A backdoor in xz

Journal files and coredumps really are a case where zstd would be a better choice than lzma: it's nearly as good compression-wise but compression is *much* faster, and both journals and coredumps are compressed much more often than they're uncompressed. (For things like .xz distro artifacts this is the other way around, so spending loads of time for slightly better compression is often a good idea -- or would be if xz wasn't also much slower and more memory-hungry than zstd at decompressing!)