comment-sender.service

## comment-sender.service
[Unit]
Description=Comment sender for example.com

[Service]
Type=simple
User=nobody
Group=nobody
ExecStart=/usr/bin/python3 /opt/comment-sender/sender.py

PrivateDevices=True
ProtectSystem=strict
ProtectHome=True
NoNewPrivileges=True

[Install]
WantedBy=multi-user.target

## nginx.conf
http {
    # ...

    limit_req_zone $binary_remote_addr zone=per_min:1m rate=1r/m;

    server {
        # ....

        location / {
            try_files $uri $uri/ $uri.html =404;
        }

        location = /ping {
            proxy_pass http://127.0.0.80:8000/ping;
        }

        location = /send {
            proxy_pass http://127.0.80.10:8000/send;
            limit_req zone=per_min burst=2 nodelay;
        }
    }
}

## sender.py
#!/usr/bin/env python3
import time
from datetime import date
from smtplib import SMTP
from email.message import EmailMessage
from flask import Flask, Response, request
from werkzeug.datastructures import Headers
app = Flask(__name__)


LIMIT_MIN_INTERVAL = 180
LIMIT_BURST = 5

limit_left = LIMIT_BURST
limit_last = time.time()

def has_token():
    return limit_left > 0 or time.time() - limit_last > LIMIT_MIN_INTERVAL

def response_limited():
    headers = Headers()
    headers.add('retry-after', LIMIT_MIN_INTERVAL)
    return Response(status=429, headers=headers)

@app.route("/ping", methods=['GET'])
def ping():
    return '', 204 if has_token() else response_limited()

@app.route("/send", methods=['POST'])
def post():
    global limit_left, limit_last
    if not has_token():
        return response_limited()
    if limit_left < LIMIT_BURST:
        new_tokens = (time.time() - limit_last) // LIMIT_MIN_INTERVAL
        limit_left = min(LIMIT_BURST, limit_left + new_tokens)
    limit_left -= 1
    limit_last = time.time()

    name = request.form.get('name') or 'anonymous'
    email = request.form.get('email') or 'no-reply@sorz.org'
    message = request.form['message']
    send_message(name, email, message)
    return 'sent', 201

def send_message(name, reply, content):
    msg = EmailMessage()
    msg.set_content(content)
    msg['Subject'] = f'Message on example.com ({date.today()})'
    msg['From'] = f'{name} <comments@example.com>'
    msg['To'] = 'you@example.com'
    msg['Reply-To'] = f"{name} <{reply}>"
    with SMTP('localhost') as smtp:
        smtp.send_message(msg)

if __name__ == "__main__":
    app.run('127.0.0.80', 8000)