Endless polling EOF socket

Bug report

Bug description:

Hello,

I have this very simple code to attempt a TCP connection to a given host. The linux machine where I run this has tsocks library installed, to transparently redirect connections over socks server. If the target IP accessed through the socks server refuses the connection, the code will end in an endless unbreakable loop.

def tcp_connect(ip, port):
    """Attempt a TCP connection to the given IP and port."""
    try:
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.settimeout(5)
        result = sock.connect_ex((ip, port))
        sock.close()

Timeout has no effect, ctrl+c on the console doesn't break it.

Connection attempt with some command line utility ends correctly.

$ ssh 1.1.1.1
ssh: connect to host 1.1.1.1 port 22: Transport endpoint is not connected
$ ldp telnet 1.1.1.1 22
Trying 1.1.1.1...
telnet: Unable to connect to remote host: Transport endpoint is not connected

I tried running strace to see what happens during that endless loop and this is the result :

getpeername(3, 0x7ffd6c12e3f0, [16])    = -1 ENOTCONN (Transport endpoint is not connected)
connect(3, {sa_family=AF_INET, sin_port=htons(2224), sin_addr=inet_addr("172.17.0.1")}, 16) = -1 EINPROGRESS (Operation now in progress)
poll([{fd=3, events=POLLOUT}], 1, 5000) = 1 ([{fd=3, revents=POLLOUT}])
connect(3, {sa_family=AF_INET, sin_port=htons(2224), sin_addr=inet_addr("172.17.0.1")}, 16) = 0
sendto(3, "\5\2\0\2", 4, 0, NULL, 0)    = 4
recvfrom(3, "\5\0", 2, 0, NULL, NULL)   = 2
sendto(3, "\5\1\0\1\225\203f\211\0\26", 10, 0, NULL, 0) = 10
recvfrom(3, 0x55944406ddd8, 10, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=3, events=POLLIN}], 1, 5000)  = 1 ([{fd=3, revents=POLLIN}])
recvfrom(3, "", 10, 0, NULL, NULL)      = 0
poll([{fd=3, events=POLLIN}], 1, 5000)  = 1 ([{fd=3, revents=POLLIN}])
recvfrom(3, "", 10, 0, NULL, NULL)      = 0
poll([{fd=3, events=POLLIN}], 1, 5000)  = 1 ([{fd=3, revents=POLLIN}])
recvfrom(3, "", 10, 0, NULL, NULL)      = 0
poll([{fd=3, events=POLLIN}], 1, 5000)  = 1 ([{fd=3, revents=POLLIN}])
recvfrom(3, "", 10, 0, NULL, NULL)      = 0
poll([{fd=3, events=POLLIN}], 1, 5000)  = 1 ([{fd=3, revents=POLLIN}])
recvfrom(3, "", 10, 0, NULL, NULL)      = 0

<continues until sigkill while hogging 100% CPU>

I don't understand it that much, but LLM seems to believe, that the socket is getting polled despite EOF, which is supposedly an error.

Edit:
For comparison, this is how the strace on that telnet looks like

getpeername(3, 0x7ffd4d54e260, [16])    = -1 ENOTCONN (Transport endpoint is not connected)
connect(3, {sa_family=AF_INET, sin_port=htons(2224), sin_addr=inet_addr("172.17.0.1")}, 16) = 0
sendto(3, "\5\2\0\2", 4, 0, NULL, 0)    = 4
recvfrom(3, "\5\0", 2, 0, NULL, NULL)   = 2
sendto(3, "\5\1\0\1\225\203f\211\0\26", 10, 0, NULL, 0) = 10
recvfrom(3, "", 10, 0, NULL, NULL)      = 0
write(2, "telnet: Unable to connect to rem"..., 78telnet: Unable to connect to remote host: Transport endpoint is not connected
) = 78
close(3)                                = 0
exit_group(1)                           = ?
+++ exited with 1 +++

Thank you for your attention.

CPython versions tested on:

3.10

Operating systems tested on:

Linux

从报告中的 tcp_connect 片段开始，使用 Python 3.10 和 tsocks 在 Linux 上重现该问题，然后将其 strace 与正常工作的 telnet 情况进行比较。跟踪重复 EOF 读取附近的 socket 和 connect_ex 行为，以确定循环是在 CPython 中，还是在透明代理层中。确定了负责该行为的组件，并且 issue 有可复现的回归测试或明确的解决方案，即视为完成。