[ python-Bugs-1759997 ] poll() on cygwin sometimes fails [PATCH]

[SourceForge.net]

Bugs item #1759997, was opened at 2007-07-24 17:58
Message generated for change (Comment added) made by warner
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1759997&group_id=5470

Please note that this message will contain a full copy of the comment thread,
including the initial issue submission, for this request,
not just the latest update.
Category: Extension Modules
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Brian Warner (warner)
Assigned to: Nobody/Anonymous (nobody)
Summary: poll() on cygwin sometimes fails [PATCH]

Initial Comment:

While trying to track down a problem with our application (http://allmydata.org) running under cygwin, I discovered that the select.poll() object sometimes returns completely bogus data. poll() returns a list of tuples of (fd, revent), but fds are supposed to be small integers, and revents are a bitmask of POLLIN/POLLOUT flags. In my tests, I saw poll() return a list that started out looking normal, but the last half of the list contained fds and revents values like fd=0x7672a646, revent=0xd819. 

It turns out that under cygwin-1.5.24 (which I believe is a pretty recent version), the poll() call sometimes violates the POSIX specification, and provides a return value which is different than the number of pollfd structures that have non-zero .revents fields (generally larger). This causes the implementation of poll_poll() (in Modules/selectmodule.c) to read beyond the end of the pollfd array, copying random memory into the python list it is building, causing the bogus values I observed during my tests.  

These bogus values were mostly ignored, because the Twisted pollreactor that I was using noticed that the fd didn't correspond to any previously-registered file descriptor. It was only when the bogus fd happened to coincide with a real one (and when that indicated that a TCP listening socket became writable, which should never happen) that an exception was raised.

 The attached patch (against 2.5.1) works around the problem by manually counting the number of non-zero .revents, rather than relying upon the return value from poll(). This version passes test_poll on both linux and cygwin.

cheers,
 -Brian Warner




----------------------------------------------------------------------

>Comment By: Brian Warner (warner)
Date: 2007-07-27 16:25

Message:
Logged In: YES 
user_id=24186
Originator: YES

We've begun the process: zooko is working on a patch for cygwin and is
working with them to figure out how to compile the thing. We've not yet
explained the poll() bug to them in detail (wanting to have a patch in hand
first).

I'll report back once we get some word from them about how likely it is
this problem will be fixed on the cygwin side.


----------------------------------------------------------------------

Comment By: Neal Norwitz (nnorwitz)
Date: 2007-07-24 22:53

Message:
Logged In: YES 
user_id=33168
Originator: NO

Has this problem been reported to cygwin?  Have they fixed the problem?

----------------------------------------------------------------------

You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1759997&group_id=5470