Bug? qemu-img convert to preallocated image makes it sparse

["Richard W.M. Jones" <rjones@redhat.com>]

I'm not necessarily saying this is a bug, but a change in behaviour in
qemu has caused virt-v2v to fail.  The reproducer is quite simple.

Create sparse and preallocated qcow2 files of the same size:

  $ qemu-img create -f qcow2 sparse.qcow2 50M
  Formatting 'sparse.qcow2', fmt=qcow2 size=52428800 cluster_size=65536 lazy_refcounts=off refcount_bits=16

  $ qemu-img create -f qcow2 prealloc.qcow2 50M -o preallocation=falloc,compat=1.1
  Formatting 'prealloc.qcow2', fmt=qcow2 size=52428800 compat=1.1 cluster_size=65536 preallocation=falloc lazy_refcounts=off refcount_bits=16

  $ du -m sparse.qcow2 prealloc.qcow2 
  1 sparse.qcow2
  51	prealloc.qcow2

Now copy the sparse file into the preallocated file using the -n
option so qemu-img doesn't create the target:

  $ qemu-img convert -p -n -f qcow2 -O qcow2 sparse.qcow2 prealloc.qcow2
      (100.00/100%)

In new qemu that makes the target file sparse:

  $ du -m sparse.qcow2 prealloc.qcow2 
  1 sparse.qcow2
  1 prealloc.qcow2         <-- should still be 51

In old qemu the target file remained preallocated, which is what
I and virt-v2v are expecting.

I bisected this to the following commit:

4d7c487eac1652dfe4498fe84f32900ad461d61b is the first bad commit
commit 4d7c487eac1652dfe4498fe84f32900ad461d61b
Author: Max Reitz <mreitz@redhat.com>
Date:   Wed Jul 24 19:12:29 2019 +0200

    qemu-img: Fix bdrv_has_zero_init() use in convert
    
    bdrv_has_zero_init() only has meaning for newly created images or image
    areas.  If qemu-img convert did not create the image itself, it cannot
    rely on bdrv_has_zero_init()'s result to carry any meaning.
    
    Signed-off-by: Max Reitz <mreitz@redhat.com>
    Message-id: 20190724171239.8764-2-mreitz@redhat.com
    Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
    Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
    Signed-off-by: Max Reitz <mreitz@redhat.com>

 qemu-img.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

Reverting this commit on the current master branch restores the
expected behaviour.

Thoughts?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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