/* CodeQ: an online programming tutor.
   Copyright (C) 2015 UL FRI

This program is free software: you can redistribute it and/or modify it under
the terms of the GNU Affero General Public License as published by the Free
Software Foundation, either version 3 of the License, or (at your option) any
later version.

This program is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
details.

You should have received a copy of the GNU Affero General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */

var engine = require('engine.io'), // web sockets communication handler, sitting on the low-level HTTP handler
    http = require('http'), // HTTP library
    https = require('https'), // HTTPS library
    http_server = http.createServer(), // the low-level HTTP handler
    querystring = require('querystring'), // encodes and decodes URL query strings
    net = require('net'), // TCP sockets library
    Promise = require('bluebird'), // the promises library
    log4js = require('log4js'), // the logger
    url = require('url'), // URL parser
    express = require('express'), // library providing the Express web framework
    http_app = express(), // web framework engine, sitting on the low-level HTTP handler
    bodyParser = require('body-parser');

var SAML_SERVICE_URL = process.env.CODEQ_SAML_SERVICE_URL || 'https://codeq.si/saml/';

var samlUrlParsed = url.parse(SAML_SERVICE_URL),
    samlIsHttps = samlUrlParsed.protocol === 'https:';

// ================================================================================
// The logger
// ================================================================================

log4js.loadAppender('file');
log4js.addAppender(log4js.appenders.file(process.env.CODEQ_WEB_LOG || 'codeq-web.log'), 'log');
var logger = log4js.getLogger('log');

var logException = function (msg, error) {
    if ((error instanceof Error) && (typeof error.stack === 'string')) msg += '\n' + error.stack;
    logger.error(msg);
};

// prevent the node from exiting because of an unhandled exception
Promise.onPossiblyUnhandledRejection(function (error) {
    logException('Unhandled promise rejection: ' + error, error);
});

process.on('uncaughtException', function (error) {
    logException('Uncaught exception: ' + error, error);
});

// ================================================================================
// Low-level HTTP handlers, just do the minimum and forward to upper layers
// ================================================================================

// instantiate the web socket communication handler
var server = new engine.Server({
    'pingTimeout': 60000, // in ms
    'pingInterval': 25000, // in ms
    'maxHttpBufferSize': 100000, // in bytes, for polling
    'transports': ['polling', 'websocket'],
    'cookie': false // no cookies
});

http_server.listen(8083, 'localhost');

http_server.on('request', function (request, response) {
    var uriParts = request.url.split('/'),  // uriParts[0] will be an empty string, because uri must start with a /
        params;

    logger.debug('HTTP server request, URL: ' + request.url);

    if ((uriParts.length >= 3) && (uriParts[1] === 'ws')) {
        // could be a web services request, check if there's a path following
        params = uriParts[2];
        if (!params || (params[0] === '?')) {
            server.handleRequest(request, response); // no path follows /ws/, only query string, forward it to engine.io
        }
        else {
            http_app(request, response); // there's a path present beyond /ws/, forward it to express
        }
    }
    else {
        http_app(request, response); // URI does not begin with /ws/, forward it to express
    }
});

http_server.on('upgrade', function (request, socket, head) {
    server.handleUpgrade(request, socket, head); // initiating web socket communication
});

// ================================================================================
// Misc
// ================================================================================

var doubleDigit = function (d) {
    var s = '' + d;
    if (s.length < 2) return '0' + s;
    return s;
};

var tripleDigit = function (d) {
    var s = '' + d;
    while (s.length < 3) s = '0' + s;
    return s;
};

var formatTimestamp = function (t) {
    var d = new Date(t);
    return '' + d.getFullYear() + '-' + doubleDigit(d.getMonth()+1) + '-' + doubleDigit(d.getDate()) + ' ' +
        doubleDigit(d.getHours()) + ':' + doubleDigit(d.getMinutes()) + ':' + doubleDigit(d.getSeconds()) + '.' + tripleDigit(d.getMilliseconds());
};

