# 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/>.

import socket
import selectors
import json
import threading
import traceback
from server.handlers import serve_request
import logging

# TODO: add a whole lot of try..except blocks, and just make it overall error resistant

_mapping_lock = threading.Lock()  # the lock guarding access to the two mappings below

_sessions_to_socket = {}  # keyed by sid: what session is bound to what socket

_transactions_to_socket = {}  # keyed by tid, used only when there is no sid available, so a request can be replied


def handle_connect(data):
    """The first packet is 'connect', providing the list of connected sessions to the peer (that's us)."""
    pass


def handle_unregister(data):
    """When the client disconnects from the peer, but doesn't logout first,
    leaving the possibility that it will reconnect in the same session,
    possibly via other channel (nodejs instance)."""
    pass


# TODO: meta handlers implement system protocol to support horizontal scaling, implement them when it becomes relevant
_meta_handlers = {
    'connect': handle_connect,
    'unregister': handle_unregister
}


def processIncomingPacket(receiving_socket, packet):
    logging.debug('Decoding JSON: {}'.format(packet))
    obj = json.loads(packet)
    req_type = obj.get('type')  # private (meta) requests have the 'type'
    if req_type is None:
        # user protocol, internally map the request so response can be delivered correctly, and forward it to the higher layer
        tid = obj.get('tid')  # transaction ID
        if tid is None:
            raise Exception('Transaction ID is missing from the request')
        sid = obj.get('sid')  # session ID
        _mapping_lock.acquire()
        try:
            if sid is None:
                _transactions_to_socket[tid] = receiving_socket
            else:
                _sessions_to_socket[sid] = receiving_socket
        finally:
            _mapping_lock.release()
        serve_request(obj)
    else:
        # system protocol, connection-specific meta data
        handler = _meta_handlers.get(req_type)
        if handler is None:
            logging.error('Received an unknown meta packet: {}'.format(req_type))
        else:
            handler(obj)


def sendPacket(tid, sid, json_obj):
    if sid is None:
        if tid is None:
            raise Exception('Cannot send a message without a tid and/or a sid')
        _mapping_lock.acquire()
        try:
            socket = _transactions_to_socket.get(tid)
            del _transactions_to_socket[tid]  # this mapping is not relevant anymore
        finally:
            _mapping_lock.release()
        if socket is None:
            raise Exception('Cannot send in transaction, it is not registered: ' + tid)
    else:
        _mapping_lock.acquire()
        try:
            socket = _sessions_to_socket.get(sid)
        finally:
            _mapping_lock.release()
        if socket is None:
            raise Exception('Cannot send to session, it is not registered: ' + sid)
    if json_obj is None:
        # a special case: this is only an acknowledgment, so any request/response accounting can be cleared
        json_obj = {}
        if sid is not None:
            json_obj['sid'] = sid
    if tid is not None:
        json_obj['tid'] = tid
    socket.send(json_obj)


class SocketHandler(object):

    def read(self):
        pass

    def destroy(self):
        pass


class ServerSocket(SocketHandler):

    def __init__(self, communication, hostname='localhost', port=19732):
        self.communication = communication
        server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        server_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        server_socket.bind((hostname, port))
        server_socket.listen(5)
        server_socket.setblocking(False)
        self.socket = server_socket

    def read(self):
        conn, addr = self.socket.accept()
        self.communication.add_socket(JsonClientSocket(self.communication, conn, addr))

    def destroy(self):
        self.communication.remove_socket(self)
        try:
            self.socket.close()
        except:
            pass


class JsonClientSocket(SocketHandler):

    def __init__(self, communication, socket, address):
        self.communication = communication
        self.socket = socket
        self.address = address
        socket.setblocking(False)
        self.status = 0  # registering
        self.receiving_header = True  # whether we're receiving the length of the next packet, or the packet itself
        self.buffer = []  # chunks of the next packet (header or body)
        self.received_length = 0  # the cumulative length of strings in buffer
        self.body_length = 0  # the length of the next packet, which was given in the header
        self._read_lock = threading.Lock()
        self._write_lock = threading.Lock()

