Mini Shell

#!/opt/imh-python/bin/python3
"""Executes a process, pausing and resuming it while monitoring server load"""
# vim: set ts=4 sw=4 expandtab syntax=python:

import sys
import argparse
import cproc

MISSING_RCODE = 127
BAD_ARGS_RCODE = 2


def parse_args() -> tuple[cproc.ProcLimit, list[str]]:
    """Parse limits used to create cproc.ProcLimit"""
    parser = argparse.ArgumentParser(
        description=__doc__,
        formatter_class=argparse.MetavarTypeHelpFormatter,
        usage='%(prog)s [-l float] [-s int -r int] -- COMMAND',
    )
    parser.add_argument(
        '--load',
        '-l',
        type=float,
        dest='load',
        help='Specify a custom maximum server load. '
        'If unset, uses number of cpu cores (1 on vps)',
    )
    grace = parser.add_argument_group(title='Grace periods')
    grace.add_argument(
        '--sleep',
        '-s',
        type=int,
        dest='sleep',
        help='Number of seconds to sleep if the server is overloaded. If not '
        'specified, sleep indefinitely until server load improves',
    )
    grace.add_argument(
        '--run',
        '-r',
        type=int,
        dest='run',
        help='Number of seconds to execute once -s/--sleep expires. '
        'Required if -s/--sleep is specified',
    )
    parser.add_argument('cmd', nargs='+', type=str, help=argparse.SUPPRESS)
    args = parser.parse_args()
    if args.sleep is not None and args.run is None:
        print('-r/--run is required if -s/--sleep is used', file=sys.stderr)
        sys.exit(BAD_ARGS_RCODE)
    limit = cproc.ProcLimit(
        value=args.load,
        grace=None if args.sleep is None else (args.sleep, args.run),
    )
    return limit, args.cmd


def main():
    """Collect args, execute command, return with the same return code"""
    limit, cmd = parse_args()
    try:
        proc = cproc.Proc(
            cmd,
            lim=limit,
            shell=False,
            encoding=None,
            stdout=None,
            stderr=None,
        )
        sys.exit(proc.wait())
    except FileNotFoundError as exc:
        print(exc, file=sys.stderr)
        sys.exit(MISSING_RCODE)


if __name__ == '__main__':
    main()