minitest-bisect¶ ↑

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Hunting down random test failures can be very very difficult, sometimes impossible, but minitest-bisect makes it easy.

minitest-bisect helps you isolate and debug random test failures.

If your tests only fail randomly, you can reproduce the error consistently by using ‘–seed <num>`, but what then? How do you figure out which combination of tests out of hundreds are responsible for the failure? You know which test is failing, but what others are causing it to fail or were helping it succeed in a different order? That’s what minitest-bisect does best.

• normal mode: runs your tests with a fixed seed and minimizes the tests to find a minimal set to reproduce the failure.

• inverted mode: runs your tests with a fixed seed and a starting failure and minimizes the tests to find a minimal set to reproduce the false positive.

• Includes Enumerable#find_minimal_combination and #find_minimal_combination_and_count.

Let’s say you have a bunch of test files and they fail sometimes, but not consistently. You get a run that fails, so you record the randomization seed that minitest displays at the top of every run.

% minitest example
Run options: --seed 42

# Running:

..............................................................................
..............................................................................
..............................................................................
..............................................................................
..............................................................................
..............................................................................
..............................................................................
..............................................................................
..............................................................................
..............................................................................
....................

Finished in 200.341744s, 3.6371 runs/s, 3.4230 assertions/s.

800 runs, 798 assertions, 0 failures, 0 errors, 0 skips

But someone sees the failure either locally or on the CI. They record the output and get the randomization seed that causes the test ordering bug:

$ minitest example --seed 314
Run options: --seed 314

# Running:

..............................................................................
..............................................................................
..............................................................................
..............................................................................
.........................................F....................................
..............................................................................
..............................................................................
..............................................................................
..............................................................................
..............................................................................
....................

Finished in 200.836561s, 3.9833 runs/s, 3.9784 assertions/s.

  1) Failure:
TestBad4#test_bad4_4 [example/helper.rb:16]:
muahahaha order dependency bug!

800 runs, 799 assertions, 1 failures, 0 errors, 0 skips

The problem is about how to efficiently figure out the minimal reproduction of the random failure so you can actually focus on the problem and not all the noise.

So, you run the tests again, but this time with minitest_bisect. Provide the seed given in the failure so the tests always run in the same order and reproduce every time.

minitest_bisect will minimize the number of methods.

% minitest_bisect example --seed 314
reproducing... in 203.83 sec
verifying... in 0.37 sec
# of culprit methods: 64 in 16.65 sec
# of culprit methods: 32 in 8.53 sec
# of culprit methods: 32 in 8.52 sec
# of culprit methods: 16 in 4.46 sec
# of culprit methods: 16 in 4.44 sec
# of culprit methods: 8 in 2.41 sec
# of culprit methods: 4 in 1.40 sec
# of culprit methods: 2 in 0.89 sec
# of culprit methods: 2 in 0.89 sec
# of culprit methods: 1 in 0.62 sec
# of culprit methods: 1 in 0.63 sec

Minimal methods found in 11 steps:

Culprit methods: ["TestBad1#test_bad1_1", "TestBad4#test_bad4_4"]

ruby -Itest:lib -e 'require "./example/test_bad1.rb" ; require "./example/test_bad2.rb" ; require "./example/test_bad3.rb" ; require "./example/test_bad4.rb" ; require "./example/test_bad5.rb" ; require "./example/test_bad6.rb" ; require "./example/test_bad7.rb" ; require "./example/test_bad8.rb"' -- --seed 314 -n "/^(?:TestBad1#(?:test_bad1_1)|TestBad4#(?:test_bad4_4))$/"

Final reproduction:

Run options: --seed 314 -n "/^(?:TestBad1#(?:test_bad1_1)|TestBad4#(?:test_bad4_4))$/"

# Running:

.F

Finished in 0.512999s, 3.8986 runs/s, 1.9493 assertions/s.

  1) Failure:
TestBad4#test_bad4_4 [/Users/ryan/Work/p4/zss/src/minitest-bisect/dev/example/helper.rb:16]:
muahahaha order dependency bug!

Voila! This reduced it from 800 tests across 8 files down to 2 tests across 2 files. Note how we went from a ~200 second test run to a ~0.5 second test run. Debugging that will be 400x faster and that much easier.

Now, it is now up to you to look at the source of both of those tests to determine what side-effects (or lack thereof) are causing your test failure when run in this specific order.

This happens in a single run. Depending on how many files / tests you have and how long they take, the minimization might take a long time, but each iteration can reduce the number of tests so it should get quicker with time.

Sometimes a test fails by itself consistently but passes most of the time in CI. This is a false-positive in that the test SHOULD be failing all the time but some other test is causing a side-effect that it making it pass. This is sometimes harder to track down and reason about. minitest-bisect now has an “inverted” mode, where you run as normal but point it at a failing test by name. It will then validate that the test fails by itself but passes when run in full with the given seed. Then it tries to isolate and answer “what is the minimal combination of tests to run to # make this test pass?”. For example:

% minitest_bisect example --seed=3 -n="/failing_test_name_regexp/"

reproducing w/ scoped failure (inverted run!)... in 0.06 sec
reproducing false positive... in 0.06 sec
# of culprit methods: 2 in 0.06 sec
# of culprit methods: 1 in 0.06 sec
# of culprit methods: 1 in 0.06 sec

Minimal methods found in 3 steps:

Culprit methods: ["TestBad1#test_make_it_go", "TestBad1#test_fail_without"]

/Users/ryan/.rubies/ruby-3.2.2/bin/ruby -Itest:lib -Ilib -e 'require "./example"' -- --seed 3  -n "/^(?:TestBad1#(?:test_make_it_go|test_fail_without))$/"

Final reproduction:

Run options: --seed 3 -n "/^(?:TestBad1#(?:test_make_it_go|test_fail_without))$/"

# Running:

..

Finished in 0.000369s, 5420.0544 runs/s, 0.0000 assertions/s.

2 runs, 0 assertions, 0 failures, 0 errors, 0 skips

• minitest 5+

• sudo gem install minitest-bisect

This is assuming the following doesn’t work for you:

minitest_bisect --seed 3911 -Itest test/*_test.rb

because you need it to span multiple sub-directories.

A: Use your shell to find the files for you

minitest_bisect --seed 3911 -Itest $(find test -type f -name \*_test.rb)

or

minitest_bisect --seed 3911 test/**/*_test.rb

$ MTB_VERBOSE=2 minitest_bisect --seed 3911 example/test*.rb
reproducing...
ruby -Ilib -e 'require "./example/test_bad1.rb" ; require "./example/test_bad4.rb"' -- --seed 3911 --server 33352

invalid option: --server

minitest options:
    -h, --help                       Display this help.
    -s, --seed SEED                  Sets random seed. Also via env. Eg: SEED=n rake
    -v, --verbose                    Verbose. Show progress processing files.
    -n, --name PATTERN               Filter run on /regexp/ or string.

Known extensions: guard_minitest, pride
    -p, --pride                      Pride. Show your testing pride!
Reproduction run passed? Aborting. Try running with MTB_VERBOSE=2 to verify.

A: Try adding “gem minitest-bisect” to your Gemfile and running “bundle install”

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Copyright © Ryan Davis, seattle.rb

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