// ================================================================================
// Express web framework, handle any AJAX here
// ================================================================================

http_app.use(bodyParser.json());

http_app.get('/ws/logout', function (req, res) {
    var sid = req.query.sid,
        session = sessions[sid];
    if (session) {
        logger.debug('Logging out via AJAX, sid=' + sid);
        guiHandlers.destroy(session, {'sid': sid, 'action': 'destroy'});
    }
    else if (sid) {
        logger.warn('Cannot logout via AJAX: no session exists with sid=' + sid);
    }
    else {
        logger.warn('No sid provided for AJAX logout');
    }
    res.set('Content-Type', 'text/plain');
    res.send('OK');
});

http_app.post('/ws/clientlog', function (req, res) {
    var b = req.body,
        logs, i, l, line, output, f;
    if (!b) {
        logger.error('Empty clientlog request.');
        return;
    }
    logs = b.logs;
    if (!logs) {
        logger.error('Empty logs in a clientlogs request.');
        return;
    }
    output = ['Client logs from ', req.ip];
    f = req.headers['x-forwarded-for'];
    if (f) output.push(', forwarded for ', f);
    output.push(':');
    for (i = 0; i < logs.length; i++) {
        l = logs[i];
        output.push('\n    [', formatTimestamp(l.t), '] [', l.l, '] ', l.m);
    }
    logger.info(output.join(''));
    res.set('Content-Type', 'text/plain');
    res.send('OK');
});


var performSamlAction = function (samlAction, params) {
    return new Promise(function (resolve, reject) {
        var aborted = false,
            options = {
                hostname: samlUrlParsed.hostname,
                port: samlUrlParsed.port || (samlIsHttps ? 443 : 80),
                path: samlUrlParsed.pathname + samlAction + '?' + querystring.encode(params),
                method: 'GET',
                rejectUnauthorized: false // allow server certificates we don't know
            },
            handleResponse = function (res) {
                var chunks = [],
                    headers, m, s;
                res.on('data', function (d) {
                    chunks.push(d);
                });
                res.on('end', function () {
                    if (aborted) return; // already handled
                    if (res.statusCode !== 200) {
                        // log error with all headers
                        headers = res.rawHeaders;
                        if (headers instanceof Array) m = headers.join('\n') + '\n\n' + chunks.join('');
                        else m = chunks.join('');
                        s = 'Received an error from the identity server: ' + res.statusCode + ' ' + res.statusMessage;
                        logger.error(s + '\n' + m);
                        reject(new Error(s));
                        return;
                    }
                    try {
                        m = JSON.parse(chunks.join(''));
                    }
                    catch (e) {
                        logException('Response from the identity server is not a JSON: ' + e + '\n' + chunks.join(''), e);
                        reject(e);
                        return;
                    }
                    resolve(m);
                });
            },
            req;
        if (samlIsHttps) req = https.request(options, handleResponse);
        else req = http.request(options, handleResponse);
        req.on('error', function (e) {
            if (aborted) return; // already handled
            logException('Error when trying to contact the identity server: ' + e, e);
            reject(e instanceof Error ? e : new Error('' + e));
        });
        req.setTimeout(600000, function () { // 10 minutes timeout
            var m = 'Timeout waiting for a reply from the identity server';
            aborted = true;
            req.abort();
            logger.error(m);
            reject(new Error(m));
        });
        req.end();
    });
};

// ================================================================================
// Web services stuff - handlers for GUI requests, usually just forwarded to Python
// ================================================================================

// connected clients
var sessions = {
    // sid: sessions description {sid: string, lastActivity: Date.now(), socket: net.Socket}
};

