#!/bin/bash
# nginx-controller.bash - Dynamic Nginx Configuration Controller for Single-Host Containers
#
# Description:
#   This script acts as a lightweight, single-host Nginx controller that dynamically manages
#   Nginx configurations for containers running on the host. It watches container lifecycle
#   events (start, stop) and updates the Nginx configuration accordingly to route traffic
#   to containers' internal IPs and ports.
#
# Features:
#   - Monitors container events and updates Nginx configurations on container start/stop.
#   - Automatically creates or removes Nginx reverse proxy configurations for containers.
#   - Supports dynamic upstream configuration and domain-based reverse proxying.
#   - Minimal dependencies: Runs with just bash and basic Linux tools (no external services).
#
# Usage:
#   Run this script as a background process to continuously watch container events and
#   update the Nginx configuration dynamically.
#
#   Example:
#     nohup /bin/bash /path/to/nginx-controller.bash &>/dev/null &
#
# Requirements:
#   - Docker or containerd with accessible CLI (`ctr`).
#   - A running Nginx instance configured with the appropriate site-enabled directory.
#
# Copyright (c) 2024 Phus Lu <phus.lu@example.com>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# Author: Phus Lu <phus.lu@example.com>

NGINX_SITES_DIRECTORY=${NGINX_SITES_DIRECTORY:-/etc/nginx/sites-enabled}

pidtree() (
	# https://superuser.com/a/784102/114255
	[ -n "$ZSH_VERSION" ] && setopt shwordsplit
	declare -A childs
	while read pid ppid; do
		childs[$ppid]+=" $pid"
	done < <(ps -e -o pid= -o ppid=)
	walk() {
		for i in ${childs[$1]}; do
			walk $i
		done
		echo $1
	}

	for i in "$@";do
		walk $i
	done
)

generate_nginx_conf () {
	local cid=$1
	local name=$2
	local ipaddr=$3
	local port=$4
	local domain=$5
	echo "# generated by $0
# registered for ${cid}
upstream ${name}.internal {
	server ${ipaddr}:${port};
}"

	if [ "${domain}" != "" ]; then
		echo "
server {
	listen 80;
	listen [::]:80;
	server_name ${domain};
	location / {
		proxy_pass http://${name}.internal;
		proxy_http_version 1.1;
		proxy_set_header Host \$http_host;
		proxy_set_header Upgrade \$http_upgrade;
		proxy_set_header Connection Upgrade;
		proxy_read_timeout 180;
		proxy_connect_timeout 180;
		proxy_send_timeout 180;
		send_timeout 180;
	}
}"
	fi
}

update_nginx_conf () {
	local ns=$1
	local cid=$2
	local pid=$3

	if [[ $(ctr -n ${ns} container info ${cid}) =~ (\.|\/|host)name\":\ \"/?([^\"]+)\", ]]; then
		local name=${BASH_REMATCH[2]}
	else
		return
	fi

	if [[ $(nsenter -t ${pid} -n ip -o -4 a) =~ [0-9]+:\ e[a-z0-9]+\ +inet\ ([0-9\.]+) ]]; then
		local ipaddr=${BASH_REMATCH[1]}
	fi

	if [ -z "$ipaddr" ]; then
		sleep 5
		if [[ $(nsenter -t ${pid} -n ip -o -4 a) =~ [0-9]+:\ e[a-z0-9]+\ +inet\ ([0-9\.]+) ]]; then
			local ipaddr=${BASH_REMATCH[1]}
		fi
	fi

	local IFS=$'\0'
	readarray -d '' environ </proc/${pid}/environ
	unset IFS

	for line in "${environ[@]}"; do
		if [[ ${line} =~ ^DOMAIN=([^[:space:]]+) ]]; then
			local domain=${BASH_REMATCH[1]}
		elif [[ ${line} =~ ^PORT=([0-9]+) ]]; then
			local port=${BASH_REMATCH[1]}
		fi
	done

	if [ "${port}" == "0" ]; then
		for i in 0 1 2 3 5 5 5 5 5 5; do
			sleep ${i}
			local lines=$(nsenter -t ${pid} -n cat /proc/net/tcp{6,})
			local regexs=(
				': (00000000000000000000000000000000|00000000):(0050) '
				': (00000000000000000000000000000000|00000000):([0-9A-F]+) '
			)
			for regex in "${regexs[@]}"; do
				if [[ $lines =~ $regex ]]; then
					local port=$((16#${BASH_REMATCH[2]}))
					break 2
				fi
			done
		done
	fi

	if [ "${port}" == "0" -o "${port}" == "" ]; then
		return
	fi

	generate_nginx_conf "${cid}" "${name}" "${ipaddr}" "${port}" "${domain}" >${NGINX_SITES_DIRECTORY}/${name}.internal

	if nginx -t; then
		nginx -s reload
	fi
}

remove_nginx_conf () {
	local ns=$1
	local cid=$2

	rm -f $(grep -l "^# registered for ${cid}" ${NGINX_SITES_DIRECTORY}/*.internal)

	if nginx -t; then
		nginx -s reload
	fi
}

list_watch_reconcile () {
	ctr namespace ls -q | while read -r ns; do
		ctr -n ${ns} task ls | while read -r cid pid status; do
			if [ "$status" != "RUNNING" ]; then
				continue
			fi
			update_nginx_conf ${ns} ${cid} ${pid}
		done
	done

	ctr events | while read -r line; do
		echo "$line" >>/tmp/ctr-events.log
		if [[ $line =~ \ ([a-z0-9\.\-_]+)\ /tasks/start\ \{\"container_id\":\"([0-9a-f]+)\",\"pid\":([0-9]+) ]]; then
			update_nginx_conf "${BASH_REMATCH[1]}" "${BASH_REMATCH[2]}" "${BASH_REMATCH[3]}"
		elif [[ $line =~ \ ([a-z0-9\.\-_]+)\ /tasks/delete\ \{\"container_id\":\"([0-9a-f]+)\" ]]; then
			remove_nginx_conf "${BASH_REMATCH[1]}" "${BASH_REMATCH[2]}"
		fi
	done
}

trap 'kill $(pidtree $$)' EXIT
list_watch_reconcile