is the standard tar archive extractor good enough?

[wind gate]

Might be because neither of those files are buffered.

Consider

var r_buffer = std.io.bufferedReader(testTar.reader());
var w_buffer = std.io.bufferedWriter(testTarFile.writer());
try extract_tarxz_to_dir(ally, w_buffer.writer(), r_buffer.reader());

Might have those the wrong way around, but you get the idea