Source: qemu Followup-For: Bug #1030545 (ack - that does seem like an unusually-timed coincidence. continuing on with some unrelated investigation, though...)
There's a commit[1] from Y2019 that appears to describe a similar set of circumstances - iothreads blocked forever for a bunch of backends, including qcow2. Something potentially relevant in the code diff is that it introduces function calls wrapped within 'assert' as part of the fix (perhaps those are optimized-out by our 'qemu-img' binary build?). [1] - https://gitlab.com/qemu-project/qemu/-/commit/4720cbeea1f42fd905fc69338fd42b191e58b412
