"""Render the nginx config for the current MONTY_UI mode, then exec nginx.

Picked up by supervisord's [program:nginx] entry. Reads the MONTY_UI env var
(default 'vue'), substitutes the chosen upstream into nginx.conf.template,
writes the result to /etc/nginx/nginx.conf, and execs nginx in foreground
mode so supervisord keeps tracking the same PID.
"""
import os
import sys


TEMPLATE_PATH = '/etc/nginx/nginx.conf.template'
TARGET_PATH = '/etc/nginx/nginx.conf'
UPSTREAM_PLACEHOLDER = '__MONTY_UPSTREAM__'
CACHE_VALID_PLACEHOLDER = '__INGESTION_CACHE_VALID__'

INGESTION_CACHE_VALID = '3m'

UPSTREAMS = {
    'vue': 'flask_backend',
    'streamlit': 'streamlit_backend',
}


def main():
    """Render the nginx config and exec nginx in foreground."""
    mode = os.environ.get('MONTY_UI', 'vue').strip().lower()
    upstream = UPSTREAMS.get(mode)
    if upstream is None:
        choices = ', '.join(sorted(UPSTREAMS))
        sys.stderr.write(
            f'nginx-entrypoint: MONTY_UI={mode!r} is not supported '
            f'(expected one of: {choices})\n',
        )
        sys.exit(1)

    with open(TEMPLATE_PATH) as src:
        rendered = (
            src.read()
            .replace(UPSTREAM_PLACEHOLDER, upstream)
            .replace(CACHE_VALID_PLACEHOLDER, INGESTION_CACHE_VALID)
        )
    with open(TARGET_PATH, 'w') as dst:
        dst.write(rendered)

    print(
        f'nginx-entrypoint: MONTY_UI={mode} -> upstream={upstream}, '
        f'ingestion_cache_valid={INGESTION_CACHE_VALID}',
        flush=True)
    os.execvp('nginx', [
        'nginx', '-c', TARGET_PATH, '-g', 'daemon off;'])


if __name__ == '__main__':
    main()