How should I unit test threaded code?

Question

I have thus far avoided the nightmare that is testing multi-threaded code since it just seems like too much of a minefield. I\'d like to ask how people have gone about test

Answer 1

Testing MT code for correctness is, as already stated, quite a hard problem. In the end it boils down to ensuring that there are no incorrectly synchronised data races in your code. The problem with this is that there are infinitely many possibilities of thread execution (interleavings) over which you do not have much control (be sure to read this article, though). In simple scenarios it might be possible to actually prove correctness by reasoning but this is usually not the case. Especially if you want to avoid/minimize synchronization and not go for the most obvious/easiest synchronization option.

An approach that I follow is to write highly concurrent test code in order to make potentially undetected data races likely to occur. And then I run those tests for some time :) I once stumbled upon a talk where some computer scientist where showing off a tool that kind of does this (randomly devising test from specs and then running them wildly, concurrently, checking for the defined invariants to be broken).

By the way, I think this aspect of testing MT code has not been mentioned here: identify invariants of the code that you can check for randomly. Unfortunately, finding those invariants is quite a hard problem, too. Also they might not hold all the time during execution, so you have to find/enforce executions points where you can expect them to be true. Bringing the code execution to such a state is also a hard problem (and might itself incur concurrency issues. Whew, it's damn hard!

Some interesting links to read:

• Deterministic interleaving: A framework that allows to force certain thread interleavings and then check for invariants
• jMock Blitzer : Stress test synchronization
• assertConcurrent : JUnit version of stress testing synronization
• Testing concurrent code : Short overview of the two primary methods of brute force (stress test) or deterministic (going for the invariants)