#!/usr/bin/python

"""
Helper script for running long-living processes.

(Name says daemon, but that is intended to mean "long-living", we
assume child process does not double-fork.)

We start the command passed as arguments, with /dev/null as stdin, and
then wait for EOF on stdin.

When EOF is seen on stdin, the child process is killed.

When the child process exits, this helper exits too.

usage: daemon-helper <signal> [--kill-group] [nostdin] command...
"""

import fcntl
import os
import select
import signal
import struct
import subprocess
import sys

end_signal = signal.SIGKILL
if sys.argv[1] == "term":
    end_signal = signal.SIGTERM

cmd_start = 2

group = False
if sys.argv[cmd_start] == "--kill-group":
    group = True
    cmd_start += 1

nostdin = False
if sys.argv[cmd_start] == "nostdin":
    nostdin = True
    cmd_start += 1

proc = None
if nostdin:
    proc = subprocess.Popen(
        args=sys.argv[cmd_start:],
        )
else:
    with file('/dev/null', 'rb') as devnull:
        proc = subprocess.Popen(
            args=sys.argv[cmd_start:],
            stdin=devnull,
            preexec_fn=os.setsid,
            )

flags = fcntl.fcntl(0, fcntl.F_GETFL)
fcntl.fcntl(0, fcntl.F_SETFL, flags | os.O_NDELAY)

saw_eof = False
while True:
    r,w,x = select.select([0], [], [0], 0.2)
    if r:
        data = os.read(0, 1)
        if not data:
            saw_eof = True
            if not group:
                proc.send_signal(end_signal)
            else:
                os.killpg(proc.pid, end_signal)
            break
        else:
            sig, = struct.unpack('!b', data)
            if not group:
                proc.send_signal(sig)
            else:
                os.killpg(proc.pid, end_signal)


    if proc.poll() is not None:
        # child exited
        break

exitstatus = proc.wait()
if exitstatus > 0:
    print >>sys.stderr, '{me}: command failed with exit status {exitstatus:d}'.format(
        me=os.path.basename(sys.argv[0]),
        exitstatus=exitstatus,
        )
    sys.exit(exitstatus)
elif exitstatus < 0:
    if saw_eof and exitstatus == -end_signal:
        # suppress error from the exit we intentionally caused
        pass
    else:
        print >>sys.stderr, '{me}: command crashed with signal {signal:d}'.format(
            me=os.path.basename(sys.argv[0]),
            signal=-exitstatus,
            )
        sys.exit(1)