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cabal-testsuite is a suite of integration tests for Cabal-based frameworks.

How to run

  1. Build cabal-tests (cabal build cabal-tests)
  2. Run the cabal-tests executable. It will scan for all tests in your current directory and subdirectories and run them. To run a specific set of tests, use cabal-tests PATH .... You can control parallelism using the -j flag.

There are a few useful flags:

  • --with-cabal PATH can be used to specify the path of a cabal-install executable. IF YOU DO NOT SPECIFY THIS FLAG, CABAL INSTALL TESTS WILL NOT RUN.

  • --with-ghc PATH can be used to specify an alternate version of GHC to ask the tests to compile with.

  • --builddir DIR can be used to manually specify the dist directory that was used to build cabal-tests; this can be used if the autodetection doesn't work correctly (which may be the case for old versions of GHC.)

How to write

If you learn better by example, just look at the tests that live in cabal-testsuite/PackageTests; if you git log -p, you can see the full contents of various commits which added a test for various functionality. See if you can find an existing test that is similar to what you want to test.

Otherwise, here is a walkthrough:

  1. Create the package(s) that you need for your test in a new directory. (Currently (2021-10-06), tests are stored in PackageTests, with the exception of one test stored in tests.)

  2. Create one or more .test.hs scripts in your directory, using the template:

    import Test.Cabal.Prelude
main = setupAndCabalTest $ do
    -- your test code here

    setupAndCabal test indicates that invocations of setup should work both for a raw Setup script, as well as cabal-install (if your test works only for one or the other, use setupTest or cabalTest).

    Code runs in the TestM monad, which manages some administrative environment (e.g., the test that is running, etc.). Test.Cabal.Prelude contains a number of useful functions for testing implemented in this monad, including the functions cabal and setup which let you invoke those respective programs. You should read through that file to get a sense for what capabilities are possible (grep for use-sites of functions to see how they are used). If you don't see something anywhere, that's probably because it isn't implemented. Implement it!

    To include parts that are supposed to fail (in the sense that a non-zero exit code is returned), there is the fails combinator, e.g.:

    main = cabalTest $ do
  fails $ cabal "bad-command" [ "bad", "args" ]
  cabal "good-command" [ "good", "args" ]
  fails $ cabal "another-bad-one" [ ... ]
  ...

  3. Run your tests using cabal-tests (no need to rebuild when you add or modify a test; it is automatically picked up). The first time you run a test, assuming everything else is in order, it will complain that the actual output doesn't match the expected output. Use the --accept flag to accept the output if it makes sense!

We also support a .multitest.hs prefix; eventually this will allow multiple tests to be defined in one file but run in parallel; at the moment, these just indicate long running tests that should be run early (to avoid straggling).

Frequently asked questions

For all of these answers, to see examples of the functions in question, grep the test suite.

Why isn't some output I added to Cabal showing up in the recorded test output? Only "marked" output is picked up by Cabal; currently, only notice, warn and die produce marked output. Use those combinators for your output.

How do I safely let my test modify version-controlled source files? Use withSourceCopy. Note that you MUST git add all files which are relevant to the test; otherwise they will not be available when running the test.

How can I add a dependency on a package from Hackage in a test? By default, the test suite is completely independent of the contents of Hackage, to ensure that it keeps working across all GHC versions. If possible, define the package locally. If the package needs to be from Hackage (e.g., you are testing the global store code in new-build), use withRepo "repo" to initialize a "fake" Hackage with the packages placed in the repo directory.

How do I run an executable that my test built? The specific function you should use depends on how you built the executable:

  • If you built it using Setup build, use runExe
  • If you installed it using Setup install or cabal install, use runInstalledExe.
  • If you built it with cabal build, use runPlanExe; note that you will need to run this inside of a withPlan that is placed after you have invoked build. (Grep for an example!)

How do I turn off accept tests? My test output wobbles too much. Use recordMode DoNotRecord. This should be a last resort; consider modifying Cabal so that the output is stable. If you must do this, make sure you add extra, manual tests to ensure the output looks like what you expect.

How can I manually test for a string in output? Use the primed variants of a command (e.g., cabal' rather than cabal) and use assertOutputContains. Note that this will search over BOTH stdout and stderr.

How do I skip running a test in some environments? Use the skipIf and skipUnless combinators. Useful parameters to test these with include hasSharedLibraries, hasProfiledLibraries, hasCabalShared, isGhcVersion, isWindows, isLinux, isOSX and hasCabalForGhc.

I programatically modified a file in my test suite, but Cabal/GHC doesn't seem to be picking it up. You need to sleep sufficiently long before editing a file, in order for file system timestamp resolution to pick it up. Use withDelay and delay prior to making a modification.

How do I mark a test as broken? Use expectBroken, which takes the ticket number as its first argument. Note that this does NOT handle accept-test brokenness, so you will have to add a manual string output test, if that is how your test is "failing."

Hermetic tests

By default, we run tests directly on the source code that is checked into the source code repository. However, some tests require programatically modifying source files, or interact with Cabal commands which are not hermetic (e.g., cabal freeze). In this case, cabal-testsuite supports opting into a hermetic test, where we first make copy of all the relevant source code before starting the test. You can opt into this mode using the withSourceCopy combinator (search for examples!) This mode is subject to the following limitations:

  • You must be running the test inside a valid Git checkout of the test suite (withSourceCopy uses Git to determine which files should be copied.)

