Unit Testing With Threads

I saw this and started thinking back on a calculus course at university.

Wouldn't we expect a theorem like this to be true:

"for all codebases C and features F, there exists a set of refactorings R and unit test U, such that U(F) is safe under R(C)"

In other words, can we not manipulate any piece of untestable thread code into a testable one, given enough refactorings and inversion of control levers (and effort!)? Seems reasonable to me.

In a situation with 2 threads and one interaction, it is easy to imagine the combinatorial complexity of test cases. It of course explodes when the numbers go up - but perhaps we could argue that for any number of threads and interactions, we can reduce the complexity of our tests by looking at each in isolation...

If my theorem holds, you can of course turn it around and use it to knock people over their heads when they screw up thread testing - hmm, how productive that would be :-)

Interesting thought, anyway.