    def read(self):
        lock = self._read_lock
        lock.acquire()
        try:
            data = self.socket.recv(8192)
            if not data:
                self.destroy()
            else:
                # packet decode loop
                logging.debug("received: {}".format(data))
                offset = 0
                N = len(data)
                while (offset < N):
                    if self.receiving_header:
                        i = data.find(b':', offset)
                        if i < 0:
                            self.buffer.append(data[offset:])
                            break
                        if len(self.buffer) == 0:
                            self.body_length = int(data[offset:i].decode('utf-8'))
                        else:
                            self.buffer.append(data[offset:i])
                            self.body_length = int(b''.join(self.buffer))
                            self.buffer.clear()
                        offset = i + 1
                        self.receiving_header = False
                        continue
                    elif (self.received_length + N - offset) >= self.body_length:
                        if self.received_length == 0:
                            # an optimized case
                            s = data[offset:offset+self.body_length]
                            offset += len(s)
                        else:
                            s = data[offset:offset+self.body_length - self.received_length]  # TODO: is this correct?
                            offset += len(s)
                            self.buffer.append(s)
                            s = b''.join(self.buffer)
                            self.buffer.clear()
                            self.received_length = 0
                        self.receiving_header = True
                        try:
                            processIncomingPacket(self, s.decode('utf-8'))
                        except Exception as e:
                            # any exception that propagates to here means a possible protocol error, we have to disconnect
                            logging.critical(traceback.format_exc())
                            self.destroy()
                            return
                    else:
                        # incomplete packet body, buffer it until the next time
                        s = data[offset:]
                        self.buffer.append(s)
                        self.received_length += len(s)
                        break
        finally:
            lock.release()

    def destroy(self):
        self.communication.remove_socket(self)
        try:
            self.socket.close()
        except:
            pass
            # TODO: unregister from any internal mechanisms

    def send(self, json_obj):
        b = json.dumps(json_obj).encode('utf-8')
        b = bytes(str(len(b)), 'utf-8') + b':' + b
        lock = self._write_lock
        lock.acquire()
        try:
            self.socket.sendall(b)
        finally:
            lock.release()


class Communication(SocketHandler):
    def __init__(self):
        # the selector to register sockets with, the serve_forever() uses it to handle any incoming data
        self._selector = selectors.DefaultSelector()  # to wait until at least on socket is readable
        self._lock = threading.Lock()  # the access lock, because of multi-threading
        self._remove_queue = []  # what sockets to remove
        self._add_queue = []  # what sockets to add
        notifier, receiver = socket.socketpair()  # the notification mechanism for the serving thread, using the selector
        receiver.setblocking(False)
        self._notifier = notifier
        self._receiver = receiver
        self._registered_handlers = set()
        self._not_signalled = True
        self._destroying = False

    def serve_forever(self):
        self._selector.register(self._receiver, selectors.EVENT_READ, self)
        while True:
            events = self._selector.select()
            for key, mask in events:
                if mask & selectors.EVENT_READ:
                    key.data.read()

    def add_socket(self, socket_handler):
        lock = self._lock
        lock.acquire()
        try:
            self._add_queue.append(socket_handler)
            self._signal()
        finally:
            lock.release()

    def remove_socket(self, socket_handler):
        lock = self._lock
        lock.acquire()
        try:
            self._remove_queue.append(socket_handler)
            self._signal()
        finally:
            lock.release()

    def _signal(self):  # invoke holding the lock
        if self._not_signalled:
            self._notifier.send(b'1')
            self._not_signalled = False

    def read(self):
        # we have been invoked to process some new additions and/or removals
        lock = self._lock
        lock.acquire()
        try:
            self._receiver.recv(1024)  # clear the queue
            self._not_signalled = True
            selector = self._selector
            for handler in self._remove_queue:
                selector.unregister(handler.socket)
                self._registered_handlers.remove(handler)
            self._remove_queue.clear()
            for handler in self._add_queue:
                selector.register(handler.socket, selectors.EVENT_READ, handler)
                self._registered_handlers.add(handler)
            self._add_queue.clear()
            if self._destroying:
                self._notifier.close()
                self._receiver.close()
                self._selector.close()
        finally:
            lock.release()

    def destroy(self):
        lock = self._lock
        # make a copy of handlers, because as we're destroying them each will invoke remove_socket(), and for that the lock must be available and the iterating set must not change
        lock.acquire()
        try:
            handlers = set(self._registered_handlers)
        finally:
            lock.release()
        for handler in handlers:
            handler.destroy()  # destroy them all, even the server socket
        # this is where everything has been destroyed but us, so what follows is self-destruct
        lock.acquire()
        try:
            self._destroying = True
            self._signal()
        finally:
            lock.release()


def serve_forever():
    communication = Communication()
    server = ServerSocket(communication)
    communication.add_socket(server)
    communication.serve_forever()