#!/usr/bin/env python3

"""
restart_service

This script is part of the feature which will restart the container if memory
usage of it is larger than the threshold value.

This script is intended to be run by Monit and is used to restart the specified
container if the memory usage of it is larger than the threshold value for X
times within Y cycles/minutes.

The following is an example in Monit configuration file to show how Monit will run
this script:

check program container_memory_<container_name> with path "/usr/bin/memory_checker <container_name> <threshold_value>"
    if status == 3 for X times within Y cycles exec "/usr/bin/restart_service <container_name>"
"""

import argparse
import sys
import syslog
import subprocess


def get_command_result(command):
    """Executes command and return the exit code, stdout and stderr.

    Args:
        command: A string contains the command to be executed.

    Returns:
        An integer contains the exit code.
        A string contains the output of stdout.
        A string contains the output of stderr.
    """
    command_stdout = ""
    command_stderr = ""

    try:
        proc_instance = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE,
                                         universal_newlines=True)
        command_stdout, command_stderr = proc_instance.communicate()
        if proc_instance.returncode != 0:
            return 1, command_stdout.strip(), command_stderr.strip()
    except (OSError, ValueError) as err:
        return 2, command_stdout.strip(), err

    return 0, command_stdout.strip(), command_stderr.strip()


def reset_failed_flag(service_name):
    """Reset the failed status of a service.

    Args:
        service_name: Name of the service.

    Returns:
        None
    """
    reset_failed_command = ["sudo", "systemctl", "reset-failed", "{}.service".format(service_name)]

    syslog.syslog(syslog.LOG_INFO, "Resetting failed status of service '{}' ..."
                  .format(service_name))

    exit_code, command_stdout, command_stderr = get_command_result(reset_failed_command)
    if exit_code == 0:
        syslog.syslog(syslog.LOG_INFO, "Succeeded to reset failed status of service '{}.service'."
                      .format(service_name))
    else:
        syslog.syslog(syslog.LOG_ERR, "Failed to reset failed status of service '{}'. Error: {}"
                      .format(service_name, command_stderr))


def restart_service(service_name):
    """Reset the failed status of a service and then restart it.

    Args:
        service_name: Name of specified service.

    Returns:
        None.
    """
    restart_command = ["sudo", "systemctl", "restart", "{}.service".format(service_name)]

    reset_failed_flag(service_name)

    syslog.syslog(syslog.LOG_INFO, "Restarting service '{}' ...".format(service_name))
    exit_code, command_stdout, command_stderr = get_command_result(restart_command)
    if exit_code != 0:
        syslog.syslog(syslog.LOG_ERR, "Failed to restart the service '{}'. Error: {}"
                      .format(service_name, command_stderr))


def main():
    parser = argparse.ArgumentParser(description="Restart a specific service",
                                     usage="/usr/bin/restart_service <service_name>")
    parser.add_argument("service_name", help="service name")
    args = parser.parse_args()

    restart_service(args.service_name)


if __name__ == "__main__":
    main()