A test case, attached.

Note that you have to look at the output of the final qemu-img info
command.  In the case where it goes wrong, the 'backing file:' and
'backing file format:' lines disappear completely.  In the case where
the bug is not reproduced, these lines are still present.

It's 100% reproducible for me when lazy_refcounts=on, and 0%
reproducible when lazy_refcounts=off.

BUT it only occurs if the backing file is a remote source (nbd:... in
this case), not if the backing file is a plain file.  Make of that
what you will.

Rich.

-- 
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