#!/usr/bin/python3

# Postfix socketmap proxy
#
# This script provides a proxy from Postfix socketmap to a variety of backends.
# For now, only HTTP backends are supported.

import asyncio
import aiohttp
import logging
import urllib
import argparse


class NetstringProtocol(asyncio.Protocol):
    """ Netstring asyncio protocol implementation.

    For protocol details, see https://cr.yp.to/proto/netstrings.txt
    """

    # Length of the smallest allocated buffer, larger buffers will be
    # allocated dynamically
    BASE_BUFFER = 1024

    # Maximum length of a buffer, will crash when exceeded
    MAX_BUFFER = 65535

    def __init__(self):
        super(NetstringProtocol, self).__init__()
        self.init_buffer()

    def init_buffer(self):
        self.len = None  # None when waiting for a length to be sent)
        self.separator = -1  # -1 when not yet detected (str.find)
        self.index = 0  # relative to the buffer
        self.buffer = bytearray(NetstringProtocol.BASE_BUFFER)

    def data_received(self, data):
        # Manage the buffer
        missing = len(data) - len(self.buffer) + self.index
        if missing > 0:
            if len(self.buffer) + missing > NetstringProtocol.MAX_BUFFER:
                raise IOError("Not enough space when decoding netstring")
            self.buffer.append(bytearray(missing + 1))
        new_index = self.index + len(data)
        self.buffer[self.index:new_index] = data
        self.index = new_index
        # Try to detect a length at the beginning of the string
        if self.len is None:
            self.separator = self.buffer.find(0x3a)
            if self.separator != -1 and self.buffer[:self.separator].isdigit():
                self.len = int(self.buffer[:self.separator], 10)
        # Then get the complete string
        if self.len is not None:
            if self.index - self.separator == self.len + 2:
                string = self.buffer[self.separator + 1:self.index - 1]
                self.init_buffer()
                self.string_received(string)

    def string_received(self, string):
        pass

    def send_string(self, string):
        """ Send a netstring
        """
        self.transport.write(str(len(string)).encode('ascii'))
        self.transport.write(b':')
        self.transport.write(string)
        self.transport.write(b',')


class SocketmapProtocol(NetstringProtocol):
    """ TCP protocol to answer Postfix socketmap and proxify lookups to
    an outside object.

    See http://www.postfix.org/socketmap_table.5.html for details on the
    protocol.

    A table map must be provided as a dictionary to lookup tables.
    """

    def __init__(self, table_map):
        self.table_map = table_map
        super(SocketmapProtocol, self).__init__()

    def connection_made(self, transport):
        logging.info('Connect {}'.format(transport.get_extra_info('peername')))
        self.transport = transport

    def string_received(self, string):
        space = string.find(0x20)
        if space != -1:
            name = string[:space].decode('ascii')
            key = string[space+1:].decode('utf8')
            return asyncio.async(self.process_request(name, key))

    def send_string(self, string):
        logging.debug("Send {}".format(string))
        super(SocketmapProtocol, self).send_string(string)

    async def process_request(self, name, key):
        """ Process a request by querying the provided map.
        """
        logging.debug("Request {}/{}".format(name, key))
        try:
            table = self.table_map.get(name)
        except KeyError:
            return self.send_string(b'TEMP no such map')
        try:
            result = await table.get(key)
            return self.send_string(b'OK ' + result.encode('utf8'))
        except KeyError:
            return self.send_string(b'NOTFOUND ')
        except Exception:
            logging.exception("Error when processing request")
            return self.send_string(b'TEMP unknown error')

    @classmethod
    def factory(cls, table_map):
        """ Provide a protocol factory for a given map instance.
        """
        return lambda: cls(table_map)


class UrlTable(object):
    """ Resolve an entry by querying a parametrized GET URL.
    """

    def __init__(self, url_pattern):
        """ url_pattern must contain a format ``{}`` so the key is injected in
        the url before the query, the ``§`` character will be replaced with
        ``{}`` for easier setup.
        """
        self.url_pattern = url_pattern.replace('§', '{}')

    async def get(self, key):
        async with aiohttp.ClientSession() as session:
            async with session.get(self.url_pattern.format(key)) as request:
                if request.status == 200:
                    result = await request.text()
                    return result


def main():
    """ Run the asyncio loop.
    """
    parser = argparse.ArgumentParser("Postfix Socketmap proxy")
    parser.add_argument("--bind", help="address to bind to", required=True)
    parser.add_argument("--port", type=int, help="port to bind to", required=True)
    parser.add_argument("--name", help="name of the table", action="append")
    parser.add_argument("--type", help="type of the table", action="append")
    parser.add_argument("--param", help="table parameter", action="append")
    args = parser.parse_args()
    # Prepare the maps
    table_types = dict(url=UrlTable)
    table_map = {name: table_types[table_type](param)
                 for name, table_type, param
                 in zip(args.name, args.type, args.param)}
    # Run the main loop
    logging.basicConfig(level=logging.DEBUG)
    loop = asyncio.get_event_loop()
    server = loop.run_until_complete(loop.create_server(
        SocketmapProtocol.factory(table_map), args.bind, args.port
    ))
    try:
        loop.run_forever()
    except KeyboardInterrupt:
        pass
    server.close()
    loop.run_until_complete(server.wait_closed())
    loop.close()


if __name__ == "__main__":
    main()