// GUI action handlers, keyed by the action name, values are functions that take the session and the message, or truthy values
var guiHandlers = {
    // special-case action handlers
    'destroy': function actionLogout(session, message) {
        // logout and destroy the session, the user quit the app
        logger.debug('Destroy GUI');
        sendDataToPython(message).finally(function () {
            //session.end({'type': 'reset', 'code': 9999, 'message': 'Bye.'});
            session.end({'code': 0, 'message': 'Bye.', 'tid': message.tid, 'sid':message.sid});
        }).done();
    },

    'saml_login': function samlLogin(session, message) {
        var samlData = session.samlData,
            p;
        if (samlData && ('upgrade_account' in message)) {
            message.saml_data = samlData.userData;
            p = sendDataToPython(message);
        }
        else {
            p = performSamlAction('WaitLogin', {'sid': session.sid})
                .then(function (jsonObj) {
                    if (jsonObj.code !== 0) return jsonObj;
                    samlData = jsonObj.data;
                    if (!samlData) return {'tid': message.tid, 'sid': message.sid, 'code': -2, 'message': 'IdP proxy did not return any SAML data'};
                    if (!samlData.userData) return {'tid': message.tid, 'sid': message.sid, 'code': -3, 'message': 'IdP proxy did not return any SAML user data'};

                    // we need the SAML data to be able to perform global logout (saml_logout action)
                    session.samlData = samlData;

                    // add the data from SAML authentication, and forward everything to Python
                    message.saml_data = samlData.userData;
                    return sendDataToPython(message);
                });
        }

        p.then(session.send)
            .catch(function (e) {
                var m = 'Error while trying to process the SAML request using the identity server: ' + e;
                logException(m, e);
                session.send({'tid': message.tid, 'sid': message.sid, 'code': -1, 'message': m});
            })
            .done();
    },

    'saml_logout': function samlLogout(session, message) {
        var samlData = session.samlData,
            keys, sd, key, i;
        if (!samlData) {
            session.send({'tid': message.tid, 'sid': message.sid, 'code': -1, 'message': 'The session was not authenticated using SAML'});
            return;
        }
        // make a copy of SAML data, excluding user data
        keys = Object.keys(samlData);
        sd = {};
        for (i = keys.length - 1; i >= 0; i--) {
            key = keys[i];
            if (key !== 'userData') sd[key] = samlData[key];
        }

        delete session.samlData;

        performSamlAction('WaitLogout', {'sid': session.sid, 'saml': JSON.stringify(sd)})
            .then(function (jsonObj) {
                if (jsonObj.code !== 0) return jsonObj;
                return sendDataToPython(message);
            })
            .then(session.send)
            .catch(function (e) {
                var m = 'Error while trying to process the SAML request using the identity server: ' + e;
                logException(m, e);
                session.send({'tid': message.tid, 'sid': message.sid, 'code': -2, 'message': m});
            })
            .done();
    },

    // actions to use default handling should define truthy values that are not functions
    // (this is to filter out unnecessary traffic before it hits Python)
    'login':                true,
    'logout':               true,
    'signup':               true,
    'change_password':      true,
    'activity':             true,
    'query':                true,
    'python_exec':          true,
    'python_push':          true,
    'python_stop':          true,
    'hint':                 true,
    'test':                 true,
    'get_attempts':         true,
    'get_solutions':        true,
    'get_current_solution': true,
    'update_settings':      true,
    'load_problem':         true,
    'end_problem':          true,
    'user_stat':            true,
    'system':               true
};

var system_handlers = {
    'create_session': function (session, message) {
        sendDataToPython({'action': 'create_session'})
            .then(function (response) {
                var sid, existingSession;
                if (response.code !== 0) {
                    logger.debug('Python rejected create_session request from GUI');
                    session.send({'type': 'reset', 'message': response.message});
                }
                else {
                    logger.debug('Python accepted create_session request from GUI');
                    session.sid = sid = response.sid;
                    existingSession = sessions[sid];
                    sessions[sid] = session;
                    if (existingSession && (existingSession !== session)) {
                        existingSession.end({'type': 'reset', 'message': 'Supplanted with a new connection for the same session', 'sid': sid});
                    }
                    session.send({'type': 'sid', 'sid': sid});
                }
            })
            .catch(function (error) {
                session.send({'type': 'reset', 'message': 'Could not create a new session: ' + (error || 'unknown error')});
            })
            .done();
    },

