@DNSB: Yep, that's it. I got the impression you were saying that *nickel* was handling part of that after the download, which is what was confusing me
.
(That and the use of "file copy" to describe what tar's doing when unpacking. But I get what you were trying to say now ^^ (i.e., unlink() -> sendfile() [well, a pair of read() + write() because the (un)compressor filter writes to a pipe]).
The Kobo vs. KoboRoot thing might be related to how a factory reset reinstall is handled (or not, I'm possibly getting my wires crossed with Kindles, there
).