I noticed recently that os-autoinst has had support for decompressing compressed disk images on-the-fly since 2018. Using this should allow us to POST .raw.xz disk images directly as HDD_X_URL rather than using HDD_X_DECOMPRESS_URL, which triggers decompression code in openQA, which we wrote (initially) and which predates the os-autoinst code.
Doing it in os-autoinst rather than openQA would make the scheduler code slightly simpler, and would also potentially allow us to drop the decompression code from openQA itself, which would be a benefit to upstream.
I'm already working on a branch that implements this; unfortunately we run into mysterious permission problems, even though I've set everything up now so the user which os-autoinst runs as should have permission to uncompress the files, and indeed it works fine in a test using runuser.
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