Python - 使用选择器的非阻塞套接字

我的问题简而言之： 我不知道选择器如何知道哪个套接字应该首先读取或写入。

它是一个可以处理多连接的服务器，其流程应该是：

1. 服务器创建监听套接字
2. 客户端创建2个套接字并将它们连接到服务器
3. 客户端2套接字发送消息
4. 服务器 2 套接字回显这些消息，客户端和服务器关闭 联系

这就是发生的情况，但是如果创建的服务器套接字首先写入，则连接将立即关闭或抛出异常（？），因为它甚至不调用 send 并且客户端套接字不会接收任何内容。那么选择器如何知道哪些套接字应该首先准备好写入/读取呢？为了理解这一点我错过了哪些信息？

Server:

import socket
import selectors
import types

host = "127.0.0.1"
port = 63210

def accept_wrapper(sock):
    conn, addr = sock.accept()
    print('accepted connection from', addr)
    conn.setblocking(False)
    data = types.SimpleNamespace(addr=addr, inb=b'', outb=b'')
    events = selectors.EVENT_READ | selectors.EVENT_WRITE
    sel.register(conn, events, data=data)

def service_connection(key, mask):
    sock = key.fileobj
    data = key.data
    if mask & selectors.EVENT_READ:
        recv_data = sock.recv(1024)
        if recv_data:
            data.outb += recv_data
        else:
            print('closing connection to', data.addr)
            sel.unregister(sock)
            sock.close()
    if mask & selectors.EVENT_WRITE:
        if data.outb:
            print('echoing', repr(data.outb), 'to', data.addr)
            sent = sock.send(data.outb)
            data.outb = data.outb[sent:]

sel = selectors.DefaultSelector()

lsock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
lsock.bind((host, port))
lsock.listen()
print('listening on', (host, port))
lsock.setblocking(False)
sel.register(lsock, selectors.EVENT_READ, data=None)

while True:
    events = sel.select(timeout=None)
    for key, mask in events:
        if key.data is None:
            accept_wrapper(key.fileobj)
        else:
            service_connection(key, mask)

Client:

import socket
import selectors
import types

host = "127.0.0.1"
port = 63210
num_conns = 2
messages = [b'Message 1 from client.', b'Message 2 from client.']

def start_connections(host, port, num_conns):
    server_addr = (host, port)
    for i in range(0, num_conns):
        connid = i + 1
        print('starting connection', connid, 'to', server_addr)
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.setblocking(False)
        sock.connect_ex(server_addr)
        events = selectors.EVENT_READ | selectors.EVENT_WRITE
        data = types.SimpleNamespace(connid=connid,
                                     msg_total=sum(len(m) for m in messages),
                                     recv_total=0,
                                     messages=list(messages),
                                     outb=b'')
        sel.register(sock, events, data=data)

def service_connection(key, mask):
    sock = key.fileobj
    data = key.data
    if mask & selectors.EVENT_READ:
        recv_data = sock.recv(1024)
        if recv_data:
            print('received', repr(recv_data), 'from connection', data.connid)
            data.recv_total += len(recv_data)
        if not recv_data or data.recv_total == data.msg_total:
            print('closing connection', data.connid)
            sel.unregister(sock)
            sock.close()
    if mask & selectors.EVENT_WRITE:
        if not data.outb and data.messages:
            data.outb = data.messages.pop(0)
        if data.outb:
            print('sending', repr(data.outb), 'to connection', data.connid)
            sent = sock.send(data.outb)
            data.outb = data.outb[sent:]

sel = selectors.DefaultSelector()
start_connections(host, port, num_conns)

while True:
    events = sel.select(timeout=None)
    for key, mask in events:
        service_connection(key, mask)

套接字实际上并不直接写入对等点，也不从对等点读取。相反，它们写入本地套接字特定的写入缓冲区并从套接字特定的读取缓冲区读取。操作系统内核关心将数据从套接字写入缓冲区传送到对等方，并将从对等方接收到的数据包放入套接字接收缓冲区。

可以使用以下函数监视这些内核套接字缓冲区的状态以及这些缓冲区的更改select, poll, kqueue。本质上：如果套接字写入缓冲区中有空间，则套接字被认为是可写的。如果套接字读取缓冲区中有数据，则认为套接字是可读的。