Test durations are inaccurate in for certain concurrent tests

[kachkaev]

Do you want to request a feature or report a bug? bug

What is the current behavior?

test.concurrent does not produce correct test durations. See MVP in https://repl.it/@kachkaev/concurrent-test-durations-in-jest

What is the expected behavior?

  works concurrently - 0.1 seconds per test
    ✓ concurrent #0 (100ms) <-- ok
    ✓ concurrent #1 (100ms) <-- currently incorrect (≈0-2ms)
    ✓ concurrent #2 (100ms) <-- currently incorrect (≈0-2ms)
    ✓ concurrent #3 (100ms) <-- currently incorrect (≈0-2ms)
    ✓ concurrent #4 (100ms) <-- currently incorrect (≈0-2ms)
  sequential tests - 0.2 seconds per test
    ✓ sequential #0 (200ms) <-- ok
    ✓ sequential #1 (200ms) <-- ok
    ✓ sequential #2 (200ms) <-- ok
    ✓ sequential #3 (200ms) <-- ok
    ✓ sequential #4 (200ms) <-- ok
  concurrent tests - 2 seconds per test
    ✓ concurrent again #0 (2000ms) <-- currently incorrect (≈1000ms), see footnote
    ✓ concurrent again #1 (2000ms) <-- currently incorrect (≈0-2ms)
    ✓ concurrent again #2 (2000ms) <-- currently incorrect (≈0-2ms)
    ✓ concurrent again #3 (2000ms) <-- currently incorrect (≈0-2ms)
    ✓ concurrent again #4 (2000ms) <-- currently incorrect (≈0-2ms)

---
1000ms = expected 2000ms - time to run previous sequential tests (5 * 100ms = 1000ms)

Looks like the a variable that stores start time is being reused by other tests and this causes incorrect durations.

[SimenB]

test.concurrent is still somewhat experimental (the reason it's not documented), but a PR fixing it would be appreciated 🙂

[AvremelM]

It seems this is now working correctly, based on running the repl.it.

[ghost]

Seemingly most of the test events emitted by jest-circus are wrong for concurrent tests, as internally even though the original test is run concurrently, all events are fired and timing generated as if the tests run sequentially. Ref: https://github.com/facebook/jest/blob/master/packages/jest-circus/src/legacy-code-todo-rewrite/jestAdapterInit.ts#L89

Looks like the implementation of concurrency at least for the moment looks hackish, unfortunately (please don't take offense, it makes sense as it is still experimental). I guess it would require a bigger refactor within jest-circus to support concurrency properly, though I am not sure as my knowledge about the codebase is very slim. I Will have a look at this over the next few days

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