Python asyncio Telnet connection is disconnected immediately: How to solve server-side blocking problem?

Python asyncio Telnet connection disconnection: Analysis and solution to server-side blocking problems

When building a web server using Python's asyncio library, you often encounter the problem of disconnecting the Telnet client immediately after connecting. This article analyzes this problem and provides effective solutions.

Problem description:

A simple asyncio TCP server, after connecting using the telnet client, immediately displays an error "connection closed by foreign host.", indicating that the server has actively closed the connection. The root cause of the problem lies in the server-side processing function blocking.

Original code (has a problem):

 import asyncio
from asyncio.base_events import server


async def handle_client(reader, writer):
    While True:
        data = await reader.readline()
        if not data:
            break
        writer.write(data)
    writer.close()


async def main():
    server: server = await asyncio.get_running_loop().create_server(
        handle_client, '127.0.0.1', 8888)

    async with server:
        await server.serve_forever()

asyncio.run(main())

reader.readline() method blocks until a newline ('\n') is read. The Telnet client will not send data immediately after the connection is successful, causing reader.readline() to block indefinitely, which will eventually cause the connection to time out and be closed by the server.

Solution:

The improved handle_client function sends welcome messages immediately after the connection is established, and uses reader.read(100) instead of reader.readline() to avoid blocking and waiting for newlines. await writer.drain() ensures that the data is sent correctly.

Improved code:

 async def handle_client(reader, writer):
    welcome_message = "Welcome to the server!\n"
    writer.write(welcome_message.encode())
    await writer.drain()

    While True:
        data = await reader.read(100)
        if not data:
            break
        writer.write(data)
        await writer.drain()
    writer.close()

Through these modifications, the Telnet client can successfully connect to the server and interact normally. reader.read(100) reads up to 100 bytes of data, and can continue to execute even if the client does not send newlines, avoiding blocking. await writer.drain() ensures that data is sent to the client completely and avoids data loss.

Through the above analysis and code improvements, we can effectively solve the problem of Python asyncio Telnet connection being immediately disconnected and build a more stable and reliable network server.

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