async_calculator_server.py

#!/usr/bin/env python3

import argparse
import asyncio
import logging
import socket

import capnp
import calculator_capnp


logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)


class Server:
    async def myreader(self):
        while self.retry:
            try:
                # Must be a wait_for so we don't block on read()
                data = await asyncio.wait_for(self.reader.read(4096), timeout=0.1)
            except asyncio.TimeoutError:
                logger.debug("myreader timeout.")
                continue
            except Exception as err:
                logger.error("Unknown myreader err: %s", err)
                return False
            await self.server.write(data)
        logger.debug("myreader done.")
        return True

    async def mywriter(self):
        while self.retry:
            try:
                # Must be a wait_for so we don't block on read()
                data = await asyncio.wait_for(self.server.read(4096), timeout=0.1)
                self.writer.write(data.tobytes())
            except asyncio.TimeoutError:
                logger.debug("mywriter timeout.")
                continue
            except Exception as err:
                logger.error("Unknown mywriter err: %s", err)
                return False
        logger.debug("mywriter done.")
        return True

    async def myserver(self, reader, writer):
        # Start TwoPartyServer using TwoWayPipe (only requires bootstrap)
        self.server = capnp.TwoPartyServer(bootstrap=CalculatorImpl())
        self.reader = reader
        self.writer = writer
        self.retry = True

        # Assemble reader and writer tasks, run in the background
        coroutines = [self.myreader(), self.mywriter()]
        tasks = asyncio.gather(*coroutines, return_exceptions=True)

        while True:
            self.server.poll_once()
            # Check to see if reader has been sent an eof (disconnect)
            if self.reader.at_eof():
                self.retry = False
                break
            await asyncio.sleep(0.01)

        # Make wait for reader/writer to finish (prevent possible resource leaks)
        await tasks


def read_value(value):
    """Helper function to asynchronously call read() on a Calculator::Value and
    return a promise for the result.  (In the future, the generated code might
    include something like this automatically.)"""

    return value.read().then(lambda result: result.value)


def evaluate_impl(expression, params=None):
    """Implementation of CalculatorImpl::evaluate(), also shared by
    FunctionImpl::call().  In the latter case, `params` are the parameter
    values passed to the function; in the former case, `params` is just an
    empty list."""

    which = expression.which()

    if which == "literal":
        return capnp.Promise(expression.literal)
    elif which == "previousResult":
        return read_value(expression.previousResult)
    elif which == "parameter":
        assert expression.parameter < len(params)
        return capnp.Promise(params[expression.parameter])
    elif which == "call":
        call = expression.call
        func = call.function

        # Evaluate each parameter.
        paramPromises = [evaluate_impl(param, params) for param in call.params]

        joinedParams = capnp.join_promises(paramPromises)
        # When the parameters are complete, call the function.
        ret = joinedParams.then(lambda vals: func.call(vals)).then(
            lambda result: result.value
        )

        return ret
    else:
        raise ValueError("Unknown expression type: " + which)


class ValueImpl(calculator_capnp.Calculator.Value.Server):

    "Simple implementation of the Calculator.Value Cap'n Proto interface."

    def __init__(self, value):
        self.value = value

    def read(self, **kwargs):
        return self.value


class FunctionImpl(calculator_capnp.Calculator.Function.Server):

    """Implementation of the Calculator.Function Cap'n Proto interface, where the
    function is defined by a Calculator.Expression."""

    def __init__(self, paramCount, body):
        self.paramCount = paramCount
        self.body = body.as_builder()

    def call(self, params, _context, **kwargs):
        """Note that we're returning a Promise object here, and bypassing the
        helper functionality that normally sets the results struct from the
        returned object. Instead, we set _context.results directly inside of
        another promise"""

        assert len(params) == self.paramCount
        # using setattr because '=' is not allowed inside of lambdas
        return evaluate_impl(self.body, params).then(
            lambda value: setattr(_context.results, "value", value)
        )


class OperatorImpl(calculator_capnp.Calculator.Function.Server):

    """Implementation of the Calculator.Function Cap'n Proto interface, wrapping
    basic binary arithmetic operators."""

    def __init__(self, op):
        self.op = op

    def call(self, params, **kwargs):
        assert len(params) == 2

        op = self.op

        if op == "add":
            return params[0] + params[1]
        elif op == "subtract":
            return params[0] - params[1]
        elif op == "multiply":
            return params[0] * params[1]
        elif op == "divide":
            return params[0] / params[1]
        else:
            raise ValueError("Unknown operator")


class CalculatorImpl(calculator_capnp.Calculator.Server):

    "Implementation of the Calculator Cap'n Proto interface."

    def evaluate(self, expression, _context, **kwargs):
        return evaluate_impl(expression).then(
            lambda value: setattr(_context.results, "value", ValueImpl(value))
        )

    def defFunction(self, paramCount, body, _context, **kwargs):
        return FunctionImpl(paramCount, body)

    def getOperator(self, op, **kwargs):
        return OperatorImpl(op)


def parse_args():
    parser = argparse.ArgumentParser(
        usage="""Runs the server bound to the\
given address/port ADDRESS. """
    )

    parser.add_argument("address", help="ADDRESS:PORT")

    return parser.parse_args()


async def new_connection(reader, writer):
    server = Server()
    await server.myserver(reader, writer)


async def main():
    address = parse_args().address
    host = address.split(":")
    addr = host[0]
    port = host[1]

    # Handle both IPv4 and IPv6 cases
    try:
        print("Try IPv4")
        server = await asyncio.start_server(
            new_connection, addr, port, family=socket.AF_INET
        )
    except Exception:
        print("Try IPv6")
        server = await asyncio.start_server(
            new_connection, addr, port, family=socket.AF_INET6
        )

    async with server:
        await server.serve_forever()


if __name__ == "__main__":
    asyncio.run(main())