bun test: wait for done() when the callback also returns a fulfilled promise

[robobun]

Problem

• A test that takes a done parameter and returns an already-fulfilled promise passes at once. done() is never awaited. test("x", done => Promise.resolve(1)) prints (pass). A done(err) that arrives later is blamed on whichever test is running then: with test("late", done => Promise.resolve().then(() => setTimeout(() => done(new Error("boom")), 30))) the next test in the file fails and late passes.
• Cause: BunTest::run_test_callback (src/runtime/test_runner/bun_test.rs:1242). The Fulfilled arm returned Some(cfg_data) before the tail check at line 1260 that waits for the done callback. Synchronous concurrent test fix #22928 (0ea4ce1) added that early return. Before it every returned promise went through .then() with the shared ref, so a fulfilled promise waited for done().

Fix

• Make the Fulfilled arm fall through to the existing tail check: if dcb_ref.is_some() { return None }. The done callback holds the only ref and adds the result when it is called, the same path a sync test body with a later done() takes.
• The Rejected arm is unchanged and now says so: it fails fast like bun_test_catch. A late done(err) after a rejection is an attribution problem in bun_test_done_callback, which bun test: fail the test or hook whose done() received an error #39112 fixes.
• Behaviour change: a test that takes done, returns a fulfilled promise, and never calls done() goes from an instant pass to the existing failure timed out ..., before its done callback was called. If a done callback was not intended, remove the last parameter from the test callback function. Jest rejects a test that takes done and returns a value. node's test runner fails it. Bun's documented shape (test/js/bun/test/bun_test.fixture.ts, "done combined with promise") is to wait for both.
• Verified: test/js/bun/test/done-async.test.ts, a new describe.each over serial and --concurrent. Both fail on main (the two broken tests print (pass), innocent prints (fail)). Also ran bun_test.test.ts, test-error-code-done-callback.test.ts, test-failing.test.ts, jest-hooks.test.ts, concurrent.test.ts, and test/js/node/test_runner/node-test.test.ts.

Background

• RefData is the runner's handle for "this test is still running". run_test_callback creates one when the callback takes done and returns a value. bun_test_done_callback and bun_test_then_or_catch each check has_one_ref() and only the last holder adds the result.
• Under --concurrent a late done(err) is still printed as Unhandled error between tests instead of failing its own test. That is the attribution bug bun test: fail the test or hook whose done() received an error #39112 fixes. The test here asserts only the exit code and the error text for that mode.

Notes

Repro on main:

import { test, expect } from "bun:test";
test("never_calls_done", done => { return Promise.resolve(1); });
test("late_done_err", done => {
  return Promise.resolve().then(() => {
    setTimeout(() => { try { expect(1).toBe(2); done(); } catch (e) { done(e); } }, 30);
  });
});
test("innocent", async () => { await new Promise(r => setTimeout(r, 100)); });

main: (pass) never_calls_done, (pass) late_done_err, (fail) innocent. With this branch: never_calls_done times out before its done callback, late_done_err fails with the expect error, innocent passes.

Self-reviewed: 3 concerns raised, 3 addressed (fall through to the tail check instead of a second guard, leave Rejected fail-fast with a comment, cite #22928 and #39112).

[coderabbitai]

Walkthrough

The test runner now waits for pending done() callbacks after fulfilled promises while preserving fail-fast behavior for rejected promises. Parameterized tests cover serial and concurrent execution, timeout handling, late completion, failures, diagnostics, and exit codes.

Changes

Async done callback handling

Layer / File(s)	Summary
Runner completion behavior src/runtime/test_runner/bun_test.rs	Fulfilled promises no longer return immediately when done() remains pending. Rejected promises still fail without waiting for done().
Serial and concurrent regression coverage test/js/bun/test/done-async.test.ts	Parameterized tests cover timeout, late success, late failure, result attribution, stderr diagnostics, and exit codes.

Suggested reviewers: jarred-sumner, alii, dylan-conway

Merge Risk: 🔵 Low · up to bac71

The test-runner fix makes fulfilled promise-returning callback tests wait for done(), but its regression fixture may run a binary that does not include the change. Updating the fixture to use the debug build will ensure the timeout and failure-attribution checks cover the modified runner.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise, specific, and accurately describes the primary change: waiting for done() when a test callback returns a fulfilled promise.
Description check	✅ Passed	The description is complete and directly addresses the problem, cause, fix, behavior change, verification steps, and known concurrent-mode limitation. It does not use the template headings exactly, bu…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[robobun]

Status: reproduced on main with the file in the Notes block (never_calls_done and late_done_err print (pass), innocent fails). With this branch never_calls_done times out before its done callback, late_done_err fails, innocent passes. Tests: test/js/bun/test/done-async.test.ts, serial and --concurrent.

