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

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

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.

FYI, this is the issue ticket on the allmydata.org Tahoe project:

http://allmydata.org/trac/tahoe/ticket/31

I've written a patch for cygwin poll and am now testing it before submitting it to the cygwin developers.

This bug was fixed in cygwin 1.5.25-7, released 2007-12-17.

Closing as 3rd party, then.