[issue42513] Socket.recv hangs

[Barney Stratford]

Barney Stratford  added the comment:

Sure. So, I'm using STOMP to connect to a messaging server. STOMP uses 
heartbeats to detect and close failed connections. The problem was that if the 
connection fails before the protocol has set up its heartbeats then there's 
nothing to stop the whole thing hanging. I had a timeout on the 
socket.create_connection, thinking this would protect against this edge-case, 
but it wasn't sufficient.

STOMP is such a simple protocol that it's often worth writing your own code to 
handle it. Indeed, this is actively encouraged. My own STOMP code is about 250 
lines in a single source file, compared to nearly 3500 lines in 13 files for 
stomp.py. I very much prefer to simplify things as much as possible, and 
smaller is almost always better in my view. Simplify!

So, I kept seeing this very occasional hang-up in my code, and probing it with 
gdb showed that the execution was always stuck inside the poll. Of course it 
was, as that's where it sits to wait for something to happen. Then I was lead 
astray by finding that website, so I saddled up to save the world, as one does 
in such situations.

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[issue42513] Socket.recv hangs

[Jakub Stasiak]

Jakub Stasiak  added the comment:

If the edge-case is vaguely socket/file descriptor-related and not 
application-specific or otherwise secret do you mind sharing what is it? (I'm 
just curious)

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[issue42513] Socket.recv hangs

[Barney Stratford]

Barney Stratford  added the comment:

Instrumented code shows that the poll is in fact working completely correctly, 
and I've run into a documented edge-case in an unrelated area. Hence, closing 
the bug report and cancelling the pull request.

--
resolution:  -> not a bug
stage: patch review -> resolved
status: open -> closed

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[issue42513] Socket.recv hangs

[Barney Stratford]

Barney Stratford  added the comment:

The instrumented code froze today, so I'm finally able to probe the bug. 
Despite what that website said, it's looking like checking the return fron the 
poll syscall isn't the problem here. I'm probably going to close this bug 
report as not a bug, but want to check fully before I do so.

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[issue42513] Socket.recv hangs

[Jakub Stasiak]

Change by Jakub Stasiak :


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nosy: +jstasiak

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[issue42513] Socket.recv hangs

[Barney Stratford]

Barney Stratford  added the comment:

Still waiting for the instrumented code to hang. It sometimes runs for a month 
or two before freezing.

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[issue42513] Socket.recv hangs

[Barney Stratford]

Change by Barney Stratford :


--
keywords: +patch
pull_requests: +22448
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/23567

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[issue42513] Socket.recv hangs

[Barney Stratford]

New submission from Barney Stratford :

import socket
self.__socket = socket.create_connection ([host, port], 1)
value = self.__socket.recv (4096)

This code sometimes hangs despite having a non-None timeout specified. GDB says:

(gdb) bt
#0  0x76d33c94 in __GI___poll (fds=0x7ea55148, nfds=1, timeout=1)
at ../sysdeps/unix/sysv/linux/poll.c:29
#1  0x001e8014 in poll (__timeout=, __nfds=, 
__fds=, __fds=, __nfds=, 
__timeout=)
at /usr/include/arm-linux-gnueabihf/bits/poll2.h:46
#2  internal_select (writing=writing@entry=0, interval=, 
connect=0, connect@entry=4156908, s=)
at ../Modules/socketmodule.c:745
#3  0x001ec588 in sock_call_ex (s=s@entry=0x76979d68, writing=writing@entry=0, 
sock_func=sock_func@entry=0x1e736c , data=0x7ea551b0, 
data@entry=0x7ea551a8, connect=connect@entry=0, err=err@entry=0x0, 
timeout=100) at ../Modules/socketmodule.c:840
#4  0x001ed394 in sock_call (data=0x7ea551a8, func=0x1e736c , 
writing=0, s=0x76979d68) at ../Modules/socketmodule.c:3287
#5  sock_recv_guts (s=s@entry=0x76979d68, cbuf=, 
len=, flags=)
at ../Modules/socketmodule.c:3287
#6  0x001ed51c in sock_recv (s=0x76979d68, args=)
at ../Modules/socketmodule.c:3318

Googling for this problem turned up this:

https://stackoverflow.com/questions/56038224/poll-waits-indefinitely-although-timeout-is-specified

If we look at socket module.c line 756 (Python 3.7.9 version), we see that 
we're indeed not checking for the pollfd.revents, and are therefore missing 
socket errors.

PR coming up in a few days.

--
components: Library (Lib)
messages: 382145
nosy: Barney Stratford
priority: normal
severity: normal
status: open
title: Socket.recv hangs
type: crash
versions: Python 3.7

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