What were you trying to do that didn't work?

Libnbd 1.16 introduced a regression (fixed in 1.18.3 and 1.20.0) where nbd_can_meta_context() fails with an error rather than returning false when talking to a server that has structured replies but lacks block status support.  This is a corner case: most servers (including qemu-nbd and nbdkit) are either so old that they support neither feature, or new enough that both features were added in the same release.  nbd-server (from the upstream nbd project) 3.24 lacked both features, making nbd 3.25 (upstream release May 2023) being the only known server in the wild that triggers the libnbd regression.

Please provide the package NVR for which bug is seen:

libnbd-1.18.1-3

How reproducible:

100% with nbd-server 3.25 as server

Steps to reproduce

1. echo hello world > file1
2. nbd-server -r 10809 $PWD/file1
3. nbdcopy nbd://localhost:10809 file2

Expected results

file1 and file2 should have identical contents

Actual results

nbdcopy fails with:

nbd://localhost:10809: nbd_block_status_64: did not negotiate any metadata contexts, either you did not call nbd_add_meta_context before connecting or the server does not support it: Operation not supported

Most of the Red Hat virtualization stack does not use nbd-server (nbdkit and qemu-nbd are more common); but since RHEL does ship the nbd project (which includes nbd-server), customers can hit this.