#!/usr/bin/env bash
# The nightly CachyOS kernel update on git.granstrm.com (the update-kernel effect,
# flake.nix): moves the nix-cachyos-kernel input to the latest commit of its master
# branch and commits flake.lock, if the kernel changed (its version, config,
# patches or how it's built: the input's kernel-cachyos directory). Upstream's own
# daily nixpkgs updates alone don't count, or the kernel would be rebuilt every
# night. gt then fast-forwards main to the commit, and CI on it builds the kernel
# into the binary cache (cache.git.granstrm.com), so the Desktop update
# (tools/nixos-update) downloads it instead of compiling.
#
# It runs in gt's effect sandbox: a clone of main, the internet, and no Nix daemon,
# so the input is fetched into a store of its own under /tmp.
set -euo pipefail

input=nix-cachyos-kernel
store=/tmp/store

nix() { command nix --extra-experimental-features 'nix-command flakes' --store "$store" "$@"; }

# The input's lock entry, by its name in the root's inputs.
locked() { jq -r --arg i "$input" '.nodes[.nodes.root.inputs[$i]].locked' flake.lock; }

# Where a lock entry's source is, on disk.
source_of() {
  local path
  path=$(nix flake prefetch --json "$(jq -r '"github:\(.owner)/\(.repo)/\(.rev)"' <<<"$1")" | jq -r .storePath)
  echo "$store$path"
}

# The kernel cachyos.nix uses, linuxPackages-cachyos-latest-lto-zen4, is
# linux-cachyos's version.
version() { jq -r '."linux-cachyos".version' "$1/kernel-cachyos/version.json"; }

old=$(locked)
nix flake update "$input"
new=$(locked)
old_rev=$(jq -r .rev <<<"$old")
new_rev=$(jq -r .rev <<<"$new")

if [ "$old_rev" = "$new_rev" ]; then
  echo "No new commit on $input." | tee /run/gt/skip
  exit 0
fi

old_src=$(source_of "$old")
new_src=$(source_of "$new")
if [ "$(nix hash path "$old_src/kernel-cachyos")" = "$(nix hash path "$new_src/kernel-cachyos")" ]; then
  echo "$input ${new_rev:0:7}: the kernel is the same, so it stays at ${old_rev:0:7}." | tee /run/gt/skip
  exit 0
fi

old_version=$(version "$old_src")
new_version=$(version "$new_src")
if [ "$old_version" = "$new_version" ]; then
  subject="CachyOS kernel $new_version, rebuilt"
else
  subject="CachyOS kernel $new_version"
fi

git add flake.lock
git -c user.name=git.granstrm.com -c user.email=gt@git.granstrm.com commit -q -F - <<EOF
$subject

$input ${old_rev:0:7} -> ${new_rev:0:7}, kernel $old_version -> $new_version.
EOF
git --no-pager log -1 --format='%s%n%n%b'