snakeoil.compression can't cleanly handle if the parallelization compressor changes during runtime

[ferringb]

See

snakeoil/src/snakeoil/compression/_bzip2.py

Lines 41 to 49 in 266d2c7

	try:
	lbzip2_path = process.find_binary("lbzip2")
	lbzip2_compress_args = (f"-n{multiprocessing.cpu_count()}",)
	lbzip2_decompress_args = lbzip2_compress_args
	parallelizable = True
	except process.CommandNotFound:
	lbzip2_path = None
	parallelizable = False
	lbzip2_compress_args = lbzip2_decompress_args = ()

Note that upon module import it detects lbzip2 and the code uses it from that point forward.

This will break if someone does something like merging bzip2 and unmerging lbzip2 without the module being reloaded. Alternatively, if one does a merge that adds lbzip2, it still won't use lbzip2 for that process w/out an explicit module reload.

This can be fixed via moving the check for the binary to invocation time; that'll also allow for pbzip2 support to be added properly.