Enough innocent images have died because users called 'qemu-img snapshot' while
the VM was still running. Educating the users doesn't seem to be a working
strategy, so this series adds locking to qcow2 that refuses to access the image
read-write from two processes.
Eric, this will require a libvirt update to deal with qemu crashes which leave
locked images behind. The simplest thinkable way would be to unconditionally
override the lock in libvirt whenever the option is present. In that case,
libvirt VMs would be protected against concurrent non-libvirt accesses, but not
the other way round. If you want more than that, libvirt would have to check
somehow if it was its own VM that used the image and left the lock behind. I
imagine that can't be too hard either.
Also note that this kind of depends on Max's bdrv_close_all() series, but only
in order to pass test case 142. This is not a bug in this series, but a
preexisting one (bs->file can be closed before bs), and it becomes apparent
when qemu fails to unlock an image due to this bug. Max's series fixes this.