Re: [PATCH 04/15] iotests/040: Don't check image pattern on zero-length image

[Eric Blake <eblake@redhat.com>]

On Fri, Mar 18, 2022 at 04:36:44PM -0400, John Snow wrote:
> qemu-io fails on read/write with zero-length raw images, so skip these
> when running the zero-length image tests.

On my first read, I wondered what we accomplish by rejecting
zero-length reads on a zero-length image, and whether entering the
rabbit hole of trying to make that corner case "work" differently
makes more sense...

> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  tests/qemu-iotests/040 | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/tests/qemu-iotests/040 b/tests/qemu-iotests/040
> index adf5815781..c4a90937dc 100755
> --- a/tests/qemu-iotests/040
> +++ b/tests/qemu-iotests/040
> @@ -86,8 +86,10 @@ class TestSingleDrive(ImageCommitTestCase):
>          qemu_img('create', '-f', iotests.imgfmt,
>                   '-o', 'backing_file=%s' % mid_img,
>                   '-F', iotests.imgfmt, test_img)
> -        qemu_io('-f', 'raw', '-c', 'write -P 0xab 0 524288', backing_img)
> -        qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0xef 524288 524288', mid_img)
> +        if self.image_len:
> +            qemu_io('-f', 'raw', '-c', 'write -P 0xab 0 524288', backing_img)
> +            qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0xef 524288 524288',
> +                    mid_img)

...but now it is obvious - one of our test cases is attempting a
non-zero-length modification to a zero-length file, and it does make
sense for that modification attempt to fail, in which case, making the
test special case the zero-length file is the right thing to do.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org