[robobun]

Problem

• When an Error's message getter throws, the exception stays pending after the error prints. Since 1.4.1, release bun test skips every later hook and body and prints (pass). With retry: 2 pass 0 fail, exit 0. bun run dies with panic(main thread): abort() called.
• An asserts build aborts in the print: ASSERTION FAILED: Unexpected exception observed, top frame Bun__JSObject__getCodePropertyVMInquiry.
• fromErrorInstance (src/jsc/bindings/ZigException.cpp:499) reads message with a real [[Get]]. Since Add jsc-exception-lint and fix the missing exception checks it finds #40410 a RETURN_IF_EXCEPTION returns with the exception still pending.

Fix

• Check the scope after the read. If the getter threw (not a termination), clear the exception, drop the value, and use ErrorInstance::sanitizedMessageString as for an absent message.
• Correct because fromErrorInstance runs under a TopExceptionScope, which swallows exceptions. The stack read below already does this. The error prints as Error: plus its frames.
• Verified: test/js/bun/util/inspect-error.test.js (13 new tests). Also ran the reportError, inspect, stack and retry suites.

Background

• fromErrorInstance fills the ZigException struct that the Rust error printer renders. Every printer entry point reaches it.
• A VMInquiry read never runs JS, so JSC asserts that no exception is pending.
• EventLoop::enter_js calls no callback while an exception is pending. bun test reads that as a test that returned.
• Considered clearing after the print in each caller: the reads inside the print still assert.

Downsides

• The getter's own error is not shown: the output is Error: with an empty message.
• No cost for errors that print today: one scope.exception() check replaces another. CI binary size: ±0.0 KB on all 12 targets.

Notes

Frames, for search. Asserts build, direct sinks: ExceptionScope::assertNoExceptionExceptTermination (ExceptionScope.h:63) under Bun__JSObject__getCodePropertyVMInquiry, called from print_error_instance_body. Release build, bun run: WTFCrashWithInfo under JSC::JSScope::abstractResolve, JSC::CodeBlock::finishCreation, JSC::Interpreter::executeCall, Bun__JSTimeout__call.

Release builds, 1.4.1 and later. Checked on stock 1.4.3-canary (367d939), Linux x64, three runs each. TWIN=1 makes the getter return a string.

// green.test.js: bun test ./green.test.js
import { test, expect } from "bun:test";
class E extends Error { get message() { if (process.env.TWIN) return "fine"; throw new TypeError("a bug in the getter"); } }
let n = 0;
test("flaky", () => { if (n++ === 0) throw new E(); }, { retry: 2 });
test("must fail", () => { console.log("BODY RAN"); expect(1).toBe(2); });

// run.mjs: bun run.mjs
class E extends Error { get message() { if (process.env.TWIN) return "fine"; throw new TypeError("a bug in the getter"); } }
setTimeout(() => { setImmediate(() => console.log("immediate ran")); throw new E(); }, 1);

run	stock 1.4.3-canary	TWIN=1, and this PR
green.test.js	(pass) flaky (attempt 2), (pass) must fail, 2 pass 0 fail, exit 0, BODY RAN never printed	(pass) flaky (attempt 2), BODY RAN, (fail) must fail, 1 pass 1 fail, exit 1
run.mjs	panic(main thread): abort() called, SIGABRT	immediate ran, exit 1

• Why the run is green: the getter's exception is still pending when the printer returns. EventLoop::enter_js (src/jsc/event_loop.rs) refuses to call a callback while an exception is pending, and run_callback_with_result_and_forcefully_drain_microtasks returns undefined for the skipped call. BunTest::run_test_callback reads that as a test that returned. Nothing clears the exception, so no later hook or body runs: with a beforeEach, afterEach and afterAll in the file, none of them runs after the first throw, and each skipped test prints (pass).
• Why bun run aborts: Bun__JSTimeout__call calls the immediate's function with the exception pending. Its first call links a CodeBlock, and JSScope::abstractResolve reaches catchScope.releaseAssertNoException() (JSScope.cpp:98). Confirmed in gdb on a release build: the line passed to WTFCrashWithInfo is 100, which is releaseAssertNoException in ExceptionScope.h.
• Where it started: before Add jsc-exception-lint and fix the missing exception checks it finds #40410 the code converted the value that the failed read returned (undefined) and then cleared the exception. That printed error: undefined and left nothing pending. Add jsc-exception-lint and fix the missing exception checks it finds #40410 put a RETURN_IF_EXCEPTION in front of the conversion, in both branches. That satisfies the exception check validation, but it returns before the clear.
• A debug build without the fix (367d939) prints the same two (pass) lines for green.test.js and then panics with a task returned Ok with a JS exception pending, a debug assertion in EventLoop::tick_turn (exit code 134). run.mjs panics with the same message. For a value thrown from a test body or a timer, the print does not hit the VMInquiry assertion.
• Not in this PR: bun test still reports a callback that enter_js skipped as a pass if any other code leaves an exception pending. That is a change to the runner and is separate work.

