#!/usr/bin/env bash
set -euo pipefail

# Enable all future paths in this script to be relative to the directory containing the script.
cd "$(dirname -- "${BASH_SOURCE[0]}")"

configPath=$(realpath -- "$HOSTNAME-configuration.nix")

if [ ! -f $configPath ]; then
    echo "aborting: $configPath doesn't exist"
    exit 1
fi

firstLine=$(head -n1 "$configPath")

if ! echo "$firstLine" | grep -E ^"# channel *=" > /dev/null; then
    echo "aborting: $configPath doesn't start with \`# channel=\"...\"\`, where ... is a pin from sources.json"
    exit 1
fi

nix-eval() {
    nix-instantiate --eval --read-write-mode "$@" \
      | tr -d \"  # nix-instantiate has no raw output yet (like the experimental nix eval)
}

channel=$(nix-eval --argstr line "$(echo $firstLine | tr -d \#)" --expr '{line}: (builtins.fromTOML line).channel')
nixpkgsPath=$(nix-eval --argstr channel "$channel" --expr '{channel}: (import ./npins).${channel}.outPath')

# nixos-rebuild always reads Nixpkgs from the NIX_PATH,
# so we need to set it explicitly to our pinned version
exec nixos-rebuild "$@" -I nixpkgs="$nixpkgsPath" -I nixos-config="$configPath"