[Nix-dev] Accepting -jX on builds

[Lluís Batlle]

I remember that someone here said that the make "-jX" parameter should
not be used at all, because some build systems produce (relevantly)
different results according to that parameter.

I argued on freenode#nixos that we could add an option to
mkDerivation, to allow derivation authors to assert, whether the pkg
build system accepts make's "-jX" or not. Defaulting to "not", of
course. I think this can be done passing an environment variable from
the nix-build calling user space.

As derivation results can also be affected by data outside the nix
store, and the only way to assert anything about that is running the
build in a chrooted environment (we have a nix parameter to force
that), we can have a similar parameter which disallows any kind of -jX
to make.

I don't think we should even think about getting *different hashes*
for different -jX parameters, when the mkDerivation parameters assert
that the build allows any -jX.

Do you like this idea comparing "-jX" to "data outside the store"?

Regards,
Lluís.