event loop: a fatal error ends the run even while immediates are queued

[robobun]

Problem

• After bun reports a fatal error (an uncaught exception or an unhandled rejection with no listener for it), a program that has a setImmediate queued keeps running. With a self-requeueing immediate it never exits: intervals keep firing, I/O keeps being served, and the error that was printed has no further effect. Node exits 1 from the error itself.

  setImmediate(function again() { setImmediate(again); });
  setInterval(() => console.log("tick"), 200);
  setTimeout(() => { throw new Error("fatal"); }, 100);

  bun x.js prints error: fatal and then prints tick forever (1.4.0 and main); node x.js exits 1 right after the error. The same happens with Promise.reject(...) in place of the throw, and with an unhandled rejection raised by a beforeExit listener that also queues an immediate.
• Cause: VirtualMachine::is_event_loop_alive() (src/jsc/VirtualMachine.rs) is is_event_loop_alive_excluding_immediates() || immediates queued, and only the first operand contains the unhandled_error_counter == 0 check. Every run loop (Run::start in src/runtime/cli/run_command.rs, on_before_exit(), WebWorker::spin) is while vm.is_event_loop_alive() { tick(); auto_tick_active(); }, so once an error has been counted, queued immediates alone keep the loop turning, and each turn also polls I/O and fires due timers. The immediate terms were split out of the gated expression in node:timers fixes #16855 (they were inside it before); the split was for unref'd immediates and moving them out of the counter gate was a side effect.

Fix

• New predicate has_fatal_unhandled_error() (unhandled_error_counter > 0 && !is_watcher_enabled()); is_event_loop_alive() returns false when it holds, before looking at anything queued. The rest of the predicate is unchanged.
• Why this is right: a counted error already means "this run is over" everywhere else in bun: it is what makes the same loops stop for timers and I/O, on_before_exit() skips beforeExit because of it (process: skip 'beforeExit' after a fatal uncaught exception #34639), and the exit code is 1. Immediates were the one kind of pending work exempt from it, and that exemption is what node never has: after a fatal error nothing else runs.
• Why the watcher exemption: --hot and --watch print the error and keep the process alive until the next reload, but they count the error too. Today the immediate terms are what still runs a hot-reloaded program's immediates after an error (timers already stop there, which hot/watch: keep driving timers and surviving errors after an unhandled error #38206 fixes by not counting errors in watcher mode). Without the exemption this change would have taken immediates away from those modes as well; with it they behave exactly as before, and once hot/watch: keep driving timers and surviving errors after an unhandled error #38206 lands the counter is never nonzero in watcher mode, so the is_watcher_enabled() half of the new predicate becomes inert and can be dropped.
• Other callers of the predicate: bun repl -e / bun repl -p (the one caller in repl.rs, a one-shot eval followed by a drain) had the same hang and now exits 1 like bun -e; the interactive REPL does not use it. Not affected: bun test (its counter is a tally, and its drive loop does not consult this predicate; the --watch idle loop has a watcher installed), workers, whose error hook already requests termination, so their loop exits either way (checked: a worker with the same requeueing immediate exits on both builds), and the inspector thread's own VM (Debugger.rs), whose script (src/js/internal/debugger.ts) schedules no immediates or timers, so the immediate terms never decide anything there.
• Still out of scope: the rest of the loop turn in which the error was reported still runs (remaining immediates of the same batch, due timers of the same auto_tick_active() pass; e.g. setTimeout(() => { Promise.reject(e); setImmediate(cb); }) still runs cb once because the rejection is only reported at the top of the next turn). That is bounded and pre-existing for timers and tasks as well; a separate change reports rejections left by immediates before the poll and returns early from auto_tick_active() once the run is over, and composes with this one.
• Tests, test/js/node/process/process.test.js, new block "fatal error while immediates are pending": an uncaught exception, an unhandled rejection, and an unhandled rejection from a beforeExit listener, each with a requeueing immediate that exits 42 if it is still being run after the error. All three fail on the current release (immediates still running after the error, exit 42) and pass with this change (exit 1, nothing after the error). A fourth test pins that --hot and --watch still run the immediates after the error; it passes before and after.
• Also run on this build: the rest of process.test.js (the only failures are process.env.USER being unset in this container and three tests that time out under the whole-file concurrency of a debug build but pass alone), test/cli/hot/hot.test.ts, test/cli/hot/watch.test.ts, test/cli/watch/watch.test.ts, test/js/node/timers/node-timers.test.ts, test/js/node/worker_threads/worker_threads.test.ts, test/js/web/workers/worker.test.ts (three failures that are this debug build's speed, not errors: two pass with a longer timeout, the third asserts within a 30ms window while a worker takes about 145ms to start here), test/js/bun/spawn/exit-code.test.ts, bun-serve-propagate-errors, serve-reused-response, test/js/bun/test/bun_test.test.ts, test/cli/test/test-changed.test.ts, and the 43 node test-promise*, test-promises-*, test-process-exit*, test-process-beforeexit*, test-timers-immediate*, test-timers-unref*, test-timers-uncaught-exception, test-worker-exit*, test-worker-uncaught* files, all passing.

