#!/usr/bin/env python2

"""Fault tolerance system for WebKit.

Contributed to Webware for Python by Jay Love.

This module is intended to provide additional assurance that the
AppServer continues running at all times. This module will be
responsible for starting the AppServer, and monitoring its health.
It does that by periodically sending a status check message to the
AppServer to ensure that it is responding. If it finds that the
AppServer does not respond within a specified time, it will start a
new copy of the AppServer, after killing the previous process.

Use::

  $ python Monitor.py start
  $ python Monitor.py stop

The default AppServer specified below will be used, or you can list
the AppServer you would like after ``start``.

You can have the whole process run as a daemon by specifying ``daemon``
after ``start`` on the command line.

To stop the processes, run ``Monitor.py stop``.
"""

# Future:
# Add ability to limit number of requests served. When some count is reached,
# send a message to the server to save its sessions, then exit. Then start
# a new AppServer that will pick up those sessions.

# It should be possible on both Unix and Windows to monitor the AppServer
# process in 2 ways:
# 1) The method used here, i.e. can it service requests?
# 2) is the process still running?

# Combining these with a timer lends itself to load balancing of some kind.

defaultServer = "ThreadedAppServer"
monitorInterval = 10  # add to config if this implementation is adopted
maxStartTime = 120

"""Module global:

`defaultServer`:
    default ``"ThreadedAppServer"``. The type of AppServer
    to start up (as listed in ``Launch.py``)
`monitorInterval`:
    default 10. Seconds between checks.
`maxStartTime`:
    default 120. Seconds to wait for AppServer to start
    before killing it and trying again.
"""

import os
import sys
import time
import socket
import signal
from marshal import dumps

# Initialize some more global variables

serverName = defaultServer
srvpid = 0
addr = None
running = False

debug = True

statstr = dumps({'format': 'STATUS'})
statstr = dumps(len(statstr)) + statstr
quitstr = dumps({'format': 'QUIT'})
quitstr = dumps(len(quitstr)) + quitstr


## Start ##

def createServer(setupPath=0):
    """Unix only, executed after forking for daemonization."""
    print "Starting Server..."

    import WebKit
    code = 'from WebKit.%s import main' % serverName
    exec code
    main(['start'])


def startupCheck():
    """Make sure the AppServer starts up correctly."""
    if os.name == 'posix':
        print "Waiting for start..."
        time.sleep(monitorInterval / 2)  # give the server a chance to start
        count = 0
        while 1:
            if checkServer(False):
                break
            count += monitorInterval
            if count > maxStartTime:
                print "Couldn't start AppServer."
                print "Killing AppServer..."
                os.kill(srvpid, signal.SIGKILL)
                sys.exit(1)
            print "Waiting for start..."
            time.sleep(monitorInterval)


def startServer(killcurrent=True):
    """Start the AppServer.

    If `killcurrent` is true or not provided, kill the current AppServer.
    """
    global srvpid
    if os.name == 'posix':
        if killcurrent:
            try:
                os.kill(srvpid, signal.SIGTERM)
            except Exception:
                pass
            try:
                os.waitpid(srvpid, 0)
            except Exception:
                pass
        srvpid = os.fork()
        if srvpid == 0:
            createServer(not killcurrent)
            sys.exit()


def checkServer(restart=True):
    """Send a check request to the AppServer.

    If restart is 1, then attempt to restart the server
    if we can't connect to it.

    This function could also be used to see how busy an AppServer
    is by measuring the delay in getting a response when using the
    standard port.
    """
    try:
        sts = time.time()
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.connect(addr)
        s.sendall(statstr)
        s.shutdown(1)
        resp = s.recv(9)  # up to 1 billion requests!
        monwait = time.time() - sts
        if debug:
            print "Processed %s Requests." % resp
            print "Delay %s." % monwait
        return True
    except Exception:
        print "No Response from AppServer."
        if running and restart:
            startServer()
            startupCheck()
        else:
            return False


def main(args):
    """The main loop.

