#!/bin/sh
# wg-dns-sync — render dnsmasq records for the wg1 mesh from wg-mesh-hosts.json
# and (re-)install them to /etc/dnsmasq.d/wg-mesh.conf on the local host.
#
# Source of truth: ../data/wg-mesh-hosts.json (relative to this script).
# Output file:     /etc/dnsmasq.d/wg-mesh.conf
# Daemon:          dnsmasq.service (restarted only if conf changed)
#
# Why a separate file (not editing lilith-local.conf):
#   lilith-local.conf returns 127.0.0.1 for *.lilith.apricot.local on purpose
#   — that's split-horizon DNS for apricot's own loopback Traefik. Other clients
#   (e.g. phone over wg1) need the LAN IP instead. This script writes a SECOND
#   conf file that targets only the wg1 listen address, so both views coexist.
#
# Idempotent: re-run is a no-op if rendered conf hashes match what's installed.
#
# Usage:
#   wg-dns-sync            # render + install + restart dnsmasq if changed
#   wg-dns-sync --dry-run  # print rendered conf, no install
#
# Exit codes:
#   0   success (or unchanged no-op)
#   1   missing dependency (jq, sudo, dnsmasq) or invalid JSON
#   2   sudo required but not available non-interactively
#   3   dnsmasq failed to start after install (rolled back)

set -eu

dry_run=0
[ "${1:-}" = "--dry-run" ] && dry_run=1

script_dir=$(cd "$(dirname "$0")" && pwd)
data_file="$script_dir/../data/wg-mesh-hosts.json"
target=/etc/dnsmasq.d/wg-mesh.conf

[ -r "$data_file" ] || { echo "missing $data_file" >&2; exit 1; }
command -v jq >/dev/null    || { echo "jq not installed" >&2; exit 1; }

# Validate JSON early so render errors don't half-write.
jq empty "$data_file" || { echo "invalid JSON in $data_file" >&2; exit 1; }

listen=$(jq -r '.listen_address' "$data_file")
[ -n "$listen" ] && [ "$listen" != "null" ] || {
    echo "wg-mesh-hosts.json missing .listen_address" >&2; exit 1
}

# Render the conf to a temp file. Header is structured so future audits can see
# it was machine-generated and from where (path, host, time, data sha).
tmp=$(mktemp /tmp/wg-mesh.conf.XXXXXX)
trap 'rm -f "$tmp"' EXIT

data_sha=$(shasum -a 256 "$data_file" 2>/dev/null | awk '{print $1}' \
    || sha256sum "$data_file" | awk '{print $1}')
when=$(date -u +%Y-%m-%dT%H:%M:%SZ)
host=$(hostname -s)

{
    printf '# Generated by session-tools/bin/wg-dns-sync — DO NOT EDIT MANUALLY\n'
    printf '# To change records: edit session-tools/data/wg-mesh-hosts.json and re-run this script.\n'
    printf '# rendered_at: %s\n' "$when"
    printf '# rendered_on: %s\n' "$host"
    printf '# source_sha256: %s\n' "$data_sha"
    printf '\n'
    printf '# Bind only to the wg1 IP so this view is invisible to LAN/loopback clients\n'
    printf '# (which are served by lilith-local.conf with split-horizon 127.0.0.1 records).\n'
    printf 'listen-address=%s\n' "$listen"
    printf 'bind-interfaces\n'
    printf '\n'
    printf '# DNS records (one per record entry in wg-mesh-hosts.json)\n'
    jq -r '.records[] | "address=/\(.name|sub("^\\.";""))/\(.ip)  # \(.comment // "")"' "$data_file"
} > "$tmp"

if [ "$dry_run" -eq 1 ]; then
    cat "$tmp"
    exit 0
fi

# Compare to existing target. If identical, no-op (avoid unneeded restart).
if [ -r "$target" ] && cmp -s "$tmp" "$target"; then
    echo "wg-dns-sync: $target unchanged (no-op)"
    exit 0
fi

# Install requires sudo. Refuse to prompt mid-script — fail loudly if non-interactive.
if ! sudo -n true 2>/dev/null; then
    echo "wg-dns-sync: sudo required (run interactively or pre-auth with 'sudo -v')" >&2
    exit 2
fi

# Back up current target (if any) before replacing — undo handled by re-running
# wg-dns-sync after editing the JSON, NOT by restoring this backup. Backup is
# audit-only; safe to delete.
if [ -r "$target" ]; then
    sudo cp "$target" "${target}.prev"
fi

sudo install -m 0644 -o root -g root "$tmp" "$target"
echo "wg-dns-sync: installed $target"

# Restart dnsmasq and verify it came back up. If it failed (e.g. listen-address
# unreachable because wg1 is down), restore the previous conf and exit 3.
if ! sudo systemctl restart dnsmasq; then
    echo "wg-dns-sync: dnsmasq failed to restart, rolling back" >&2
    if [ -r "${target}.prev" ]; then
        sudo install -m 0644 -o root -g root "${target}.prev" "$target"
    else
        sudo rm -f "$target"
    fi
    sudo systemctl restart dnsmasq || true
    exit 3
fi

echo "wg-dns-sync: dnsmasq restarted, listening on $listen"