    'connect_session': function (session, message) {
        sendDataToPython({'action': 'login', 'sid': session.sid, 'username': message.username, 'password': message.password})
            .then(function (response) {
                var sid, existingSession;
                if (response.code !== 0) {
                    logger.debug('Python rejected connect_session request from GUI');
                    session.send({'type': 'reset', 'message': response.message});
                }
                else {
                    logger.debug('Python accepted connect_session request from GUI');
                    session.sid = sid = response.sid;
                    existingSession = sessions[sid];
                    sessions[sid] = session;
                    if (existingSession && (existingSession !== session)) {
                        existingSession.end({'type': 'reset', 'message': 'Supplanted with a new connection for the same session', 'sid': sid});
                    }
                    session.send({'type': 'sid', 'sid': sid});
                }
            })
            .catch(function (error) {
                session.send({'type': 'reset', 'message': 'Could not connect the existing session: ' + (error || 'unknown error')});
            })
            .done();
    }
};

server.on('connection', function (socket) {
    var session,
        fatal = function (jsonObj) {
            // we need to preserve the socket object in a variable, so we can invoke close() on it later
            var s = socket, m;
            if (s === null) return;
            if (jsonObj instanceof Error) jsonObj = {'code': -50, 'message': jsonObj.toString()};
            m = JSON.stringify(jsonObj);
            logger.debug('Sending to GUI from fatal(): ' + m);
            try {
                s.send(m, function () {
                    s.close();
                    s = null;
                });
                setTimeout(function () { // give it a second, then close it forcefully if it's still open
                    if (s) {
                        try {
                            s.close();
                        }
                        catch (e) {} // ignore
                        s = null;
                    }
                }, 1000);
            }
            catch (e) {
                logException('Failed to send reply to GUI about a fatal error: ' + e, e);
                try {
                    s.close();
                    s = null;
                }
                catch (e) {} // ignore
            }
            socket = null; // this socket will no longer be processed
            session.socket = null;
        },
        reply = function (jsonObj) {
            var m;
            if (!socket) return false;
            m = JSON.stringify(jsonObj);
            logger.debug('Sending to GUI from reply(): ' + m);
            try {
                socket.send(m);
                return true;
            }
            catch (e) {
                socket = null; // this socket will no longer be processed
                session.socket = null;
                return false;
            }
        };

    logger.debug('A new client connection established');

    session = {
        'sid': null, // session id, null means the python hasn't authenticated us yet
        'lastActivity': Date.now(),
        'socket': socket,
        'lastTID': -1, // last transaction ID, anything less than this we've already handled
        'send': reply,
        'end': fatal
    };

    socket.on('close', function (reason, description) { // description is optional
        logger.debug('GUI socket closed: ' + reason);
        if (session.sid !== null) {
            if (sessions[session.sid] === session) delete sessions[session.sid];
            sendDataToPython({'type': 'unregister', 'sid': session.sid}).done();
        }
    });

    socket.on('message', function (data) {
        var m, tid, action, handler;

        if (socket === null) {
            // this socket is not being processed anymore
            logger.debug('Received something from GUI, but the socket is not being processed anymore');
            return;
        }

        if (typeof data !== 'string') {
            logger.debug('Received from GUI a binary message');
            fatal({"code": -1, "message": "Can handle only text messages."});
            return;
        }

        logger.debug('Received from GUI: ' + data);

        try {
            m = JSON.parse(data);
        }
        catch (e) {
            fatal({"code": -2, "message": "Not a JSON message."});
            return;
        }

        if (typeof m.type === 'string') {
            // a system (meta) protocol message
            handler = system_handlers[m.type];
            if (!handler) logger.error('Received an unknown system message from client: ' + m.type);
            else handler(session, m);
            return;
        }

