Python3标准库：selectors I/O多路复用抽象

Z时代
2024-01-10
分类：综合

python

selectors模块在select中平台特定的I/O监视函数之上提供了一个平台独立的抽象层。

1.1 操作模型

selectors中的API是基于事件的，与select中的poll()类似。它有多个实现，并且这个模块会自动设置别名DefaultSelector来指示对当前系统配置最为高效的一个实现。
选择器对象提供了一些方法，可以指定在一个套接字上查找哪些事件，然后以一种平台独立的方式让调用者等待事件。注册对事件的兴趣会创建一个SelectorKey，其中包含套接字、所注册事件的有关信息，可能还有可选的应用数据。选择器的所有者调用它的select()方法来了解事件。返回值是一个键对象序列和一个指示发生了哪些事件的位掩码。使用选择器的程序要反复调用select()，然后适当地处理事件。

1.2 回送服务器

这里给出的回送服务器例子使用了Selectorkey中的应用数据来注册发生新事件时要调用的一个回调函数。主循环从键得到这个回调，并把套接字和事件掩码传递给该回调。服务器启动时，其会注册当主服务器套接字上发生读事件时要调用的accept()函数。接受连接会产生一个新的套接字，然后注册read()函数作为读事件的一个回调。

import selectors
import socket
mysel = selectors.DefaultSelector()
keep_running = True
def read(connection, mask):
    "Callback for read events"
    global keep_running
    client_address = connection.getpeername()
    print('read({})'.format(client_address))
    data = connection.recv(1024)
    if data:
        # A readable client socket has data
        print('  received {!r}'.format(data))
        connection.sendall(data)
    else:
        # Interpret empty result as closed connection
        print('  closing')
        mysel.unregister(connection)
        connection.close()
        # Tell the main loop to stop
        keep_running = False
def accept(sock, mask):
    "Callback for new connections"
    new_connection, addr = sock.accept()
    print('accept({})'.format(addr))
    new_connection.setblocking(False)
    mysel.register(new_connection, selectors.EVENT_READ, read)
server_address = ('localhost', 9999)
print('starting up on {} port {}'.format(*server_address))
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.setblocking(False)
server.bind(server_address)
server.listen(5)
mysel.register(server, selectors.EVENT_READ, accept)
while keep_running:
    print('waiting for I/O')
    for key, mask in mysel.select(timeout=1):
        callback = key.data
        callback(key.fileobj, mask)
print('shutting down')
mysel.close()

如果read()没有从套接字接收到任何数据，那么当连接的另一端关闭时，它会中断读事件而不是发送数据。之后，会从选择器删除这个套接字，并将其关闭。由于这只是一个示例程序，所以这个服务器与唯一的客户结束通信后还会关闭服务器自身。

1.3 回送客户

下面的回送客户例子会处理主循环中的所有I/O事件，而不是使用回调。它会建立选择器来报告套接字上的读事件，并报告套接字什么时候准备好可以发送数据。由于它查看两种类型的事件，所以客户必须通过查看掩码值来检查发生了哪个事件。所有数据都发出后，它会修改选择器配置，只在有可读取的数据时才会报告。

import selectors
import socket
mysel = selectors.DefaultSelector()
keep_running = True
outgoing = [
    b'It will be repeated.',
    b'This is the message.  ',
]
bytes_sent = 0
bytes_received = 0
# Connecting is a blocking operation, so call setblocking()
# after it returns.
server_address = ('localhost', 9999)
print('connecting to {} port {}'.format(*server_address))
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(server_address)
sock.setblocking(False)
# Set up the selector to watch for when the socket is ready
# to send data as well as when there is data to read.
mysel.register(
    sock,
    selectors.EVENT_READ | selectors.EVENT_WRITE,
)
while keep_running:
    print('waiting for I/O')
    for key, mask in mysel.select(timeout=1):
        connection = key.fileobj
        client_address = connection.getpeername()
        print('client({})'.format(client_address))
        if mask & selectors.EVENT_READ:
            print('  ready to read')
            data = connection.recv(1024)
            if data:
                # A readable client socket has data
                print('  received {!r}'.format(data))
                bytes_received += len(data)
            # Interpret empty result as closed connection,
            # and also close when we have received a copy
            # of all of the data sent.
            keep_running = not (
                data or
                (bytes_received and
                 (bytes_received == bytes_sent))
            )
        if mask & selectors.EVENT_WRITE:
            print('  ready to write')
            if not outgoing:
                # We are out of messages, so we no longer need to
                # write anything. Change our registration to let
                # us keep reading responses from the server.
                print('  switching to read-only')
                mysel.modify(sock, selectors.EVENT_READ)
            else:
                # Send the next message.
                next_msg = outgoing.pop()
                print('  sending {!r}'.format(next_msg))
                sock.sendall(next_msg)
                bytes_sent += len(next_msg)
print('shutting down')
mysel.unregister(connection)
connection.close()
mysel.close()