Background

• unhandled_error_counter: a field on the VM, incremented in uncaught_exception() and unhandled_rejection() when no uncaughtException/unhandledRejection listener took the error (the error is printed at the same time). For bun run it is the fatal-error state: the run loops stop when it is nonzero, on_before_exit() skips beforeExit, and the process exits 1. bun test increments it on separate branches and only compares it before and after a test to attribute errors; --hot/--watch increment it too but their run loop never returns.
• Immediates and liveness: setImmediate callbacks sit in the event loop's immediate_tasks/next_immediate_tasks lists and are run at the start of auto_tick_active(), before the I/O poll. A ref'd immediate also holds a ref on the I/O loop, but an unref'd one does not, so is_event_loop_alive() checks the lists themselves in addition to is_event_loop_alive_excluding_immediates() (refs, queued tasks, in-flight work), which run_immediate_task uses to decide whether an unref'd immediate should run at all. This change does not touch that function.
• The run loop: tick() runs queued tasks and microtasks; auto_tick_active() runs immediates, polls I/O for as long as the timer heap allows (not at all while immediates are queued), then fires due timers. The condition is only re-checked between turns, which is why the turn that reports the error still completes.

Probes (release 1.4.0 vs this build, linux x64)

$ cat x.js
let n = 0;
function loop() { n++; setImmediate(loop); }
loop();
setInterval(() => console.log("interval tick, immediates so far:", n), 200);
setTimeout(() => { throw new Error("fatal"); }, 100);

$ timeout 2 bun x.js            # 1.4.0: error: fatal, then 9 interval lines, killed (124)
$ timeout 2 bun-debug x.js      # this build: error: fatal, exit 1
$ node x.js                     # exit 1 right after the error

# same with Promise.reject(new Error("fatal rejection")) in the timer: 124 -> 1

# worker_threads variant (same script inside a Worker): both builds report the
# error to the parent and the worker exits; the worker's error hook requests
# termination, which spin() checks separately from is_event_loop_alive().

# bun repl -e, the caller in repl.rs:
$ timeout 5 bun repl -e 'setImmediate(function again(){ setImmediate(again) }); setTimeout(() => console.log("kept running"), 300); setTimeout(() => { throw new Error("boom") }, 1)'
# 1.4.0: error: boom, kept running, killed (124);  this build: error: boom, exit 1

# remaining same-turn tail, unchanged by this PR:
$ cat batch.js
setImmediate(() => { throw new Error("boom"); });
setImmediate(() => console.log("same batch"));
setImmediate(() => setImmediate(() => console.log("next batch")));
setTimeout(() => console.log("timer"), 0);
$ bun batch.js        # 1.4.0:      same batch, timer, next batch; exit 1
$ bun-debug batch.js  # this build: same batch, timer; exit 1
$ node batch.js       # timer (fires before the check phase); exit 1

[robobun]

Status: closed, superseded by #34661 (see the closing comment below).

Reproduced on 1.4.0 and on main (032b8dbf) with a self-requeueing setImmediate plus a fatal throw, a fatal rejection, and a fatal rejection from a beforeExit listener: the error is printed and the process keeps running until killed. This branch fixed it by gating is_event_loop_alive() on the counted error; #34661 fixes the whole class by exiting from the fatal path itself (#35313), so this gate is not needed. Tests for the immediates shape are in this branch's test/js/node/process/process.test.js and were offered to #34661.

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📒 Files selected for processing (2)

• src/jsc/VirtualMachine.rs
• test/js/node/process/process.test.js

Walkthrough

Changes

Event-loop lifecycle

Layer / File(s)	Summary
VM liveness check src/jsc/VirtualMachine.rs	Non-watcher VMs now stop when unhandled errors exist before pending immediate tasks are checked. Watcher modes continue evaluating activity.
Fatal error and watcher coverage test/js/node/process/process.test.js	Added tests for uncaught exceptions, unhandled rejections, beforeExit rejections, and --hot or --watch behavior with requeueing immediates.

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

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly summarizes the main change: fatal errors now stop the event loop even when immediates remain queued.
Description check	✅ Passed	The description explains the problem, fix, scope, watcher behavior, tests, and verification results, although it does not use the template headings exactly.

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[claude]

I reviewed this and didn't find any bugs. Because is_event_loop_alive() is the core liveness predicate consulted by ~10 run loops (run_command, on_before_exit, workers, bun test --watch, REPL, debugger), a maintainer look would still be worthwhile.