        tid = m['tid']; // transaction ID
        if (typeof tid !== 'number') {
            fatal({"code": -3, "message": "Transaction ID is missing or is not an integer."});
            return;
        }

        if (Math.floor(tid) !== tid) {
            fatal({"code": -4, "message": "Transaction ID is not an integer."});
            return;
        }

        session.lastActivity = Date.now();

        if (tid <= session.lastTID) {
            // we already met this message, ignore it
            return;
        }

        action = m['action'];
        if (typeof action !== 'string') {
            if (typeof action === 'undefined') {
                reply({"code": -10, "tid": tid, "message": "Action is not set."});
            }
            else if (action === null) {
                reply({"code": -11, "tid": tid, "message": "Action is null."});
            }
            else {
                reply({"code": -12, "tid": tid, "message": "Action is not a string."});
            }
            return;
        }

        if (!action) {
            reply({"code": -13, "tid": tid, "message": "Action is an empty string."});
            return;
        }

        handler = guiHandlers[action];
        if (!handler) {
            reply({"code": -14, "tid": tid, "message": "Action is not understood."});
            return;
        }

        try {
            if (typeof handler === 'function') {
                // special case processing
                handler(session, m);
            }
            else {
                // default processing: just proxy it through to Python
//                logger.debug((session.sid || '[no session]') + ': Received ' + action + ' from GUI');
                sendDataToPython(m).then(session.send, session.end).done();
            }
            session.lastTID = tid;
        }
        catch (e) {
            reply({"code": -20, "tid": tid, "message": "Processing failed: " + e.toString()});
        }
    });
});


// ================================================================================
// Python server connection - sending to and receiving data from Python
// ================================================================================

var pythonClient = null, // the Socket for communication with the python server
    pythonSaturated = false, // whether the kernel-level buffer is full, in this case only push into the pythonQueue
    pythonQueue = [], // queue of packets, waiting to be sent
    pythonPromises = {}; // promises waiting to be resolved with incoming data from python; key is TID, or TID:SID if SID is available, value is {'created': Date.now(), resolve: function, reject: function}

var pythonPromisesTimeout = setInterval(function () {
    var floor = Date.now() - 300000, // timeout whatever is waiting for more than 5 minutes
        key, resolver;
    for (key in pythonPromises) {
        if (pythonPromises.hasOwnProperty(key)) {
            resolver = pythonPromises[key];
            if ((typeof resolver === 'object') && (resolver !== null) && (resolver.created < floor)) {
                delete pythonPromises[key];
                resolver.reject(new Error('Timeout waiting for reply from server'));
                logger.warn('Python promise timed out at key ' + key);
            }
        }
    }
}, 60000); // once every minute

var processPacketFromPython = function processPacketFromPythonFunc(packetString) {
    var m, session, isDummy,
        key = null,
        resolver = null;

    logger.debug('Received from Python: ' + packetString);

    try {
        m = JSON.parse(packetString);
    }
    catch (e) {
        logger.error('Failed to parse JSON data from Python: ' + e.toString() + '\n  * The packet that caused the error: ' + packetString);
        return;
    }

    if (typeof m.type === 'string') {
        // system (meta) protocol message
        switch (m.type) {
            case 'session_expire':
                if (typeof m.sid === 'string') {
                    session = sessions[m.sid];
                    if (session) session.end({'type': 'reset', 'message': 'Session expired'}); // send a system message to force the client to reset
                    else logger.warn('Received the session_expire system message from Python for a sid not handled by me: ' + m.sid);
                }
                else {
                    logger.error('Received the session_expire system message from Python, but no sid was included');
                }
                break;

            default:
                logger.warn('An unknown system message from Python: ' + m.type);
                break;
        }
        return;
    }

