entry-point

#!/usr/bin/env python3

import argparse as ap
import contextlib as cl
import os
import pathlib
import shlex
import sys
import tarfile
import urllib.request as ur

def parse_args():
    parser = ap.ArgumentParser(
        description="Runs application packages in the current container.",
    )
    parser.add_argument(
        '--url',
        nargs='?',
        default=os.environ.get('APP_PACKAGE_URL'),
        help='Application package URL (optional; read from APP_PACKAGE_URL if set)',
    )
    parser.add_argument(
        '--program',
        nargs='?',
        default=os.environ.get('APP_PACKAGE_PROGRAM'),
        help='Program to run (optional; read from APP_PACKAGE_PROGRAM if set; if not set, runs a shell)',
    )
    return parser.parse_args()

def install_package(source_url):
    with cl.ExitStack() as stack:
        response = stack.enter_context(ur.urlopen(source_url))
        archive = stack.enter_context(tarfile.open(fileobj=response, mode='r:gz'))
        archive.extractall(path='/')

WHITELIST_ENV_VARS = {
    'HOSTNAME',
    'PWD',
    'HOME',
    'TERM',
    'LC_CTYPE',
    'PATH',
}

BASH_INIT_FILE = pathlib.Path(__file__).parent / 'bash-init'

def base_environment():
    return dict(
        (k, v) for (k, v) in os.environ.items() if k in WHITELIST_ENV_VARS
    )

def program_environment():
    return dict(
        **base_environment(),
        BASH_ENV=BASH_INIT_FILE,
    )

def start_program(path):
    environ = program_environment()
    os.execle('/bin/bash', '/bin/bash', '-c', f"exec {shlex.quote(path)}", environ)

def start_interactive_shell():
    environ = base_environment()
    os.execle('/bin/bash', '/bin/bash', '--rcfile', BASH_INIT_FILE, environ)

def main():
    args = parse_args()

    if args.url is not None:
        install_package(args.url)
    if args.program is not None:
        start_program(args.program)

    start_interactive_shell()

if __name__ == '__main__':
    sys.exit(main())