bun:test: fixtures and around hooks wait on what await waits on, through the engine - #47800

Order and scope

• The stack is bun:test: do not call then() on what a callback with a done parameter returns #47730, bun:test: call then() of a returned thenable before the microtask drain, and once for a fixture #47760, this PR, bun:test: wait on a Promise subclass through its own then() #41008. The base branch of this PR is the branch of bun:test: call then() of a returned thenable before the microtask drain, and once for a fixture #47760, so the diff here is this step only.
• This PR does not need the decision that bun:test: wait on a Promise subclass through its own then() #41008 asks for. JSPromise::promise_resolve moved here from bun:test: wait on a Promise subclass through its own then() #41008.
• No issue is linked. A review of bun:test: call then() of a returned thenable before the microtask drain, and once for a fixture #47760 and bun:test: wait on a Promise subclass through its own then() #41008 raised the then read at the fixture site twice. test.extend and the around hooks are in no release.
• bun:test: call then() of a returned thenable before the microtask drain, and once for a fixture #47760 left the fixture site on the then read. With it, a builder fixture whose function returns a thenable gets the thenable itself while Promise.prototype.then is not callable. This PR repairs that.

Behaviour changes

Each row was run on release builds of main (3f3a9ff), of #47760 (55b02d8) and of this branch. "Function" means a fixture function or an around hook.

Not changed by this step: a thenable from a fixture function gets one then() call, one turn later (#47760). A Promise subclass with its own then() is awaited through that then() at both sites, as on main.

The route of an error

• The old attach called then(), which made a derived promise. An error that left a reaction rejected that promise, and nothing handled it.
• then_with_value makes no derived promise. The error is thrown out of the reaction job. It is reported when it is thrown, and the entry that runs then gets it.
• What reaches this: a synchronous throw of an onCleanup callback when teardown runs at once. That is when the fixture function settles after its test ended, or after its file ended for a file scope. The other errors that can leave the two fixture reactions take the same route. The reactions of an around hook call no user code.
• With only a process.on("unhandledRejection") listener, main gives that error to the listener and the running test passes. This PR fails the running test with it. With only an uncaughtException listener it is the reverse.
• The new test pins the event with both listeners. A better report (one that blames no test) belongs in provide(). It is not part of this step.

Not changed here

• vi.waitFor, vi.waitUntil and the callback of expect.poll (src/runtime/test_runner/vi_wait.rs:324) keep the then read and the direct call. Vitest 5.0.3 makes the same call in waitFor and waitUntil (thenable.then(...) in its dist). A then that is replaced or not callable still reaches these three. After this PR it is the only read of then in src/runtime/test_runner.
• The rule of the runner for a value that a test or a hook returns. That is bun:test: wait on a Promise subclass through its own then() #41008.
• An around hook still ignores the value that its promise resolves with.

Implementation

• A value that await does not wait on is passed on, as before. awaited_promise returns None for a value that is not an object. It also returns None when PromiseResolve made a new promise that is fulfilled at once: the value has no callable then. Otherwise it returns the promise: the value itself when its constructor is the Promise of the realm, or a new promise that follows then().
• The reactions take (value, wrapper). The four entries in test_context.bind.ts no longer pass this.
• The fixture site caches the promise that it waits on. The teardown entry gives it to the runner, which calls no then() on a native promise.
• JSPromise::promise_resolve wraps the existing binding of JSC::JSPromise::resolvedPromise. VirtualMachine.rs had the one private declaration of it, and now calls the wrapper. There is no new C++.

Measurements

Release builds of #47760 and of this branch from one build directory, linux-x64, JIT off. Method: a gdb single-step from the entry of the function to its return, at the fifth call.

• Instructions inside Bun__ActiveFixture__setUp, the call of the fixture function included: async fixture function 10622 to 8516. A function that returns a thenable: about 10800 to about 8600. That pair moves by up to 70 between runs.
• Instructions inside Bun__AroundHook__setUp, the call of the hook included: async hook 9392 to 7320, a hook that returns a thenable 8468 to 7686.
• Each site no longer makes two bound functions and the promise that then() derives.
• size text: 140288367 to 140287343, -1024 B. run_test_callback is 2489 B on both. New JSPromise::promise_resolve: 109 B.
• perf and valgrind are not in the container, so there is no cache or branch number.

Tests

All in test/js/bun/test/vitest-flavor.test.ts:

• "the promise of a fixture function is awaited without a then() call": Promise.prototype.then is replaced, or not callable, while the runner attaches.
• "the promise of a hook is awaited without a then() call": the same for aroundEach and aroundAll.
• "a promise whose own then is not callable is awaited or passed on, as await does".
• "a fixture function that settles after its test has ended runs its cleanup right away": both process listeners log the event.
• A new block in "a hook that returns a thenable, or doesn't wait for runTest()": the turn of then(), and a second thenable.
• Debug build: vitest-flavor.test.ts 201 pass. bun_test.test.ts, done-async.test.ts, concurrent.test.ts, jest-hooks.test.ts, test-error-code-done-callback.test.ts and expect-deferred.test.ts: 98 pass. node-module-module.test.js: 80 pass. The new paths are clean with BUN_JSC_validateExceptionChecks=1.
• Release build: test/js/bun/test/ 4196 pass, 1 fail. The failure is "error snapshots" in snapshot.test.ts, which expects a TTY and fails on the release build of main too. The release build of bun:test: call then() of a returned thenable before the microtask drain, and once for a fixture #47760 fails the 5 tests above.
• Reference runs with Vitest 5.0.3: the turn of then() in aroundEach and aroundAll, the second thenable, and the two promises with no callable then give the results of this PR.

Other open PRs in this code

• bun test: hold the event loop across a test callback's drain and inside fake timer controls #47726 conflicts with bun:test: call then() of a returned thenable before the microtask drain, and once for a fixture #47760 in src/jsc/event_loop.rs. This step adds no conflict there.
• bun test: follow-ups to Vitest compatibility #47727 and bun test: do not charge a test or hook for a promise rejected before it started #47731 merge with this step without a conflict.

Self-review

20 concerns raised, 16 addressed:

• One helper serves both sites, so the fixture site and the around hooks do not differ.
• A value with no callable then keeps the result of await: a test pins both cases.
• The route of an error that leaves a reaction is stated and pinned.
• vi.waitFor is named with its reason.
• The change is its own PR below bun:test: wait on a Promise subclass through its own then() #41008, so it does not wait for the decision there.

Not taken:

• Read a Promise subclass with no callable then by its state. await and Vitest 5.0.3 pass it on.
• Keep main's unhandledRejection for a cleanup that throws, with a catch in the reaction. The thrown error now goes where an error thrown from a job goes.
• A PR on main that also takes the fixture lines of bun:test: call then() of a returned thenable before the microtask drain, and once for a fixture #47760. This PR replaces those lines one step later.
• The change as part of bun:test: wait on a Promise subclass through its own then() #41008.