    if ((typeof m.tid !== 'undefined') && (m.tid !== null)) {
        if ((typeof m.tid === 'string') && (m.tid.charAt(0) === 'i')) {
            // internal TID: only one way to key this one
            key = m.tid;
            resolver = pythonPromises[key];
        }
        else if (m.sid) {
            key = m.tid + ':' + m.sid;
            resolver = pythonPromises[key];
            if (!resolver) {
                key = '' + m.tid;
                resolver = pythonPromises[key];
            }
        }
        else {
            key = '' + m.tid;
            resolver = pythonPromises[key];
        }
        if (resolver) delete pythonPromises[key];
    }

    if (!m.tid || !resolver) {
        // no promise is waiting, because this is a generated message, not a response to a previously sent message
        isDummy = true;
        for (key in m) {
            if (m.hasOwnProperty(key) && (key !== 'tid') && (key != 'sid')) {
                isDummy = false;
                break;
            }
        }
        if (isDummy) {
            logger.debug('Message from Python is a dummy message, ignoring');
            return;
        }
        if (!m.sid) {
            // no session ID -> nowhere to send the message
            logger.error('Message from Python dropped because there is no outstanding promise for it, and no session ID has been set');
            return;
        }
        session = sessions[m.sid];
        if (session) {
            logger.debug('Proxying a message from Python directly to GUI, as there is no outstanding promise for it (probably a generated message)');
            session.send(m);
        }
        else {
            logger.error('Message from Python dropped because there is no outstanding promise for it, and no active session with ID ' + m.sid);
        }
        return;
    }

    logger.debug('Invoking the promise that is waiting for this Python message');
    resolver.resolve(m);
};

// this is internal function to python connection handler
var sendPacketToPython = function sendPacketToPythonFunc(packet) {
    if (pythonSaturated || (pythonClient === null)) return false;
    logger.debug('Sending to Python: ' + packet);
    try {
        pythonSaturated = !pythonClient.write(packet);
        return true;
    }
    catch (e) {
        // an error occurred: disconnect; the close event handler will trigger a connection retry
        logger.error('Failed to write to the Python client connection: ' + e.toString());
        try {
            pythonClient.close();
        }
        catch (e2) {
            logger.error('Failed to close() the Python client connection: ' + e2.toString());
        }
        pythonClient = null;
        return false;
    }
};

var internalTID = 0;
var sendDataToPython = function sendDataToPythontFunc(packetObj) {
    var s, key;
    if ((typeof packetObj.tid !== 'undefined') && (packetObj.tid !== null)) {
        if ((typeof packetObj.tid === 'string') && (packetObj.tid.charAt(0) === 'i')) key = packetObj.tid; // it bears a TID that is marked as internal one, there's only one way to key this
        else if (packetObj.sid) key = packetObj.tid + ':' + packetObj.sid;
        else key = '' + packetObj.tid;
    }
    else {
        // no TID exists: set the internal one, not being a number, so it differs from GUI TIDs
        packetObj.tid = key = 'i' + internalTID;
        internalTID++;
    }

    s = JSON.stringify(packetObj);
    s = new Buffer(Buffer.byteLength(s) + ':' + s);

    return new Promise(function (resolve, reject) {
        var existingResolver = pythonPromises[key];
        pythonPromises[key] = {
            'created': Date.now(),
            'resolve': resolve,
            'reject': reject
        };
        if (!sendPacketToPython(s)) {
            // we can't send right now, queue it for later
            pythonQueue.push(s);
        }
        if (existingResolver) {
            existingResolver.reject(new Error('Supplanted by a new request with the same TID'));
        }
    });
};

// this is internal function to python connection handler
var sendPythonQueue = function sendPythonQueueFunc() {
    while (pythonQueue.length > 0) {
        if (sendPacketToPython(pythonQueue[0])) {
            // remove the packet only after it has been successfully sent
            pythonQueue.shift();
        }
        else {
            // no more sending possible
            break;
        }
    }
};

var connectPython = function connectPythonFunc() {
    var receivingHeader = true, // whether we're receiving the length of the next packet, or the packet itself
        receiveBuffer = [], // chunks of the next packet (header or body), Buffer objects
        bodyLength = 0, // the required length of the next packet
        receivedLength = 0; // the size of the received body inside receiveBuffer up to now

    logger.debug('Attempting to connect to Python');

    pythonClient = net.connect({
        'host': 'localhost',
        'port': 19732
    });

    pythonClient.on('connect', function () {
        // login to the python server by sending it the list of currently handled sessions
        var sid, sessionList = [], packet;
        logger.debug('Connection to Python established');
        for (sid in sessions) {
            if (sessionList.hasOwnProperty(sid)) {
                sessionList.push({'sid': sid, 'lastActivity': sessionList[sid].lastActivity});
            }
        }
        // this should/must succeed in sending right now, as it is the first write ever made on the socket
        packet = JSON.stringify({'type': 'connect', 'sessions': sessionList});
        packet = new Buffer(Buffer.byteLength(packet) + ':' + packet);
        sendPacketToPython(packet);
    });

    // handler for incoming data, chunk is a Buffer object
    pythonClient.on('data', function (chunk) {
        var i, s,
            offset = 0,
            N = chunk.length;

        logger.debug('Received a chunk from Python: ' + chunk);
        while (offset < N) { // while there's data left to process
            if (receivingHeader) {
                // receiving the length of the packet
                for (i = offset; i < N; i++) {
                    if (chunk[i] === 58) break; // found a colon?
                }
                if (i >= N) {
                    // incomplete header, buffer it for the next 'data' event
                    s = chunk.slice(offset);
                    receiveBuffer.push(s);
                    receivedLength += s.length;
                    break;
                }
                if (receiveBuffer.length == 0) bodyLength = +chunk.toString('utf8', offset, i);
                else {
                    s = chunk.slice(offset, i);
                    receiveBuffer.push(s);
                    bodyLength = +Buffer.concat(receiveBuffer, receivedLength + s.length).toString('utf8');
                    receiveBuffer.length = 0;
                    receivedLength = 0;
                }
                offset = i + 1;
                receivingHeader = false;
                continue; // process the packet body in the next iteration
            }
            else if ((receivedLength + N - offset) >= bodyLength) {
                if (receivedLength == 0) {
                    // an optimization in case the buffer is empty
                    s = chunk.slice(offset, offset + bodyLength);
                    offset += s.length;
                }
                else {
                    s = chunk.slice(offset, offset + bodyLength - receivedLength);
                    offset += s.length;
                    receiveBuffer.push(s);
                    s = Buffer.concat(receiveBuffer, receivedLength + s.length);
                    receiveBuffer.length = 0;
                    receivedLength = 0;
                }
                receivingHeader = true; // process the packet header in the next iteration
                try {
                    processPacketFromPython(s.toString('utf8'));
                }
                catch (e) {
                    logException('Failed to process a packet from Python: ' + e, e);
                }
            }
            else {
                // incomplete packet body, buffer it for the next 'data' event
                s = chunk.slice(offset);
                receiveBuffer.push(s);
                receivedLength += s.length;
                break;
            }
        }
    });

    pythonClient.on('drain', function () {
        logger.debug('The Python client connection has been drained, resuming with sending any queued packets');
        pythonSaturated = false;
        sendPythonQueue();
    });

    pythonClient.on('end', function () {
        logger.debug('The Python peer closed the connection');
        pythonClient = null; // make it unavailable for further communication
    });

    pythonClient.on('error', function () {
        logger.debug('There was an error on Python client connection');
        pythonClient = null; // make it unavailable for further communication
    });

    pythonClient.on('close', function () {
        logger.debug('The Python client connection was closed, attempting to reconnect after half a second');
        setTimeout(connectPython, 500); // reconnect after half a second
    });
};

connectPython(); // the initial connection attempt