@DNSB: Well, that's not terribly efficient, but, oh, well.
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In any case, that's definitely not the step I'm concerned with (which is partly why it completely skipped my mind ^^), but thanks for the details
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AFAIK, nickel itself never deals with the KoboRoot (by which I mean unpacking it), that's left entirely to the init script, so I'm mightily confused by you two final sentences.
The naively sane approach to the zip unpack failing would be to clean up and forget about it (or retry a fresh and/or delta download), not continue to attempt an update from half-broken assets?