CI (build 111252): the new test passes on every lane. The red lanes are verify-baseline (static scan hit in llint_op_wide16, also red on main at the base commit) and test-crypto-dh-leak.js on x64-asan (pre-existing on main). The rest are retried flakes. Nothing in this diff touches them. Ready for review.

[robobun]

^{Updated 8:34 AM PT - Sep 6th, 2026}

❌ @robobun, your commit bac7114 has 2 failures in Build #111252 (All Failures):

• test/js/node/test/parallel/test-crypto-dh-leak.js - code 1 on 🐧 13 x64-asan
• ❌ CPU instruction violation on Linux x64 — 1 check(s) failed

The baseline build contains instructions not available on Nehalem (SSE4.2, no AVX/AVX2/AVX512).

• Static instruction scan

Static scan violations

llint_op_wide16  [INVLPGB]  (1 insns)
If these are runtime-dispatched behind a CPUID gate: add each symbol to
scripts/verify-baseline-static/allowlist-x64.txt with a comment pointing at the gate.
If there's no gate: this is a real bug — a -march leaked into a subbuild.
❌ CPU instruction violation on Linux x64 — 1 check(s) failed
The baseline build contains instructions not available on Nehalem (SSE4.2, no AVX/AVX2/AVX512).
Static instruction scan
Static scan violations
llint_op_wide16  [INVLPGB]  (1 insns)
If these are runtime-dispatched behind a CPUID gate: add each symbol to
scripts/verify-baseline-static/allowlist-x64.txt with a comment pointing at the gate.
If there's no gate: this is a real bug — a -march leaked into a subbuild.

---

🧪   To try this PR locally:

bunx bun-pr 41613

That installs a local version of the PR into your bun-41613 executable, so you can run:

bun-41613 --bun

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@test/js/bun/test/done-async.test.ts`:
- Line 60: Update the command constructed in the test fixture to invoke the test
runner through “bun bd test” instead of “bun test”, while preserving the
existing mode arguments and “done.test.ts” fixture target.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between d316760 and bac7114.

📒 Files selected for processing (2)

• src/runtime/test_runner/bun_test.rs
• test/js/bun/test/done-async.test.ts

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[claude]

LGTM — the follow-up commit switched stdout to "ignore", which addresses the pipe-drain nit from the earlier pass.

What was reviewed:

• The Fulfilled arm now falls through to the existing if dcb_ref.is_some() { return None } tail check, so no new state handling is introduced — the sync-body-with-done path already owns this case.
• Confirmed Rejected still early-returns (fail-fast unchanged) and Pending is untouched.
• Test uses tempDir/bunExe/bunEnv, drains piped streams via Promise.all, asserts stderr before exit code, and covers both serial and --concurrent.

Extended reasoning...

Overview

The PR changes one control-flow line in src/runtime/test_runner/bun_test.rs: the PromiseStatus::Fulfilled arm no longer early-returns Some(cfg_data), so execution falls through to the pre-existing if dcb_ref.is_some() { return None } check at line 1261. This means a test that both takes a done callback and returns an already-fulfilled promise now waits for done() instead of passing immediately. The Rejected arm keeps its early return with a one-line comment noting parity with bun_test_catch. A new describe.each block in test/js/bun/test/done-async.test.ts spawns a fixture exercising four cases (never-calls-done → timeout, late done(err) → fails its own test, late done() → pass, innocent bystander → pass) under both serial and --concurrent modes.

Security risks

None. This is test-runner control flow with no parsing of untrusted input, no auth/crypto/permissions surface, and no network or filesystem effects beyond the harness tempDir.

Level of scrutiny

Low-to-moderate. The Rust diff is a single deleted return that reuses an existing, already-exercised code path (the same tail check that handles a sync test body with a pending done). No new allocations, refcounts, or JSC calls are introduced. The behavior change (never-called done now times out instead of passing) is intentional, documented in the PR body, and matches Jest/Node semantics as well as Bun's own bun_test.fixture.ts "done combined with promise" contract. No CODEOWNERS entry covers src/runtime/test_runner/.

Other factors

The one prior inline comment (undrained stdout: "pipe") was addressed in bac7114 by switching to stdout: "ignore". The new test follows harness conventions: tempDir with using, bunExe()/bunEnv, Promise.all over the piped stream and exited, stderr asserted before exit code, describe.each for the serial/concurrent matrix, and an explicit note that --concurrent late-done(err) attribution is tracked separately. The 50ms timeout is a per-test argument inside the fixture (testing timeout behavior itself), not a harness setTimeout wait. No other reviewer has posted CHANGES_REQUESTED.