I’ve been using Clojure for about two years. Much of that time has been spent using Zach Tellman’s Manifold library that makes it easier to build event-driven systems. I tend to prefer a test driven approach to development. This means that I like to write tests that are deterministic and quick to fail or succeed.

Much of the Clojure code that I write for network based systems ends up using manifold streams to build processing pipelines. My tests frequently have some assertions that require that I pull a value off of a stream.

One of the slightly frustrating things about Clojure when coming from a language like python is how long it takes for the JVM to start up and for leiningen to be ready to run tests. This can be mitigated by using something like lein auto. It keeps the JVM up, reloads your code, and can rerun your tests. This is extremely helpful assuming you haven’t written code that blocks forever.

All of the source code to be discussed and run is available at derwolfe/test-restarts-examples.

To the problem: I have accidentally written part of a test backwards. The following test will start once and block forever. lein auto test won’t track further changes to it until the process is killed and restarted. Note, this isn’t lein auto test’s fault, the test simply blocks forever due to a mistake I’ve made.

(deftest ^:slow example-test-blocks-forever
  (let [a (ms/stream)]
    (is (= 1 @(ms/take! a)))))

Source for this test is here.

If you have pulled down the project, you should be able to run lein auto test :slow to run this test. This test will never complete. To continue developing you will need to kill and restart the test runner/JVM.

What can we do to avoid this?

1. Pay better attention when writing the test.
2. Add a way to timeout an entire test (this is what twisted does and I miss it).
3. Don’t use function that can block indefinitely in the tests.

In the longer term, I’d like to write a function/test macro that will timeout a test after a specified number of seconds (that is, I’d like two use #2). But, as I don’t have that right now, I have settled on using option #3.

(deftest example-test-timeout
  (let [a (ms/stream)]
    (is (= 1 @(ms/try-take! a ::drained (mt/seconds 1) ::timeout)))))

Source for this test is here

This works well enough: the test fails immediately and obviously.

If you are using manifold streams and have tests that involve pulling or pushing data to them, I’d suggest using try-put! and/or try-take! with timeouts in your tests, instead of simply using put! and take!.