Thanks, Alan.  I realised my answer was a shallow one after reading (too
late!) the thread you started on python-dev.

However, I have a suspicion that that particular array test
(test_alloc_overflow) was merely meant to test the code in
newarrayobject, at around line 427 of arraymodule.c, which looks like:

 	/* Check for overflow */
	if (nbytes / descr->itemsize != (size_t)size) {
		return PyErr_NoMemory();
	}

and that the test dated from an era when it was fairly safe to assume
that a size_t was at most 32 bits.  I'd guess that test_alloc_overflow
was never intended to be a test of OS malloc failure behaviour.

So the array test is wrong, and I think this patch should be applied
anyway.  I admit this doesn't help with the much more interesting
question of what's going on with malloc on FreeBSD.