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Description of problem:

A data corrupter bug has been identified in the following
nbdkit filters:

https://libguestfs.org/nbdkit-cow-filter.1.html
https://libguestfs.org/nbdkit-cache-filter.1.html

It's quite subtle, but it affects modular virt-v2v and was
picked up by Ming Xie during testing.

The full bug is described in this link, as well as a simple way
to reproduce it, and the fix:

https://listman.redhat.com/archives/libguestfs/2021-August/msg00044.html

Version-Release number of selected component (if applicable):

All versions of nbdkit since 1.14 up to 1.27.4

How reproducible:

100%

Steps to Reproduce:
1. See link above.

Reproduce the bug with nbdkit-1.26.2-2.el9.x86_64 

Steps to reproduce:

1. # nbdkit --filter=cow data "33 * 100000" --run 'nbdsh -u $uri -c "h.trim(100000, 0)" ; nbdcopy $uri - | hexdump -C'
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00010000  21 21 21 21 21 21 21 21  21 21 21 21 21 21 21 21  |!!!!!!!!!!!!!!!!|
*
000186a0


Verify the bug with nbdkit-1.26.3-1.el9.x86_64

Steps:
1.# nbdkit --filter=cow data "33 * 100000" --run 'nbdsh -u $uri -c "h.trim(100000, 0)" ; nbdcopy $uri - | hexdump -C'
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000186a0


Result:
   The bug has been fixed, so move the bug from ON_QA to VERIFIED