[libvirt] external snapshots bug

[Khasan Khafizov]

Suppose we have a disk with several external snapshots of it. This disk is
attached to some domain. I found that after destroying and starting the
domain the xml of attached disk doesn't contain all backingStore that it
has to.

Before the restart the part of domain xml that relates to attached disk
looks like this:

    <disk type='file' device='disk'>
>       <driver name='qemu' type='qcow2'/>
>       <source file='...'/>
>       <backingStore type='file' index='1'>
>         <format type='qcow2'/>               <!-- here is qcow2 -->
>         <source file='...'/>
>         <backingStore type='file' index='2'>
>           <format type='qcow2'/>
>           <source file='...'/>
>           <backingStore type='file' index='3'>
>             <format type='qcow2'/>
>             <source file='...'/>
>             <backingStore/>
>           </backingStore>
>         </backingStore>
>       </backingStore>
>     </disk>


And after the restart like this:

    <disk type='file' device='disk'>
>       <driver name='qemu' type='qcow2'/>
>       <source file='...'/>
>       <backingStore type='file' index='1'>
>         <format type='raw'/>               <!-- here is raw -->
>         <source file='...'/>
>         <backingStore/>
>       </backingStore>
>     </disk>

So, the difference in two xml not only in depth of backing stores but in
format type also.
The result of it is that you can't run blockcommit that will commit overlay
into base image if they don't have direct relations (if for example overlay
is child of child of base image). Virsh says it can't find base in chain (
exception
<https://github.com/libvirt/libvirt/blob/cdecfbed027ab242467580a897a636be82d5d411/src/util/virstoragefile.c#L1667>
).
Then I got that libvirt stops calling virStorageFileGetMetadataRecurse
<https://github.com/libvirt/libvirt/blob/ed8661a309c96e4a1573dc0ca2c76ea7ff19469a/src/storage/storage_source.c#L394>
function
after first call because it thinks that format of first backingStore of
disk is raw instead of qcow2. I solved it  by adding to the  qemu config
the following line:

>  allow_disk_format_probing = 1

 But in qemu config it was marked as secure hole.
Idea of fix is that we can try assume that backing store of qcow is qcow
insted of raw, but I really don't know what to do to implement this idea if
it at all possible

Thanks!
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