Okay the patch has been worked over in the bundled fork of setuptools that Tahoe-LAFS uses ("zetuptoolz"), and applying the patch made the build go from red to green on many different platforms and did not introduce a regression from green to red on any builder. It has now gone into Tahoe-LAFS trunk and will be released in Tahoe-LAFS v1.8.1 soon.

http://tahoe-lafs.org/trac/tahoe-lafs/ticket/1233

Hooray! We have a patch and the deterministic test of this on the Tahoe-LAFS buildslave herd shows that it works:
http://tahoe-lafs.org/trac/tahoe-lafs/ticket/1233

We now have an automated test of this issue on the tahoe-lafs buildbot:

http://tahoe-lafs.org/trac/tahoe-lafs/ticket/1233#comment:8

It is often the case that a binary .egg is usable and an sdist is not (when the
package requires compiling C or C++ code).  Here is an example where this is
currently happening:

http://allmydata.org/trac/tahoe/ticket/773#comment:7

Tahoe-LAFS requires zfec>=1.1.0.  There are two distributions found during build
a binary .egg of zfec v1.4.1 for the current platform and python version and an
sdist of zfec v1.4.5.  Setuptools is choosing the latter, which then fails to
build because the C/C++ compiler isn't set up on the target system.

If setuptools would prefer binaries over sdists when both satisfy the versioning
requirements, then this would be better.