[Libguestfs] [nbdkit PATCH 3/3] RFC swab: Re-enable .extents

[Eric Blake]

On 7/8/20 6:32 AM, Richard W.M. Jones wrote:
> On Tue, Jul 07, 2020 at 05:22:47PM -0500, Eric Blake wrote:
>> This reverts commit 2c5aec42cf04c567639bd885cf12a57192229215.
>>
>> Now that we have an easy way to generate aligned extents from the
>> plugin, where the client will never see an unaligned extent
>> transition, it's time to put it to use.
>>
>> Signed-off-by: Eric Blake <eblake at redhat.com>
>>
>> ---
>> Here's what I used on the command line; I need to turn it into a formal test:
>>
>> $ ./nbdkit -U - eval pread='exit 1' get_size='echo 8' can_extents='exit 0' \
>>   extents='printf "0 3\n3 4 hole,zero\n7 1\n"' --run \
>>   'qemu-img map -f raw --output=json $uri'
>> $ ./nbdkit -U - --filter=swab eval pread='exit 1' get_size='echo 8' can_extents='exit 0' extents='printf "0 3\n3 4 hole,zero\n7 1\n"' --run 'qemu-img map -f raw --output=json $uri'

The fact that I had to supply can_extents='exit 0' was another bug that 
I just fixed in the eval plugin.

>>
>> except it's currently failing:
>> nbdkit: eval[1]: error: swab: requests to this filter must be aligned
>> nbdkit: backend.c:621: backend_extents: Assertion `*err' failed.
>> qemu-img: Could not read file metadata: Input/output error
> 
> I would add -v to the command line, as it will show you exactly what
> requests qemu-img is making.  The assert failure seems to be a
> separate and worrying problem.

Yes, the assert failure is a pre-existing bug in the swab filter, which 
I just fixed: basically, filters MUST set *err when failing, so the 
swab.c:is_aligned() helper needed an additional parameter.

But I hit the assert because of my bug in patch 2 of using a copy 
instead of editing through a pointer, such that the client wasn't seeing 
the edited aligned values after all.  With that fixed, I'm now ready to 
formalize the test, and push this patch and a similar one for the 
blocksize filter.

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