[Libguestfs] [PATCH libnbd] copy/copy-file-to-qcow2-compressed.sh: Skip test for all zeroes disk

[Laszlo Ersek]

On 8/8/23 00:28, Richard W.M. Jones wrote:
> Occasionally this test will choose a random seed which results in an
> all-zeroes disk.  The test tries to convert this to a compressed qcow2
> file, and fails because no compressed clusters are detected in the
> resulting file.  This happens because qcow2 stores zero clusters with
> a special sparse representation, they are never stored compressed, so
> a disk with only zeroes in it will never contain compressed clusters.
> 
> To fix this, detect an all-zeroes disk and skip.
> 
> Reported-by: Eric Blake
> ---
>  copy/copy-file-to-qcow2-compressed.sh | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/copy/copy-file-to-qcow2-compressed.sh b/copy/copy-file-to-qcow2-compressed.sh
> index 018c8bba2f..2706eadd66 100755
> --- a/copy/copy-file-to-qcow2-compressed.sh
> +++ b/copy/copy-file-to-qcow2-compressed.sh
> @@ -25,6 +25,7 @@ requires $QEMU_NBD --version
>  requires nbdkit --exit-with-parent --version
>  requires nbdkit sparse-random --dump-plugin
>  requires qemu-img --version
> +requires nbdinfo --version
>  #requires stat --version
>  
>  # Check the compress driver is supported by this qemu-nbd.
> @@ -45,6 +46,15 @@ cleanup_fn rm -f $file1 $file2 $out1 $out2
>  size=1G
>  seed=$RANDOM
>  
> +# Occasionally we will choose a seed which results in a completely
> +# empty file.  Skip this case.
> +if nbdinfo --map --totals -- \
> +        [ nbdkit --exit-with-parent sparse-random $size seed=$seed ] |
> +    grep -sq '100.0%.*hole,zero'; then
> +    echo "$0: bad seed chosen, skipping the test"
> +    exit 77
> +fi
> +
>  # Create a compressed qcow2 file1.
>  #
>  # sparse-random files should compress easily because by default each

Nbdkit uses its own PRNG from "common/include/random.h", not the one
from libc. That means the seed=... parameter entirely determines the
random series -- not only does it become time-independent, but also
platform-independent.

And based on that, could we just eliminate "seed=$RANDOM" from this test
case, and use simply "seed=1" (for example)? What we want is a
sparse-random disk image, generated both times with the same
not-fully-zero contents. seed=1 satisfies that, and makes sure the test
is always "armed" (and, not least, deterministic).

Laszlo