Cannot use high-numbered sockets in 2.4.3

[mlrsmith]

BPO	1494314
Nosy	@tim-one, @loewis, @akuchling, @gpshead
Files	socket.patch: Proposed simple patch poll-in-socketmodule.diff: Patch to use poll() in preference to select() high-socket.diff: use poll in socket and ssl modules high-socket.diff: v2 of using poll

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2006-10-11.02:40:04.000>
created_at = <Date 2006-05-24.13:51:05.000>
labels = ['extension-modules']
title = 'Cannot use high-numbered sockets in 2.4.3'
updated_at = <Date 2006-10-11.02:40:04.000>
user = 'https://bugs.python.org/mlrsmith'

bugs.python.org fields:

activity = <Date 2006-10-11.02:40:04.000>
actor = 'nnorwitz'
assignee = 'anthonybaxter'
closed = True
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2006-05-24.13:51:05.000>
creator = 'mlrsmith'
dependencies = []
files = ['2011', '2012', '2013', '2014']
hgrepos = []
issue_num = 1494314
keywords = []
message_count = 21.0
messages = ['28607', '28608', '28609', '28610', '28611', '28612', '28613', '28614', '28615', '28616', '28617', '28618', '28619', '28620', '28621', '28622', '28623', '28624', '28625', '28626', '28627']
nosy_count = 8.0
nosy_names = ['tim.peters', 'loewis', 'akuchling', 'nnorwitz', 'anthonybaxter', 'gregory.p.smith', 'mlrsmith', 'gabrielwicke']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1494314'
versions = ['Python 2.4']

[mlrsmith]

Python 2.4.3 introduced (in Modules/socketmodule.c) the
IS_SELECTABLE() macro, to check whether calling
select() on a given file descriptor is permissible.

However, it executes this check even when it won't
actually call select(). Specifically, select() is
called ONLY when s->sock_timeout > 0 (in
internal_select mostly, but also internal_connect).

I have a twisted application which uses many FDs
(several thousand), and upgrading python to 2.4.3 makes
it hit this limit (at 1024 file descriptors),
regardless of ulimit settings. Twisted itself always
uses non-blocking I/O on sockets, so with older
versions of python this worked fine.

A simple solution relies on the fact that select is
only ever actually called, and changes the
IS_SELECTABLE macro as in the attached fairly trivial
patch. This is sufficient to restore my application to
its previous state (where it works fine).

This doesn't solve the more general problem in
socketmodule - that we simply can't do all operations
properly with the current reliance on select(), though,
and it seems like a bit of a hack... If I wrote up a
patch to use poll() on systems where it's available,
throughout socketmodule.c, in preference to select(),
would this be accepted?

Mike

[gabrielwicke]

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Try "ulimit -n 8096" (only permitted in a root shell) to set
your max socket usage to something larger. Opening more than
1024 sockets works for me in python 2.4.3 after changing the
limit for the terminal in question and restarting the
interpreter. Just ulimit -n will show you your current
limit. Ulimits are enforced by the Linux kernel and can
likely be configured system-wide in /etc/sysctl.cfg.

[gabrielwicke]

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Never mind- you already tried ulimit. It still works for me
though, 2.4.3 from Ubuntu Dapper.

[gabrielwicke]

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Please disregard my comments completely- just opening more
than 1024 files does not trigger this bug, but doing a
socket.send() on the 1025th socket does.

[mlrsmith]

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Yes, I had my ulimit set appropriately.

There's no problem with _opening_ many sockets (at least, I
don't think there is) - the problem is with trying to do
something (like call socket.recv()) with them.

The code in socketmodule.c is pretty clear - and having
upgraded to 2.4.3 with ubuntu dapper, I _am_ running into this.

For now, we're having to keep all our production machines on
2.4.2.

[akuchling]

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I expect such a patch would be acceptable. The largest
issue is probably whether poll() is available everywhere, or
if we'd be stuck maintaining both select() and poll() based
versions of internal_select(). Does Windows support poll()
at all?

[mlrsmith]

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That, of course, is the problem - select() is available more
or less universally, but poll() is not. However, it's not
terribly difficult to write a poll() wrapper using select(),
though doing so in general would have serious performance
issues (the specific uses in socketmodule.c do not hit this
problem), and retains the limitations of select.

It's not clear that the complexity of doing this is
worthwhile compared to just implementing the simple API
needed internally to socketmodule.c using both APIs (i.e.
just adding a poll() option).

Regardless, it'd be nice if at least the basic fix I wrote
up went in - it solves the immediate problem, at least.

[tim-one]

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akuchling: No, poll() is not part of the Windows socket API.

[loewis]

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A patch to replace internal_select with poll(2) where
available is acceptable. The current version should be
conditionally kept. That Windows doesn't have poll(2) is no
problem: its select implementation supports arbitrarily
large FDs (just like poll).

Relaxing the IS_SELECTABLE usage to cases where select is
actually going to be called is optional: there shouldn't be
too many systems left without select where people still want
to open many file descriptors.

[gpshead]

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eew yuck. yes use poll at the very least.

we should also consider using epoll (linux) and kevent (bsd)
in the future as both of those scale better than O(n) unlike
select and poll.

[mlrsmith]

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Ok, I'll attach a patch that uses poll when available
(HAVE_POLL is already being set by the configure stuff
appropriately).

It replaces one of the two uses of select() (specifically,
the internal_select() function) in socketmodule.c. The other
is win32-specific, so replacing it with poll() wouldn't make
sense.

greg: epoll/kevent don't make sense for replacing the use of
select/poll in this particular case - socketmodule.c always
selects/polls precisely one file descriptor.

I've tested this locally, and it fixes the problem
(linux/x86). I don't have a windows system to test it on,
but it shouldn't change behaviour in any way for windows.

[nnorwitz]

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I've added a more complete patch (against 2.5, hopefully
applies to 2.4 too). It cleans up some things and adds
support for SSL sockets too. Can people please review/test
this? I manually tested this with regular sockets and it
seemed to work. All the tests pass, but this is somewhat
tricky. I hate the duplicated code between socket and ssl
modules, but added to it. It would be great to clean this
up for 2.6.

If you are forced to use select with a high socket, the
exception on connect is operation in progress rather than
can't select on socket. I guess that's ok, it doesn't
actually change the existing behaviour. That would have
been more involved and I'm not sure it's worth it.

[loewis]

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The __APPLE__ stuff looks wrong in file 184131. You would
have to use selectmodule.c:select_have_broken_poll at
run-time to be sure you can use poll(2) on OS X (you can use
it on 10.4, but can't on 10.3, IIRC).

[nnorwitz]

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I think you're right Martin. Looking at what it means to
have a broken poll, I don't think we can in this instance.
So I removed all refs to HAVE_BROKEN_POLL. What do you
think of the new version?

[nnorwitz]

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I meant I don't think we *care* in this case (not can).

[loewis]

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The patch is fine, please apply.

[nnorwitz]

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Anthony checked this in to 2.5 as 50567. I will backport at
least part of it later.

[anthonybaxter]

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Applied. Patch will be in 2.5b2 (to be released shortly).

[anthonybaxter]

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Re-opening to remind myself to apply this to release24-maint
for the eventual 2.4.4 release.

[anthonybaxter]

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This is applied and will be in 2.3.3c1

[nnorwitz]

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I assume 2.3.3c1 means 2.4.4c1.