History of this PR. It first carried a fix for the same site under BUN_JSC_validateExceptionChecks=1 with a non-string message (e.message = 42). Main then gained the RETURN_IF_EXCEPTION from #40410. The branch was replaced with a version based on current main. The non-string cases stay covered by the second describe block.

The four direct sinks.

class E extends Error { get message() { throw new RangeError("m"); } }
console.error(new E()); // or Bun.inspect(new E()); throw new E(); Promise.reject(new E());

A release build prints error with no name and no frames. Node prints [object Error].

Workers. web_worker.rs on_unhandled_rejection formats the uncaught error with format2 on the worker thread before it dispatches it to the parent, so on an asserts build the abort took the whole process down. With the fix the parent gets what a release build already delivers: a cloned Error with an empty message (worker_threads) or an ErrorEvent (Web Worker), then exit code 1. Checked with the throw at the top level and in a timer for both worker kinds, and as an unhandled rejection for worker_threads.

Other readers. exceptionFromString clears after each read and only converts strings. The stack read in fromErrorInstance clears and only uses strings. sanitizedNameString and the syscall, code, path, fd, errno reads are VMInquiry and never run getters.

Hostile variants run under BUN_JSC_validateExceptionChecks=1 with no assertion: throwing name, stack, all three at once, message as an object whose toString throws, a throwing getter on a cause, inside an AggregateError, as an own accessor, nested in objects and arrays, through %o, and through the bun test reporters (a thrown and a rejected hostile error, toEqual, toThrow, toMatchInlineSnapshot).

Binary size. Measured by CI's binary-size check on the previous head of this PR (9df1721, the same src/ diff) against main: within 0.0 KB on all 12 targets.

Unchanged. console.error("%s", e) calls String(e) and propagates the getter's error to the caller. That is a separate path.

Tests. The children run with BUN_JSC_validateExceptionChecks=1. Stock 1.4.3-canary fails 7 of the 13 new tests: the four direct sinks, the timer, the retried bun test, and the Proxy has trap case. The two worker tests and four of the non-string cases describe output that a release build already had. A debug build without the fix aborts in nine: every test except those four non-string cases. The abort is the VMInquiry assertion, or for the timer and the retried test the tick_turn assertion. The existing error.cause inline snapshot changes only because it contains the file's import line.

Suites run on a debug ASAN build of this branch (main bf42a52), all green: test/js/bun/util/inspect-error.test.js (52 tests), inspect.test.js, reportError.test.ts, error-name-preservation.test.ts, error-code-mirror.test.ts, test/js/bun/test/stack.test.ts, test/js/bun/test/test-retry-repeats-basic.test.ts, test/cli/test/retry-flag.test.ts.

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---

Walkthrough

Changes

Error message handling

Layer / File(s)	Summary
Message extraction fallback src/jsc/bindings/ZigException.cpp	fromErrorInstance now handles exceptions from the observable message getter. It clears non-termination exceptions, discards returned values, and uses sanitized error text when required.
Error output coverage test/js/bun/util/inspect-error.test.js	Subprocess tests cover throwing getters, non-string messages, proxy failures, output continuation, and exit statuses. Inline snapshots include the updated harness import.

Suggested reviewers: jarred-sumner

Merge Risk: 🟡 Moderate · up to 8889b

Error printing can still produce incorrect message output when a non-string message value throws during conversion because the sanitized fallback is not restored. The change is localized, but this bounded correctness issue should be fixed before merging.

🚥 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 and concisely describes the main fix: clearing the exception from a throwing Error.message getter before subsequent property reads.
Description check	✅ Passed	The description explains the problem, implementation, risks, and verification results in detail. It does not use the template headings, but it provides the required content under equivalent sections.

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[robobun]

Status: reproduced and fixed, waiting on CI for 2a9543b.

