> This type of failure appears again in current builds:

Unfortunately, I think you mean 'still' rather than 'again'.  :)
As far as I can tell, the failure's never gone away, though it may have been obscured by other failures from time to time.

Maybe it's time to do something.  I propose we:

  1. create a new branch py3k-issue4970
  2. Hack Lib/test/regrtest.py in that branch so that it runs
     *only* test_os and test_wait3, in that order (ignoring the -r
     flag).  Check that we're still getting the failure.
  3. Do a binary search (remove half the test_os tests; trigger
     buildbot run; see if we're still getting the signal; rinse;
     repeat) to narrow down the cause to a particular test.
  4. While doing 3, ruthlessly kill all other non-trunk
     checkin-triggered buildbot runs on this machine
     to speed up the search process a bit.  (Keeping trunk builds
     for the sake of the upcoming 2.7 release.)

I'll try to start this this evening (no ssh access at the moment) unless someone else beats me to it.