prefetch-nixpkgs.sh

prefetch-nixpkgs.sh

The nixpkgs = import (import ./pinned-nixpkgs.nix) ... portion of release.nix uses builtins.fetchGit underneath, which will perform a synchronous git checkout of Nixpkgs at evaluation time. This is a big issue in terms of first-time experience as in practice it means the user will run nix-shell and the command will hang in silence for minutes since it will be downloading the whole Nixpkgs repository.

You can alleviate this pain a little by using the following script:

#!/usr/bin/env bash
echo "Prefetching nixpkgs"
if RES=$(nix-instantiate --json --eval -E '"${import ./pinned-nixpkgs.nix}"'); then
  echo "Nixpkgs prefetched succesfully!"
  echo "Path:"
  echo "$RES" | sed -r 's/"//g'
else
  echo "prefetch failure"
  exit 1
fi

The user will then have to run 2 commands:

./prefetch-nixpkgs.sh
nix-shell

But now it runs the first one with a clear expectation of wait-time. While the nix-shell invocation will pickup the shell build process straight away as the result of the above checkout is cached in the store.

POSSIBLE NIX IMPROVEMENT

It would probably make sense for nix to cache a Nixpkgs checkout somewhere internally and resolve calls like the one above by simply first performing a fetch thus making sure all the new commits are present and then cloning into the store from it. This can be completely transparent to the user.

NOTE

In https://youtu.be/J4DgATIjx9E?t=1539 it is suggested to use fetchTarball instead of fetchGit as the download of the git repo represents roughly 600MB of data vs. 12MB of the tarball.

This is unfortunately not possible since fetchTarball is not available in restricted evaluation mode.

This concludes the tutorial (for now), I hope you found it helpful!