RFC: tracking valid backing chain issue

[Nikolay Shirokovskiy]

Hi, all.

I recently found a corner case when it is impossible AFAIK to find out valid
backing chain after block commit operation. Imagine committing top image. After
commit ready state pivot is sent and then mgmt crashed. So far so good. Upon
next start mgmt can either check block job status for non-autodissmised job or
inspect backing chain to infer was pivot was successful or not in case of older
qemu.

But imagine after mgmt crash qemu process was destroyed too. In this case there
is no option to know now what is valid backing chain. Yeah libvirt starts qemu
process with -no-shutdown flags so process is not destroyed in case of shutdown
but still process can crash.

So corner case is very rare. Mgmt crash in a specific short moment and then
qemu crash before mgmt is up again.

I guess some 'invalidated' flag for image would help. And also qemu itself
could check that mgmt is not trying to run on invalid backing chain based
on this flag.

Nikolay