fix(jest-core): always use workers in watch mode (jestjs#14059)

Author: benjaminjkraft

CHANGELOG.md

@@ -7,6 +7,7 @@
 ### Fixes
 
 - `[jest-config]` Handle frozen config object ([#14054](https://github.com/facebook/jest/pull/14054))
+- `[jest-core]` Always use workers in watch mode to avoid crashes ([#14059](https://github.com/facebook/jest/pull/14059)).
 - `[jest-environment-jsdom, jest-environment-node]` Fix assignment of `customExportConditions` via `testEnvironmentOptions` when custom env subclass defines a default value ([#13989](https://github.com/facebook/jest/pull/13989))
 - `[jest-matcher-utils]` Fix copying value of inherited getters ([#14007](https://github.com/facebook/jest/pull/14007))
 - `[jest-mock]` Tweak typings to allow `jest.replaceProperty()` replace methods ([#14008](https://github.com/facebook/jest/pull/14008))

packages/jest-core/src/__tests__/testSchedulerHelper.test.js

@@ -24,8 +24,8 @@ const getTestsMock = () => [getTestMock(), getTestMock()];
 
 test.each`
   tests              | timings        | detectOpenHandles | maxWorkers   | watch    | workerIdleMemoryLimit | expectedResult
-  ${[getTestMock()]} | ${[500, 500]}  | ${false}          | ${undefined} | ${true}  | ${undefined}          | ${true}
-  ${getTestsMock()}  | ${[2000, 500]} | ${false}          | ${1}         | ${true}  | ${undefined}          | ${true}
+  ${[getTestMock()]} | ${[500, 500]}  | ${false}          | ${undefined} | ${true}  | ${undefined}          | ${false}
+  ${getTestsMock()}  | ${[2000, 500]} | ${false}          | ${1}         | ${true}  | ${undefined}          | ${false}
   ${getTestsMock()}  | ${[2000, 500]} | ${false}          | ${2}         | ${true}  | ${undefined}          | ${false}
   ${[getTestMock()]} | ${[2000, 500]} | ${false}          | ${undefined} | ${true}  | ${undefined}          | ${false}
   ${getTestMock()}   | ${[500, 500]}  | ${false}          | ${undefined} | ${true}  | ${undefined}          | ${false}
@@ -36,8 +36,8 @@ test.each`
   ${new Array(45)}   | ${[500]}       | ${false}          | ${undefined} | ${false} | ${undefined}          | ${false}
   ${getTestsMock()}  | ${[2000, 500]} | ${false}          | ${undefined} | ${false} | ${undefined}          | ${false}
   ${getTestsMock()}  | ${[2000, 500]} | ${true}           | ${undefined} | ${false} | ${undefined}          | ${true}
-  ${[getTestMock()]} | ${[500, 500]}  | ${false}          | ${undefined} | ${true}  | ${'500MB'}            | ${true}
-  ${getTestsMock()}  | ${[2000, 500]} | ${false}          | ${1}         | ${true}  | ${'500MB'}            | ${true}
+  ${[getTestMock()]} | ${[500, 500]}  | ${false}          | ${undefined} | ${true}  | ${'500MB'}            | ${false}
+  ${getTestsMock()}  | ${[2000, 500]} | ${false}          | ${1}         | ${true}  | ${'500MB'}            | ${false}
   ${getTestsMock()}  | ${[2000, 500]} | ${false}          | ${1}         | ${false} | ${'500MB'}            | ${false}
   ${[getTestMock()]} | ${[2000]}      | ${false}          | ${undefined} | ${false} | ${'500MB'}            | ${false}
   ${getTestsMock()}  | ${[500, 500]}  | ${false}          | ${undefined} | ${false} | ${'500MB'}            | ${false}

packages/jest-core/src/testSchedulerHelper.ts

@@ -27,25 +27,26 @@ export function shouldRunInBand(
   }
 
   /*
-   * Run in band if we only have one test or one worker available, unless we
-   * are using the watch mode, in which case the TTY has to be responsive and
-   * we cannot schedule anything in the main thread. Same logic applies to
-   * watchAll.
+   * If we are using watch/watchAll mode, don't schedule anything in the main
+   * thread to keep the TTY responsive and to prevent watch mode crashes caused
+   * by leaks (improper test teardown).
+   */
+  if (watch || watchAll) {
+    return false;
+  }
+
+  /*
+   * Otherwise, run in band if we only have one test or one worker available.
    * Also, if we are confident from previous runs that the tests will finish
    * quickly we also run in band to reduce the overhead of spawning workers.
    * Finally, the user can provide the runInBand argument in the CLI to
-   * force running in band.
-   * https://github.com/facebook/jest/blob/700e0dadb85f5dc8ff5dac6c7e98956690049734/packages/jest-config/src/getMaxWorkers.js#L14-L17
+   * force running in band, which sets maxWorkers to 1 here:
+   * https://github.com/facebook/jest/blob/d106643a8ee0ffa9c2f11c6bb2d12094e99135aa/packages/jest-config/src/getMaxWorkers.ts#L27-L28
    */
-  const isWatchMode = watch || watchAll;
   const areFastTests = timings.every(timing => timing < SLOW_TEST_TIME);
   const oneWorkerOrLess = maxWorkers <= 1;
   const oneTestOrLess = tests.length <= 1;
 
-  if (isWatchMode) {
-    return oneWorkerOrLess || (oneTestOrLess && areFastTests);
-  }
-
   return (
     // When specifying a memory limit, workers should be used
     !workerIdleMemoryLimit &&