[PATCH v3 4/4] qemu-img: Deprecate use of -b without -F

[Eric Blake]

On 3/10/20 5:57 AM, Kashyap Chamarthy wrote:
> On Mon, Mar 09, 2020 at 10:42:25AM -0500, Eric Blake wrote:
> 
> [...]
> 
>>>> +qemu-img backing file without format (since 5.0.0)
>>>> +''''''''''''''''''''''''''''''''''''''''''''''''''
>>>> +
>>>> +The use of ``qemu-img create``, ``qemu-img rebase``, ``qemu-img
>>>> +convert``, or ``qemu-img amend`` to create or modify an image that
>>>> +depends on a backing file now recommends that an explicit backing
>>>> +format be provided.
> 
> Also for the `qemu-img amend` case, I'm not clear if the following scenario
> ought to throw the warning:
> 
>      $> ~/build/qemu/qemu-img info --backing-chain overlay1.qcow2
>      image: overlay1.qcow2
>      file format: qcow2
>      virtual size: 4 GiB (4294967296 bytes)
>      disk size: 196 KiB
>      cluster_size: 65536
>      backing file: base.raw
>      Format specific information:
>          compat: 1.1
>          lazy refcounts: false
>          refcount bits: 16
>          corrupt: false
>      
>      image: base.raw
>      file format: raw
>      virtual size: 4 GiB (4294967296 bytes)
>      disk size: 778 MiB
> 
>      $> ~/build/qemu/qemu-img amend -o compat=v3 overlay1.qcow2

This particular amend is not changing the backing file, and since I did 
not add warnings on the opening of an existing unsafe image, it is 
silent.  Instead, you need to test a case where amend provokes a path 
that would change the backing file (perhaps as simple as '-o 
backing_file=./base.raw'), while omitting a format for the new backing 
file string.

> 
>      $> echo $?
>      0
> 
> I'm just trying to work out the scenarios where the warnings ought to
> show up (for all the four cases: create, rebase, convert, amend).

Look at patch 2/4 - that patch was written AFTER this patch in order to 
silence every warning that was introduced because of this patch, then 
rebased to occur first.  My experience in writing 2/4 was that I indeed 
hit warnings through all four sub-commands.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org