#!/bin/sh
# The fallback kernel's place on this machine's boot menu (module kernel, novox/hq ADR 0508 §9 and §10,
# issue 1841). Written by the mesh and run as root by mesh-kernel-boot-entries.service at boot and when the
# module is applied, and by the pacman hook 90-mesh-kernel-boot-entries.hook after a kernel is installed,
# upgraded or removed. Replaced on every push; change the module, never this file.
#
#   boot-entries run [<root> [<this module's state> [<room needed in /boot, KiB>]]]
#
# The three are arguments only so the module's tests can run this against a stand-in tree.
#
# It reads from the machine which loader booted it, never from a setting:
#   systemd-boot  the EFI variable LoaderEntrySelected is present: the entry mesh-linux-lts.conf is written
#                 from the command line the machine booted with.
#   GRUB          /boot/grub/grub.cfg exists and that variable does not: the menu is generated again, only
#                 when the set of kernels in /boot differs from the menu's.
#   neither       it fails loudly. The default entry is never touched.
# And a pacman hook of someone else's that copies kernel images to one fixed name in /boot is moved into this
# module's state first (ADR 0508 §10): it would put the fallback kernel in the default kernel's place.
set -eu

root=/
state='${dir:state}'
[ $# -ge 2 ] && root=$(printf '%s' "$2" | sed 's|/$||')
[ $# -ge 3 ] && state=$3
boot=$root/boot
hooks=$root/etc/pacman.d/hooks
efivar=$root/sys/firmware/efi/efivars/LoaderEntrySelected-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f
ours=90-mesh-kernel-boot-entries.hook
# The fallback kernel and its initramfs are about 72 MB in /boot (issue 1836): less room fails the unit
# before a package writes half of them.
need_kib=98304
[ $# -ge 4 ] && need_kib=$4

[ $# -ge 1 ] && [ "$1" = run ] || {
	echo "usage: boot-entries run [<root> [<state directory> [<KiB needed>]]]" >&2
	exit 2
}

# A hook that is not ours, triggered by kernel images, whose action copies to one fixed name in /boot.
copies_kernels() {
	grep -Eqi '^[[:space:]]*Target[[:space:]]*=.*(vmlinuz|usr/lib/modules)' "$1" &&
		grep -Eq '^[[:space:]]*Exec[[:space:]]*=.*(cp|install|ln|rsync)[[:space:]].*/boot/[A-Za-z0-9._+-]+([^A-Za-z0-9._+/%$*-]|$)' "$1"
}

if [ -d "$hooks" ]; then
	for hook in "$hooks"/*.hook; do
		[ -f "$hook" ] || continue
		[ "$(basename "$hook")" = "$ours" ] && continue
		copies_kernels "$hook" || continue
		mkdir -p "$state/taken-over"
		mv -f "$hook" "$state/taken-over/$(basename "$hook")"
		echo "taken over: the pacman hook $(basename "$hook") copied kernel images to one fixed name in /boot; moved to $state/taken-over/ so each kernel keeps its own package's name"
	done
fi

if [ -e "$efivar" ]; then
	loader=systemd-boot
elif [ -f "$boot/grub/grub.cfg" ]; then
	loader=grub
else
	echo "no boot loader found: the machine was not booted by systemd-boot (no LoaderEntrySelected variable) and there is no $boot/grub/grub.cfg; the fallback kernel has no menu to be placed on" >&2
	exit 1
fi

# Room first, only while the fallback kernel is not in /boot yet.
if [ ! -e "$boot/vmlinuz-linux-lts" ]; then
	free=$(df -Pk "$boot" | awk 'NR==2 {print $4}')
	[ -n "$free" ] || free=0
	if [ "$free" -lt "$need_kib" ]; then
		echo "$boot has $free KiB free and the fallback kernel needs $need_kib KiB: free room there before the kernel is installed" >&2
		exit 1
	fi
fi

case "$loader" in
systemd-boot)
	entries=$boot/loader/entries
	cmdline=$(sed -e 's/BOOT_IMAGE=[^ ]*//g' -e 's/initrd=[^ ]*//g' "$root/proc/cmdline" | tr -s ' \n' ' ' | sed -e 's/^ //' -e 's/ $//')
	[ -n "$cmdline" ] || {
		echo "$root/proc/cmdline holds no command line to boot the fallback kernel with" >&2
		exit 1
	}
	want=$(
		echo "title Arch Linux (linux-lts, fallback)"
		echo "linux /vmlinuz-linux-lts"
		for ucode in intel-ucode.img amd-ucode.img; do
			[ -e "$boot/$ucode" ] && echo "initrd /$ucode"
		done
		echo "initrd /initramfs-linux-lts.img"
		echo "options $cmdline"
	)
	mkdir -p "$entries"
	if [ "$(cat "$entries/mesh-linux-lts.conf" 2>/dev/null || true)" = "$want" ]; then
		echo "systemd-boot: mesh-linux-lts.conf, as it was"
	else
		printf '%s\n' "$want" >"$entries/mesh-linux-lts.conf"
		echo "systemd-boot: wrote $entries/mesh-linux-lts.conf from the command line the machine booted with; the default entry is as it was"
	fi
	;;
grub)
	cfg=$boot/grub/grub.cfg
	menu=$(sed -n 's|^[[:space:]]*linux[[:space:]]*\(/boot\)\{0,1\}/\(vmlinuz-[^[:space:]]*\).*|\2|p' "$cfg" | sort -u)
	have=$(cd "$boot" && ls -d vmlinuz-* 2>/dev/null | sort -u || true)
	if [ "$menu" = "$have" ]; then
		echo "grub: the menu lists the kernels in /boot, left as it is"
	else
		grub-mkconfig -o "$cfg"
		echo "grub: the kernels in /boot changed ($(echo $have | tr '\n' ' ')); the menu is generated again; the default entry is as it was"
	fi
	;;
esac