  • You must git add all files which are relevant to the test, otherwise they will not be copied.

  • The source copy is still made at a well-known location, so running a test is still not reentrant. (See also Known Limitations.)

Design notes

This is the second rewrite of the integration testing framework. The primary goal was to use Haskell as the test language (letting us take advantage of a real programming language, and use utilities provided to us by the Cabal library itself), while at the same time compensating for two perceived problems of pure-Haskell test suites:

  • Haskell test suites are generally compiled before they run (for example, this is the modus operandi of cabal test). In practice, this results in a long edit-recompile cycle when working on tests. This hurts a lot when you would like to experimentally edit a test when debugging an issue.

  • Haskell's metaprogramming facilities (e.g., Template Haskell) can't handle dynamically loading modules from the file system; thus, there ends up being a considerable amount of boilerplate needed to "wire" up test cases to the central test runner.

Our approach to address these issues is to maintain Haskell test scripts as self-contained programs which are run by the GHCi interpreter. This is not altogether trivial, and so there are a few important technical innovations to make this work:

  • Unlike a traditional test program which can be built by the Cabal build system, these test scripts must be interpretable at runtime (outside of the build system.) Our approach to handle this is to link against the same version of Cabal that was used to build the top-level test program (by way of a Custom setup linked against the Cabal library under test) and then use this library to compute the necessary GHC flags to pass to these scripts.

  • The startup latency of runghc can be quite high, which adds up when you have many tests. To solve this, in Test.Cabal.Server we have an implementation an GHCi server, for which we can reuse a GHCi instance as we are running test scripts. It took some technical ingenuity to implement this, but the result is that running scripts is essentially free.

Here is the general outline of how the cabal-tests program operates:

  1. It first loads the cached LocalBuildInfo associated with the host build system (which was responsible for building cabal-tests in the first place.) This information lets us compute the flags that we will use to subsequently invoke GHC.

  2. We then recursively scan the current working directory, looking for files suffixed .test.hs; these are the test scripts we will run.

  3. For every thread specified via the -j, we spawn a GHCi server, and then use these to run the test scripts until all test scripts have been run.

The new cabal-tests runner doesn't use Tasty because I couldn't figure out how to get out the threading setting, and then spawn that many GHCi servers to service the running threads. Improvements welcome.

Expect tests

An expect test (aka golden test) is a test where we read out the output of the test and compare it directly against a saved copy of the test output. When test output changes, you can ask the test suite to "accept" the new output, which automatically overwrites the old expected test output with the new.

Supporting expect tests with Cabal is challenging, because Cabal interacts with multiple versions of external components (most prominently GHC) with different variants of their output, and no one wants to rerun a test on four different versions of GHC to make sure we've picked up the correct output in all cases.

Still, we'd like to take advantage of expect tests for Cabal's error reporting. So here's our strategy:

  1. We have a new verbosity flag +markoutput which lets you toggle the emission of -----BEGIN CABAL OUTPUT----- and -----END CABAL OUTPUT----- stanzas.

  2. When someone requests an expect test, we ONLY consider output between these flags.

The expectation is that Cabal will only enclose output it controls between these stanzas. In practice, this just means we wrap die, warn and notice with these markers.

An added benefit of this strategy is that we can continue operating at high verbosity by default (which is very helpful for having useful diagnostic information immediately, e.g. in CI.)

We also need to deal with nondeterminism in test output in some situations. Here are the most common ones:

  • Dependency solving output on failure is still non-deterministic, due to its dependence on the global package database. We're tracking this in haskell#4332 but for now, we're not running expect tests on this output.

  • Tests against Custom setup will build against the Cabal that shipped with GHC, so you need to be careful NOT to record this output (since we don't control that output.)

  • We have some munging on the output, to remove common sources of non-determinism: paths, GHC versions, boot package versions, etc. Check normalizeOutput to see what we do. Note that we save normalized output, so if you modify the normalizer you will need to rerun the test suite accepting everything.

  • The Setup interface gets a --enable-deterministic flag which we pass by default. The intent is to make Cabal more deterministic; for example, with this flag we no longer compute a hash when computing IPIDs, but just use the tag -inplace. You can manually disable this using --disable-deterministic (as is the case with UniqueIPID.)

Some other notes:

  • It's good style to put default-language in all your stanzas, so Cabal doesn't complain about it (that warning is marked!). Ditto with cabal-version at the top of your Cabal file.

  • If you can't get the output of a test to be deterministic, no problem: just exclude it from recording and do a manual test on the output for the string you're looking for. Try to be deterministic, but sometimes it's not (easily) possible.

Non-goals

Here are some things we do not currently plan on supporting:

  • A file format for specifying multiple packages and source files. While in principle there is nothing wrong with making it easier to write tests, tests stored in this manner are more difficult to debug with, as they must first be "decompressed" into a full folder hierarchy before they can be interacted with. (But some of our tests need substantial setup; for example, tests that have to setup a package repository. In this case, because there already is a setup necessary, we might consider making things easier here.)

Known limitations

  • Tests are NOT reentrant: test build products are always built into the same location, and if you run the same test at the same time, you will clobber each other. This is convenient for debugging and doesn't seem to be a problem in practice.