    Starts the server with `startServer(False)`,
    checks it's started up (`startupCheck`), and does a
    loop checking the server (`checkServer`).
    """
    global running
    running = True

    with open('monitor.pid', 'w') as f:
        if os.name == 'posix':
            f.write(str(os.getpid()))
        f.flush()
    startServer(False)
    try:
        startupCheck()
    except Exception as e:
        if debug:
            print "Startup check exception:", e
            print "Exiting monitor..."
        try:
            os.kill(srvpid, signal.SIGTERM)
        except Exception:
            pass
        sys.exit()

    while running:
        try:
            if debug:
                print "Checking server..."
            checkServer()
            time.sleep(monitorInterval)
        except Exception as e:
            if debug:
                print "Exception:", e
            if not running:
                return
            print "Exiting Monitor..."
            try:
                os.kill(srvpid, signal.SIGTERM)
            except Exception:
                sys.exit(0)
            try:
                os.waitpid(srvpid, 0)  # prevent zombies
            except Exception:
                sys.exit(0)


def shutDown(signum, frame):
    """Shutdown handler.

    For when Ctrl-C has been hit, or this process is being cleanly killed.
    """
    global running
    print "Monitor Shutdown Called."
    sys.stdout.flush()
    running = False
    try:
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.connect(addr)
        s.sendall(quitstr)
        s.shutdown(1)
        resp = s.recv(10)
        s.close()
        print "AppServer response to shutdown request:", resp
    except Exception as e:
        print e
        print "No Response to shutdown request, performing hard kill."
        os.kill(srvpid, signal.SIGINT)
        os.waitpid(srvpid, 0)
    sys.stdout.flush()
    sys.stderr.flush()
    return False

signal.signal(signal.SIGINT, shutDown)
signal.signal(signal.SIGTERM, shutDown)


## Stop ##

def stop():
    """Stop the monitor.

    This kills the other monitor process that has been opened
    (from the PID file ``monitor.pid``).
    """
    pid = int(open("monitor.pid", "r").read())
    # this goes to the other running instance of this module
    os.kill(pid, signal.SIGINT)


## Command line interface ##

def usage():
    print """
This module serves as a watcher for the AppServer process.
The required command line argument is one of:

  start: Starts the monitor and default appserver.

  stop: Stops the currently running Monitor process and the AppServer
        if is running. This is the only way to stop the process other
        than hunting down the individual process ID's and killing them.

Optional arguments:

"AppServer": The AppServer class to use (currently only ThreadedAppServer)
daemon:      If "daemon" is specified, the Monitor will run
             as a background process.
"""

arguments = ["start", "stop"]
servernames = ["ThreadedAppServer"]
optionalargs = ["daemon"]

if __name__ == '__main__':

    if os.name != 'posix':
        print "This service can only be run on Posix machines (UNIX)."
        sys.exit()

    if len(sys.argv) == 1:
        usage()
        sys.exit()

    args = sys.argv[1:]
    if args[0] not in arguments:
        usage()
        sys.exit()

    if True:  # setup path:
        if '' not in sys.path:
            sys.path = [''] + sys.path
        try:
            import WebwarePathLocation
            wwdir = os.path.abspath(os.path.join(os.path.dirname(
                WebwarePathLocation.__file__), '..'))
        except Exception as e:
            print e
            usage()
        if not wwdir in sys.path:
            sys.path.insert(0, wwdir)
        sys.path.remove('')
        try:
            sys.path.remove('.')
        except Exception:
            pass

    cfgfile = open(os.path.join(wwdir, 'WebKit',
        'Configs/AppServer.config'), 'rU').read()
    cfg = dict(WebwarePath=wwdir)
    if cfgfile.lstrip().startswith('{'):
        cfg = eval(cfgfile, cfg)
    else:
        exec cfgfile in cfg
    if not cfg.get('EnableMonitor'):
        print "Monitoring has not been enabled in AppServer.config!"
        sys.exit()
    host = cfg.get('Host', '127.0.0.1')
    port = cfg.get('MonitorPort', 8085)
    addr = (host, port)

    if 'stop' in args:
        stop()
        sys.exit()

    for i in servernames:
        if i in args:
            serverName = i

    if 'daemon' in args:  # fork and become a daemon
        daemon = os.fork()
        if daemon:
            sys.exit()

    main(args)
