The appended code is a simple call of the SCSI Inquiry
ioctl, based on what scsi-tools scsiinfo.c does.  The
intended behaviour is simple - pass it /dev/sg0 (or any
scsi device, really) and it pulls out the vendor,
product and revision from the returned buffer.

However, if you change PAD_SIZE, the behaviour changes:
at 1023 and 1025, it does what is expected; at 1024,
the second assertion fires, because the array b on
output hasn't changed at all.

Not sure if there's a different ioctl that can
demonstrate this more easily (this needs root and a
scsi device, though it does explicitly open it
readonly) but it points in the direction of a fencepost
error in the "if the supplied buffer is less than 1024
bytes long it is first copied into a static buffer 1024
bytes long which is then passed to ioctl and copied
back into the supplied buffer" behaviour in current
documentation.

observed under:
Python 2.4.1 (#2, May  5 2005, 11:32:06)
[GCC 3.3.5 (Debian 1:3.3.5-12)] on linux2
(on debian sarge.)

I didn't see it mentioned among the ioctl bugs in the
sourceforge tracker...

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user_id=33168

This code was recently changed.  It is not in any released
version AFAIK.  Can you try 2.5b2 which recently came out? 
If not, can you try 2.4.3+ from SVN?  This is the version
that will become 2.4.4.  I think the problem may be fixed. 
The patch was for http://python.org/sf/1433877 .

Closing due to lack of feedback.

The bug still exists in Python 2.6 (2.6.7-4ubuntu2). Python 2.7 (2.7.3-5ubuntu4) works correctly. I don't have Python 2.5 on hand to check.

Here is a very simple test case:

import array, fcntl
buf = array.array('B', [0]*1024)
fcntl.ioctl(0, termios.TIOCGPGRP, buf, 1)
assert any(by != 0 for by in buf)

Also happens in 2.5.2-2ubuntu6 and 2.4.5-1ubuntu4.2, FWIW.