bug#20428: git-fetch does not always validate hash

[ludo@gnu.org (Ludovic Courtès)]

Ricardo Wurmus <rekado@elephly.net> skribis:

> Upon rebuilding the package from a new commit I would expect the build
> to fail with a hash validation error as I have not updated the hash yet.
> However, the build procedure just continues.  I noticed that the git
> checkout is still the very same as before I updated the value of
> "commit".  I cannot seem to reliably force a new git checkout.

Indeed, origins compile to fixed-output derivations, meaning that these
are special derivations whose output is known in advance (rather, the
SHA256 is the output is known in advance.)  Thus, it doesn’t matter how
the derivation produces its result as long as its output has the correct
hash.

When you leave the ‘sha256’ unchanged, the daemon notices that there’s
already an item with this name and hash in the store, so it does
nothing.

This is expected behavior, though I understand this can be error-prone
here.

To avoid that, you could add a ‘file-name’ field to the origin that
includes the commit:

  (file-name (string-append name "-checkout-" commit))

Thus, if you change the commit without changing the hash, the daemon
will still want to build the origin (because of the changed name) and
will eventually complain about the hash mismatch.

Thanks,
Ludo’.