Re: 01/03: guix: store: Add ensure-path.

[Mathieu Othacehe <othacehe@gnu.org>]

Hey,

> Looking at LocalStore::ensurePath, I don’t think a GC root is made.
> You have to call ‘add-temp-root’ if you want that to happen, no?

Yes, I think we do indeed.

> However ‘ensure-path’ can substitute the store item, which I suppose
> means you can substitute .drv files without building them, am I right?

Yes I'm using it this way:

--8<---------------cut here---------------start------------->8---
(define (add-to-store outputs url)
  (with-store store
    (for-each (lambda (output)
                (add-to-locks-held store output)
                (add-substitute-url store url)
                (ensure-path store output))
              (map derivation-output-path outputs))))
--8<---------------cut here---------------end--------------->8---

to fetch substitutes without building derivations. The painful part is
that I have to use my own custom RPC: AddToLocksHeld, so that I can add
the substitute to the store even if the output lock is held.

At first, I planned to use "restore-file-set" from (guix nar) but it
only handles nar files produced by "guix archive --export".

Thanks,

Mathieu