#!/usr/bin/env bash
# vps-dns-failover.sh - Phase 0.5 dual-site DNS failover watcher
#
# Runs ON THE VPS as a systemd timer (see vps-dns-failover.timer/.service in this
# directory) - deliberately plain systemd, not a k8s CronJob, so it works even if
# the VPS's own k3s is unhealthy. It must never depend on anything inside the home
# cluster (that's the thing it's watching) or the VPS's own k3s (that's a separate
# failure domain from this box's basic OS-level networking).
#
# Health-checks home.kube.huskypup.net (kept current by an in-cluster CronJob while
# home is up - see infrastructure/cert-manager/manifests/home-ip-ddns-cronjob.yaml)
# and flips Cloudflare A records for the standby-service hostnames between home's
# public IP and this VPS's own public IP, with a consecutive-check threshold so a
# single blip doesn't cause a flap.
#
# State (current active site + streak counters) persists in $STATE_DIR between
# runs since each systemd timer firing is a fresh process.
#
# Install:
#   sudo mkdir -p /etc/vps-dns-failover
#   sudo sh -c 'echo "<dns-edit-scoped-cloudflare-token>" > /etc/vps-dns-failover/cloudflare-token'
#   sudo chmod 600 /etc/vps-dns-failover/cloudflare-token
#   sudo cp vps-dns-failover.sh /usr/local/bin/vps-dns-failover.sh
#   sudo chmod +x /usr/local/bin/vps-dns-failover.sh
#   sudo cp vps-dns-failover.service vps-dns-failover.timer /etc/systemd/system/
#   sudo systemctl daemon-reload
#   sudo systemctl enable --now vps-dns-failover.timer

set -euo pipefail

TOKEN_FILE="/etc/vps-dns-failover/cloudflare-token"
STATE_DIR="/var/lib/vps-dns-failover"
ZONE_NAME="huskypup.net"  # Cloudflare zone is the parent domain - kube.huskypup.net is just a record within it, not its own zone
HOME_CHECK_HOST="home.kube.huskypup.net"
HOME_CHECK_PORT=443
FAILURE_THRESHOLD=3   # consecutive failed checks before flipping to the VPS
SUCCESS_THRESHOLD=3   # consecutive successful checks before flipping back to home
STANDBY_HOSTNAMES=(
  vault.kube.huskypup.net
  auth.kube.huskypup.net
  gitea.kube.huskypup.net
  n8n.kube.huskypup.net
  nextcloud.kube.huskypup.net
)

mkdir -p "$STATE_DIR"
TOKEN="$(cat "$TOKEN_FILE")"
STATE_FILE="${STATE_DIR}/state"   # format: "<active-site> <fail-streak> <success-streak>"

if [ -f "$STATE_FILE" ]; then
  read -r ACTIVE FAIL_STREAK SUCCESS_STREAK < "$STATE_FILE"
else
  ACTIVE="home"
  FAIL_STREAK=0
  SUCCESS_STREAK=0
fi

# --- health check ------------------------------------------------------------
HOME_IP="$(dig +short A "$HOME_CHECK_HOST" @1.1.1.1 | tail -n1)"
if [ -n "$HOME_IP" ] && timeout 5 bash -c "cat < /dev/null > /dev/tcp/${HOME_IP}/${HOME_CHECK_PORT}" 2>/dev/null; then
  HEALTHY=1
else
  HEALTHY=0
fi

if [ "$HEALTHY" = 1 ]; then
  FAIL_STREAK=0
  SUCCESS_STREAK=$((SUCCESS_STREAK + 1))
else
  SUCCESS_STREAK=0
  FAIL_STREAK=$((FAIL_STREAK + 1))
fi

echo "$(date -u +%FT%TZ) active=${ACTIVE} healthy=${HEALTHY} fail_streak=${FAIL_STREAK} success_streak=${SUCCESS_STREAK} home_ip=${HOME_IP:-none}"

# --- Cloudflare helpers --------------------------------------------------------
cf_zone_id() {
  curl -sf -H "Authorization: Bearer ${TOKEN}" \
    "https://api.cloudflare.com/client/v4/zones?name=${ZONE_NAME}" | jq -r '.result[0].id'
}

cf_set_record() {
  local zone_id="$1" hostname="$2" target_ip="$3"
  local record_json record_id
  record_json="$(curl -sf -H "Authorization: Bearer ${TOKEN}" \
    "https://api.cloudflare.com/client/v4/zones/${zone_id}/dns_records?name=${hostname}&type=A")"
  record_id="$(echo "$record_json" | jq -r '.result[0].id // empty')"
  local body="{\"type\":\"A\",\"name\":\"${hostname}\",\"content\":\"${target_ip}\",\"ttl\":60,\"proxied\":false}"
  if [ -n "$record_id" ]; then
    curl -sf -X PATCH -H "Authorization: Bearer ${TOKEN}" -H "Content-Type: application/json" \
      -d "$body" "https://api.cloudflare.com/client/v4/zones/${zone_id}/dns_records/${record_id}" >/dev/null
  else
    curl -sf -X POST -H "Authorization: Bearer ${TOKEN}" -H "Content-Type: application/json" \
      -d "$body" "https://api.cloudflare.com/client/v4/zones/${zone_id}/dns_records" >/dev/null
  fi
  echo "  ${hostname} -> ${target_ip}"
}

flip_to() {
  local target="$1"
  local target_ip
  if [ "$target" = "vps" ]; then
    target_ip="$(curl -sf https://cloudflare.com/cdn-cgi/trace | grep -o '^ip=.*' | cut -d= -f2)"
  else
    target_ip="$HOME_IP"
  fi
  if [ -z "$target_ip" ]; then
    echo "ERROR: could not determine target IP for '${target}', not flipping"
    return 1
  fi
  echo "Flipping standby hostnames to ${target} (${target_ip})..."
  local zone_id
  zone_id="$(cf_zone_id)"
  for h in "${STANDBY_HOSTNAMES[@]}"; do
    cf_set_record "$zone_id" "$h" "$target_ip"
  done
}

# --- decide ------------------------------------------------------------------
if [ "$ACTIVE" = "home" ] && [ "$FAIL_STREAK" -ge "$FAILURE_THRESHOLD" ]; then
  flip_to "vps"
  ACTIVE="vps"
  FAIL_STREAK=0
  SUCCESS_STREAK=0
elif [ "$ACTIVE" = "vps" ] && [ "$SUCCESS_STREAK" -ge "$SUCCESS_THRESHOLD" ]; then
  flip_to "home"
  ACTIVE="home"
  FAIL_STREAK=0
  SUCCESS_STREAK=0
fi

echo "${ACTIVE} ${FAIL_STREAK} ${SUCCESS_STREAK}" > "$STATE_FILE"