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barrucadu / dejafu

Systematic concurrency testing meets Haskell.
https://dejafu.docs.barrucadu.co.uk/
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concurrency haskell testing

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dejafu

[Déjà Fu is] A martial art in which the user's limbs move in time as well as space, […] It is best described as "the feeling that you have been kicked in the head this way before"

-- Terry Pratchett, Thief of Time

Déjà Fu is a unit-testing library for concurrent Haskell programs. Tests are deterministic and expressive, making it easy and convenient to test your threaded code.

Available on GitHub, Hackage, and Stackage.

Features:

There are a few different packages under the Déjà Fu umbrella:

Version Summary
concurrency 1.11.0.3 Typeclasses, functions, and data types for concurrency and STM.
dejafu 2.4.0.5 Systematic testing for Haskell concurrency.
hunit-dejafu 2.0.0.6 Deja Fu support for the HUnit test framework.
tasty-dejafu 2.1.0.1 Deja Fu support for the Tasty test framework.

See the latest package documentation.

Each package has its own README and CHANGELOG in its subdirectory.

There is also dejafu-tests, the test suite for dejafu. This is in a separate package due to Cabal being bad with test suite transitive dependencies.

Installation

Install from Hackage globally:

$ cabal install dejafu

Or add it to your cabal file:

build-depends: ...
             , dejafu

Or to your package.yaml:

dependencies:
  ...
  - dejafu

Quick start guide

Déjà Fu supports unit testing, and comes with a helper function called autocheck to look for some common issues. Let's see it in action:

import Control.Concurrent.Classy

myFunction :: MonadConc m => m String
myFunction = do
  var <- newEmptyMVar
  fork (putMVar var "hello")
  fork (putMVar var "world")
  readMVar var

That MonadConc is a typeclass abstraction over concurrency, but we'll get onto that shortly. First, the result of testing:

> autocheck myFunction
[pass] Never Deadlocks
[pass] No Exceptions
[fail] Consistent Result
    "hello" S0----S1--S0--

    "world" S0----S2--S0--
False

There are no deadlocks or uncaught exceptions, which is good; but the program is (as you probably spotted) nondeterministic!

Along with each result, Déjà Fu gives us a representative execution trace in an abbreviated form. Sn means that thread n started executing, and Pn means that thread n pre-empted the previously running thread.

Why Déjà Fu?

Testing concurrent programs is difficult, because in general they are nondeterministic. This leads to people using work-arounds like running their testsuite many thousands of times; or running their testsuite while putting their machine under heavy load.

These approaches are inadequate for a few reasons:

Déjà Fu addresses these points by offering complete testing. You can run a test case and be guaranteed to find all results with some bounds. These bounds can be configured, or even disabled! The underlying approach used is smarter than merely trying all possible executions, and will in general explore the state-space quickly.

If your test case is just too big for complete testing, there is also a random scheduling mode, which is necessarily incomplete. However, Déjà Fu will tend to produce much more schedule variety than just running your test case in IO the same number of times, and so bugs will tend to crop up sooner. Furthermore, as you get execution traces out, you can be certain that a bug has been fixed by simply following the trace by eye.

Contributing

See the "contributing" page on the website.

If you'd like to get involved with Déjà Fu, check out the "good first issue" label on the issue tracker.

Release notes

See the CHANGELOG.markdown file.

Questions, feedback, discussion

Bibliography

These libraries wouldn't be possible without prior research, which I mention in the documentation. Haddock comments get the full citation, whereas in-line comments just get the shortened name:

There are also a few papers on dejafu itself: