Asyncio如何重用套接字

Question

如何在 asyncio 中重用服务器的套接字？而不是为每个查询创建一个新连接？

这是我的代码；

async def lookup(server, port, query, sema):
    async with sema as sema:
        try:
            reader, writer = await asyncio.open_connection(server, port)
        except:
            return {}
        writer.write(query.encode("ISO-8859-1"))
        await writer.drain()
        data = b""
        while True:
            d = await reader.read(4096)
            if not d:
                break
            data += d
        writer.close()
        data = data.decode("ISO-8859-1")
        return data

Answer 1

您只需调用asyncio.open_connection(server, port)协程一次，然后继续使用读取器和写入器（当然，前提是服务器不只是关闭其一端的连接）。

我将在一个单独的异步上下文管理器对象中为您的连接执行此操作，并使用连接池来管理连接，以便您可以为许多并发任务创建和重用套接字连接。通过使用（异步）上下文管理器，Python 确保在代码完成时通知连接，以便可以将连接释放回池中：

import asyncio
import contextlib

from collections import OrderedDict
from types import TracebackType
from typing import Any, List, Optional, Tuple, Type


try:  # Python 3.7
    base = contextlib.AbstractAsyncContextManager
except AttributeError:
    base = object  # type: ignore

Server = str
Port = int
Host = Tuple[Server, Port]


class ConnectionPool(base):
    def __init__(
        self,
        max_connections: int = 1000,
        loop: Optional[asyncio.AbstractEventLoop] = None,
    ):
        self.max_connections = max_connections
        self._loop = loop or asyncio.get_event_loop()

        self._connections: OrderedDict[Host, List["Connection"]] = OrderedDict()
        self._semaphore = asyncio.Semaphore(max_connections)

    async def connect(self, server: Server, port: Port) -> "Connection":
        host = (server, port)

        # enforce the connection limit, releasing connections notifies
        # the semaphore to release here
        await self._semaphore.acquire()

        connections = self._connections.setdefault(host, [])
        # find an un-used connection for this host
        connection = next((conn for conn in connections if not conn.in_use), None)
        if connection is None:
            # disconnect the least-recently-used un-used connection to make space
            # for a new connection. There will be at least one.
            for conns_per_host in reversed(self._connections.values()):
                for conn in conns_per_host:
                    if not conn.in_use:
                        await conn.close()
                        break

            reader, writer = await asyncio.open_connection(server, port)
            connection = Connection(self, host, reader, writer)
            connections.append(connection)

        connection.in_use = True
        # move current host to the front as most-recently used
        self._connections.move_to_end(host, False)

        return connection

    async def close(self):
        """Close all connections"""
        connections = [c for cs in self._connections.values() for c in cs]
        self._connections = OrderedDict()
        for connection in connections:
            await connection.close()

    def _remove(self, connection):
        conns_for_host = self._connections.get(connection._host)
        if not conns_for_host:
            return
        conns_for_host[:] = [c for c in conns_for_host if c != connection]

    def _notify_release(self):
        self._semaphore.release()

    async def __aenter__(self) -> "ConnectionPool":
        return self

    async def __aexit__(
        self,
        exc_type: Optional[Type[BaseException]],
        exc: Optional[BaseException],
        tb: Optional[TracebackType],
    ) -> None:
        await self.close()

    def __del__(self) -> None:
        connections = [repr(c) for cs in self._connections.values() for c in cs]
        if not connections:
            return

        context = {
            "pool": self,
            "connections": connections,
            "message": "Unclosed connection pool",
        }
        self._loop.call_exception_handler(context)


class Connection(base):
    def __init__(
        self,
        pool: ConnectionPool,
        host: Host,
        reader: asyncio.StreamReader,
        writer: asyncio.StreamWriter,
    ):
        self._host = host
        self._pool = pool
        self._reader = reader
        self._writer = writer
        self._closed = False
        self.in_use = False

    def __repr__(self):
        host = f"{self._host[0]}:{self._host[1]}"
        return f"Connection<{host}>"

    @property
    def closed(self):
        return self._closed

    def release(self) -> None:
        self.in_use = False
        self._pool._notify_release()

    async def close(self) -> None:
        if self._closed:
            return
        self._closed = True
        self._writer.close()
        self._pool._remove(self)
        try:
            await self._writer.wait_closed()
        except AttributeError:  # wait_closed is new in 3.7
            pass

    def __getattr__(self, name: str) -> Any:
        """All unknown attributes are delegated to the reader and writer"""
        if self._closed or not self.in_use:
            raise ValueError("Can't use a closed or unacquired connection")
        if hasattr(self._reader, name):
            return getattr(self._reader, name)
        return getattr(self._writer, name)

    async def __aenter__(self) -> "Connection":
        if self._closed or not self.in_use:
            raise ValueError("Can't use a closed or unacquired connection")
        return self

    async def __aexit__(
        self,
        exc_type: Optional[Type[BaseException]],
        exc: Optional[BaseException],
        tb: Optional[TracebackType],
    ) -> None:
        self.release()

    def __del__(self) -> None:
        if self._closed:
            return
        context = {"connection": self, "message": "Unclosed connection"}
        self._pool._loop.call_exception_handler(context)

然后将池对象传递给您的查找协程；连接对象为读取器和写入器部分生成代理：

async def lookup(pool, server, port, query):
    try:
        conn = await pool.connect(server, port)
    except (ValueError, OSError):
        return {}

    async with conn:
        conn.write(query.encode("ISO-8859-1"))
        await conn.drain()
        data = b""
        while True:
            d = await conn.read(4096)
            if not d:
                break
            data += d
        data = data.decode("ISO-8859-1")
        return data

请注意，标准 WHOIS 协议（RFC 3912 或前身）规定每次查询后连接都会关闭。如果您要连接到端口 43 上的标准 WHOIS 服务，则重复使用套接字是没有意义的。

在这种情况下发生的情况是，读取器将达到 EOF（reader.at_eof()为真），并且任何进一步的读取尝试都不会返回任何内容（reader.read(...)将始终返回空b''值）。在套接字连接超时后被远程端终止之前，写入写入器不会出现错误。您可以将您想要的所有内容写入连接，但 WHOIS 服务器将忽略查询。