Libguestfs - Jun 2022 - [nbdkit PATCH 3/3] tests: Add regression test for NBDKIT_EMULATE_CACHE fix

On Fri, Jun 10, 2022 at 10:55:27AM -0500, Eric Blake
wrote:
> Add testsuite coverage for the bug fixed in the previous patch (done
> separately, to make it easier to prove the test fails without the
> patch).
> ---
>  tests/Makefile.am        |  2 +
>  tests/test-eval-cache.sh | 85 ++++++++++++++++++++++++++++++++++++++++
> +# This plugin requests nbdkit to emulate caching with pread. When the
witness
> +# file exists, cache reads; when absent, reads fail if not already cached.
> +export witness="$PWD/eval-cache.witness"
> +export cache="$PWD/eval-cache.cache"
> +export script='
> +import os
> +import errno
> +
> +witness = os.getenv("witness")
> +
> +def touch(path):
> +    open(path, "a").close()
> +
> +# Test that uncached read fails
> +try:
> +    h.pread(1024 * 1024, 0)
> +except nbd.Error as ex:
> +    assert ex.errnum == errno.EIO
> +
> +# Cache the entire image; nbdkit should break it into 64k preads
64M preads
> +touch(witness)
> +h.cache(h.get_size(), 0)
> +os.unlink(witness)
> +
> +# Now read should succeed
> +buf = h.pread(64 * 1024 * 1024, 64 * 1024 * 1024)
> +if hasattr(buf, "is_zero"):
> +    assert buf.is_zero()
> +'
> +nbdkit -U - -v eval \
> +    get_size='echo 128M' can_cache='echo emulate'
open='touch "$cache"' \
> +    pread='
> +      if test -f "$witness"; then
> +        echo "$3 $4" >> "$cache"
> +      elif ! grep -q "^$3 $4$" "$cache"; then
> +        echo EIO >&2; exit 1
> +      fi
> +      dd if=/dev/zero count=$3 iflag=count_bytes
> +    ' --run 'nbdsh -u "$uri" -c "$script"'
-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Series looks good:

Acked-by: Richard W.M. Jones <rjones at redhat.com>

I think nbdkit-ext2-filter is still wrong (although at least it should
no longer corrupt disk images) because it unnecessarily calls
next->can_zero, so it might be worth dropping those two lines.

Regression test for the LUKS case might be useful.  But note that the
test should not only check that zeroes work, but also that the result
is still a LUKS image.  The reason is that a naive fix for the earlier
problem can (as I discovered) corrupt the disk image, because it will
forward zeroes (eg to offset 0) straight through to the plugin.  If
this is too much, then I can write this test case tomorrow.

Thanks!

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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