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

[Eric Blake]

On Fri, Jun 10, 2022 at 05:21:19PM +0100, Richard W.M. Jones wrote:
> 
> 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.

ext2 does not have an interface (yet?) for emulating fallocate()
operations (or other bulk-zeroing operation) against a file; if it
_did_ have one, then that's what ext2's .zero should do.  But we _do_
want to allow the compressed transmission effects of
NBD_CMD_WRITE_ZEROES over the network, so having .can_zero return
NBDKIT_ZERO_EMULATE is right.  On the other hand, ext2 DOES have a way
for the file system itself to request bulk-zeroing of a portion of the
underlying disk, so we _do_ call into the plugin's next->zero(), which
means we _do_ need to check next->can_zero() up front (see the io.c
callback io_zeroout).  I don't see any bugs in this area, once this
series is in.

> 
> 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.

I'll let you handle that one, then.  These three patches pushed as:

09a0e4dc..6ced998b

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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