-vm01.img 10G
qemu-img create -f qcow2 -b middle-vm01.img top-vm02.img 10G

Start two VM each using its own top-vm{id}.img

# cat runvm
#!/bin/bash

qemu-system-x86_64 -nographic -qmp unix:./qmp-1.sock,server,nowait -enable-kvm -device virtio-scsi-pci,id=scsi -device sga -nodefaults -monitor none -m 256M -drive file=./top-vm01.img,if=virtio,id=disk0 -smp 1 -smbios type=1,manufacturer=test&
qemu-system-x86_64 -nographic -qmp unix:./qmp-2.sock,server,nowait -enable-kvm -device virtio-scsi-pci,id=scsi -device sga -nodefaults -monitor none -m 256M -drive file=./top-vm02.img,if=virtio,id=disk0 -smp 1 -smbios type=1,manufacturer=test&

Create a snapshot

./scripts/qmp/qmp-shell ./qmp-1.sock
Welcome to the QMP low-level shell!
Connected to QEMU 2.11.1

(QEMU) blockdev-snapshot-sync device=disk0 snapshot-file=tmp.qcow2 format=qcow2
Formatting 'tmp.qcow2', fmt=qcow2 size=10737418240 backing_file=./top-vm01.img backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
{"return": {}}

Commit the snapshot
(QEMU) block-commit device=disk0 base=top-vm01.img
{"error": {"class": "GenericError", "desc": "Failed to get \"write\" lock"}}

Expected Behavior
=================
The commit should complete succesfully as the base.img backing store is opened read-only so no write lock is required

Current Behavior
================
The commit fails with "Failed to get "write" lock