Python 中的套接字错误：BlockingIOError：[Errno 35] 资源暂时不可用

Question

So Here is something wrong. I try to implement a simple Web Server in Python use socket . The single-thread version runs well. But when I try to implement more threads with non-blocking mode , it comes with errors.I try to search in Stackoverflow and Google, but no answers. Here is my code:

# coding:utf-8

import errno
import socket
import threading
import time

EOL1 = b'\n\n'
EOL2 = b'\n\r\n'
body = '''<h1>Hello, world!</h1> - from {thread_name}'''
response_params = [
    'HTTP/1.0 200 OK',
    'Date: Mon, 01 jan 2022 01:01:01 GMT'
    'Content-Type: text/plain; charset=utf-8',
    'Content_Length: {length}\r\n',
    body,
]
response = '\r\n'.join(response_params)


def handle_connection(conn, addr):
    # print(conn, addr)
    # time.sleep(60)
    request = b""
    while EOL1 and EOL2 not in request:
        request += conn.recv(1024)     # ERROR HERE!

    print(request)
    current_thread = threading.currentThread()
    content_length = len(body.format(thread_name=current_thread.name).encode())
    print(current_thread.name)
    conn.send(response.format(thread_name=current_thread.name,
                              length = content_length).encode())
    conn.close()


def main():
    serversocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    serversocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    serversocket.bind(('127.0.0.1', 8000))
    serversocket.listen(10)
    print('http://127.0.0.1:8000')
    serversocket.setblocking(0)

    try:
        i = 0
        while True:
            try:
                conn, address = serversocket.accept()
            except socket.error as e:
                if e.args[0] != errno.EAGAIN:
                    raise
                continue
            i += 1
            print(i)
            t = threading.Thread(target=handle_connection, args=(conn, address),
                                 name = 'thread-%s' % i)
            t.start()
    finally:
        serversocket.close()


if __name__ == '__main__':
    main()

The Error message is here:

1
Exception in thread thread-1:
2
Traceback (most recent call last):
  File "/Users/tdeveloper/opt/anaconda3/lib/python3.9/threading.py", line 973, in _bootstrap_inner
Exception in thread thread-2:
Traceback (most recent call last):
  File "/Users/tdeveloper/opt/anaconda3/lib/python3.9/threading.py", line 973, in _bootstrap_inner
    self.run()
    self.run()
  File "/Users/tdeveloper/opt/anaconda3/lib/python3.9/threading.py", line 910, in run
  File "/Users/tdeveloper/opt/anaconda3/lib/python3.9/threading.py", line 910, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/tdeveloper/Development/Learning/Python_Simple_WSGI/socket/thread_socketserver.py", line 26, in handle_connection
    self._target(*self._args, **self._kwargs)
  File "/Users/tdeveloper/Development/Learning/Python_Simple_WSGI/socket/thread_socketserver.py", line 26, in handle_connection
    request += conn.recv(1024)
BlockingIOError: [Errno 35] Resource temporarily unavailable
    request += conn.recv(1024)
BlockingIOError: [Errno 35] Resource temporarily unavailable

Please help me. Thanks a lot.

Answer 1

This is apparently an issue with the macos implementation of accept being different than that in other platforms with respect to inheritance of the non-blocking flag. It has nothing to do with threading per se.

Here's a trimmed-down single-threaded test program that demonstrates.

#!/usr/bin/env python3
import select
import socket

ssocket = socket.socket()
ssocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
ssocket.bind(('127.0.0.1', 7000))
ssocket.listen(10)

ssocket.setblocking(0)       # <<<<<<<===========

select.select([ssocket], [], [])
csocket, caddr = ssocket.accept()
csocket.recv(10)

If you run this on linux, and connect to it with nc localhost 7000 , the csocket.recv blocks as you're expecting. Run the same program on macos and the recv immediately triggers the BlockingIOException you're seeing.

Looking at the manual page accept(2) on macos shows:

[...] creates a new socket with the same properties of socket

In this case, the non-blocking file descriptor flag ( O_NONBLOCK ) is being "inherited" by the new socket. So if you don't want it, you'll need to disable it on the accepted socket with conn.setblocking(1) . Apparently this behavior is due to MacOS being descended from the BSD flavor of Unix.

All of that being said, you have no need to disable blocking anyway unless there is more to your actual program than shown. Ie if your main thread is doing nothing but accepting a connection and then spinning off a separate thread to handle the connection, there's no reason not to let the main thread just block in accept . If you allow the listening socket to remain in blocking mode, the accepted sockets should also be in blocking mode. (By the way, as is, you're wasting a ton of CPU time in that main thread loop: calling accept , trapping the exception, then doing continue to start the loop over.)

(For clarity, my specs: python 3.7.3 downloaded from https://www.python.org/ftp/python/3.7.3/python-3.7.3-macosx10.9.pkg running on MacOS Catalina 10.15.7)