@Serhiy

I have noticed that the failure is reproducible in the buildbot workers only when startup (of buildbot) is invoked via sudo, and not when started on first-boot (rc runs as root). 

In both situations, twistd then drops privs to --uid=buildbot --gid=buildbot). 

However, I *cannot* reproduce the failure (python -m test.regrtest test_os) on a clean checkout and build of 3.x either under a normal user, *or* under sudo.

I think we need some instrumentation added to test to see whats happening. 

We can add that instrumentation and also test your patch committed to a private hg branch using the 'custom' builder.

I can also provide SSH access to the buildbot hosts.