What was reviewed:

• Traced the new early-return against every caller of is_event_loop_alive(); the watcher exemption covers the --hot/--watch and bun test --watch loops, and workers already terminate via their error hook.
• Checked that is_event_loop_alive_excluding_immediates() (used by run_immediate_task for unref'd immediates) is unchanged.
• The repl.rs:1540 and Debugger.rs:521 drain loops aren't mentioned in the PR body — with a counted error they now fall through to the next statement / tick_possibly_forever() respectively, which reads harmless but wasn't explicitly probed.

Extended reasoning...

Overview

A 7-line early return in VirtualMachine::is_event_loop_alive() (src/jsc/VirtualMachine.rs:1205): once unhandled_error_counter > 0 and no watcher is installed, the predicate is false regardless of pending immediates. Four new subprocess tests in test/js/node/process/process.test.js cover uncaught exception, unhandled rejection, unhandled rejection from beforeExit, and a --hot/--watch guard that pins the exemption.

Security risks

None. This only tightens when the run loop stops after an already-printed fatal error; no new inputs are parsed and no privilege boundary is crossed.

Level of scrutiny

High. is_event_loop_alive() is the shared liveness check for every top-level run loop in bun: Run::start (both watcher and non-watcher arms), on_before_exit()'s inner drain, WebWorker::spin, wait_for_worker_entry_evaluation, run_event_loop_for_watch (bun test --watch), the REPL per-statement drain, and the debugger's idle loop. Changing it changes when every one of those exits. The PR body is unusually thorough — it enumerates the run_command / on_before_exit / worker / bun-test paths, explains why the watcher exemption preserves --hot/--watch behavior pending #38206, and lists the test suites re-run — but the REPL (src/runtime/cli/repl.rs:1540) and debugger (src/jsc/Debugger.rs:521) callers aren't discussed. In both cases the fall-through looks benign (REPL prints the result and prompts again; debugger drops to tick_possibly_forever()), but a maintainer who owns the event loop should confirm.

Other factors

The tests follow harness conventions (subprocess spawns with bunEnv, concurrent Promise.all pipe reads, tempDir for the watcher case, forEachLine to await the observable line rather than sleeping, exit-42 self-limiter so a regression fails fast instead of timing out). is_event_loop_alive_excluding_immediates() — the operand run_immediate_task uses to gate unref'd immediates — is untouched, so #16855's unref'd-immediate semantics are preserved. The interaction with #38206 (watcher mode stops counting errors, making the exemption inert) is a design coupling a human should acknowledge.

[robobun]

Probed the two callers the review called out:

• repl.rs: that drain loop belongs to eval_script, which only the non-interactive bun repl -e / bun repl -p path calls (one-shot eval, drain, exit); the interactive loop does not use the predicate. It had the same hang and this change fixes it the same way: bun repl -e 'setImmediate(function again(){ setImmediate(again) }); setTimeout(() => console.log("kept running"), 300); setTimeout(() => { throw new Error("boom") }, 1)' prints kept running and has to be killed on 1.4.0; on this branch it exits 1 right after error: boom.
• Debugger.rs: that loop runs the inspector thread's own VM, and its script (src/js/internal/debugger.ts) schedules no immediates or timers, so the immediate terms never decide anything there; after a counted error it falls through to tick_possibly_forever() exactly as it does today.

Added both to the PR body.

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[robobun]

Closing: this is already covered, at a better level, by #34661.

• The bug is one instance of "user code still runs after a fatal error" (Fatal uncaught exception runs already-queued fs callbacks before exiting (node exits immediately) #35313, closed by @cirospaciari with "fixing directly in domain: route fs callback throws to uncaughtException, unblocking 4 tests (domain 88%→96%) #34661"). domain: route fs callback throws to uncaughtException, unblocking 4 tests (domain 88%→96%) #34661's VirtualMachine.rs change makes the fatal path itself exit: uncaught_exception_fatal calls process_exit(1) on the main thread outside --hot/--watch, and both report_unhandled_error (timer and immediate throws) and the default unhandled-rejection tail are routed through it. All three scenarios here exit before the next immediate batch under that change, without any liveness gate, and so does the same-turn tail this PR listed as out of scope.
• domain: route fs callback throws to uncaughtException, unblocking 4 tests (domain 88%→96%) #34661 also removes the unhandled_error_counter == 0 check from is_event_loop_alive_excluding_immediates(); this PR adds a second one next to it, so landing it would extend the model that PR is deleting and then conflict with it.
• The tests are the reusable part; I have pointed to them from domain: route fs callback throws to uncaughtException, unblocking 4 tests (domain 88%→96%) #34661. The branch stays up in case domain: route fs callback throws to uncaughtException, unblocking 4 tests (domain 88%→96%) #34661 does not land and a stopgap is wanted after all. event loop: report immediates' rejections before polling, and stop polling once a fatal error ended the run #38524 is the neighbouring case (the turn that reports the error) and is affected by domain: route fs callback throws to uncaughtException, unblocking 4 tests (domain 88%→96%) #34661 in the same way.