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test_io.check_interrupted_write() has two threads and a pipe:
 - reader (thread): read one byte from the pipe
 - writer (main thread): write 1 MB into the pipe
An alarm (SIGALRM) is scheduled in one second. The writer blocks because the pipe buffer is smaller than 1 MB, but it is supposed to fill the pipe. The reader is supposed to exit quickly: the writer wrote a least one byte.
The test fails *sometimes* on FreeBSD 6:
[ 85/352] test_io
Exception in thread Thread-274:
Traceback (most recent call last):
 File "/usr/home/db3l/buildarea/3.2.bolen-freebsd/build/Lib/threading.py", line 736, in _bootstrap_inner
 self.run()
 File "/usr/home/db3l/buildarea/3.2.bolen-freebsd/build/Lib/threading.py", line 689, in run
 self._target(*self._args, **self._kwargs)
 File "/usr/home/db3l/buildarea/3.2.bolen-freebsd/build/Lib/test/test_io.py", line 2660, in _read
 s = os.read(r, 1)
OSError: [Errno 4] Interrupted system call
It is a race condition, this buildbot is very slow. The reader has maybe not enough time to read 1 byte.
The test was fixed in Python 3.3 (#11859) by adding pthread_sigmask() (issue #8407) and using it in the test: commit 28b9702a83d1.
The problem looks to be specific of FreeBSD 6 and 7 (according to #11859). The easiest solution is to skip the test on these platforms.
To workaround the lack of pthread_sigmark(), we can use two processes instead of two threads. But it is maybe too much work just to fix a bug in a test (the bug is not in Python).

Patch to skip the test on FreeBSD 5, 6 and 7.
I was unable to reproduce #11859 on my FreeBSD 8 VM, so I didn't add freebsd8.

New changeset a624b86264a4 by Victor Stinner in branch '2.7':
Issue #12429: Skip interrupted write tests on FreeBSD <= 7
http://hg.python.org/cpython/rev/a624b86264a4
New changeset e924e51e9447 by Victor Stinner in branch '3.2':
Issue #12429: Skip interrupted write tests on FreeBSD <= 7
http://hg.python.org/cpython/rev/e924e51e9447
New changeset 79d7bcbe88f6 by Victor Stinner in branch 'default':
null merge 3.2
http://hg.python.org/cpython/rev/79d7bcbe88f6 

New changeset ad16e4a0ef80 by Victor Stinner in branch '3.2':
Issue #12429: Skip interrupted write tests on FreeBSD <= 7
http://hg.python.org/cpython/rev/ad16e4a0ef80