Reproduced on stock bun 1.4.3-canary (367d939) and on a debug build of main:

class E extends Error { get message() { throw new RangeError("m"); } }
console.error(new E()); // or Bun.inspect(new E()); throw new E(); Promise.reject(new E());

• An asserts build aborts in the print: ASSERTION FAILED: Unexpected exception observed in Bun__JSObject__getCodePropertyVMInquiry.
• A release build (1.4.1 and later) leaves the getter's exception pending after the print. bun test then skips every later hook and test body and prints (pass). With retry on the test that throws, the run is 2 pass 0 fail, exit 0. bun run aborts with panic(main thread): abort() called when the next immediate runs. The two repro files are in the Notes of the PR body.

The branch was rebased on main bf42a52 on Sep 30. The fix did not change. Two tests are new: a test that bun test retries, and a timer callback. The tests cover eight entry points with a throwing getter and five kinds of non-string message.

[claude]

LGTM — small, well-scoped exception-scope fix that mirrors the existing stack read pattern in the same function.

What was reviewed:

• The new check after getIfPropertyExists correctly clears non-termination exceptions and drops message before toStringRef/sanitizedMessageString open their own scopes; the pre-existing check after the branch still covers conversion failures (Symbol, throwing toString).
• The normalizeError regex was checked against real path frames like ([dir]/file.js:88:7) — it only matches file-less positions, so it won't strip legitimate frames.
• Tests follow harness conventions (test.concurrent.each, bunEnv spread, concurrent pipe drain, combined-object assertion) and cover the variant matrix including throwing getters and Proxy traps.

Extended reasoning...

Overview

This PR fixes an unchecked-exception-scope violation in fromErrorInstance (src/jsc/bindings/ZigException.cpp). The message property lookup via getIfPropertyExists (which runs getters and Proxy traps) was not checked before Bun::toStringRef or err->sanitizedMessageString opened their own throw scopes, tripping BUN_JSC_validateExceptionChecks=1 in debug builds whenever message was non-string or absent from the prototype chain. The fix inserts a scope.exception() check immediately after the lookup, clears non-termination exceptions, and drops the value on throw — the same shape the function already uses for its stack read a few lines below. Net production change is ~7 lines restructuring an else if/else into an else with an intermediate check.

The test file gains a 9-case describe block spawning bun -e under BUN_JSC_validateExceptionChecks=1 and asserting the printed header, plus a fix to the normalizeError helper (its debug-frame pattern had gone stale) and mechanical inline-snapshot line-number shifts from the added import.

Security risks

None. The error printer already runs arbitrary user code via the message getter and toString; this change only reorders when the exception scope is checked. No new user-reachable surface, no privilege change, no allocation-size or path handling.

Level of scrutiny

Low-to-medium. The production change is small, mechanical, and copies an established pattern from the same function. The only user-visible behavior change in release builds is that a throwing message getter now prints the name-only header instead of error: undefined, which is strictly better. The debug-build abort fix is what unblocks other tests on ASAN CI lanes. I traced the control flow: after message = {} on a cleared exception, the fallback to sanitizedMessageString is entered with the scope satisfied; the pre-existing post-branch clearExceptionExceptTermination still handles conversion failures.

Other factors

• Test coverage is thorough for the input-space guidance in REVIEW.md: Symbol, undefined, number, object with toString, throwing toString, no message on chain, throwing getter, Proxy has trap, and the uncaught-exception path. The PR description confirms all 9 fail on the unfixed debug build and the throwing-getter case fails on release (USE_SYSTEM_BUN=1 would fail that one).
• The normalizeError regex change was verified not to over-match: it requires ( followed only by optional native, optional :, and \d+:\d+) at end-of-line, so real path frames (([dir]/file.js:88:7)) survive.
• Tests use test.concurrent, spread bunEnv, and drain pipes with Promise.all per harness conventions.
• No prior reviewer comments to address; the bug-hunting system found nothing.

[robobun]

Superseded: this branch was force-pushed over the PR head on Aug 30. The PR carries the rebased fix and the tests described above.

[robobun]

^{Updated 6:43 PM PT - Sep 30th, 2026}

✅ @robobun, your commit 2a9543b51bfd4eebdb86228fff9c464ed0050cb5 passed in Build #122074! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 38586

That installs a local version of the PR into your bun-38586 executable, so you can run:

bun-38586 --bun

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

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Inline comments:
In `@test/js/bun/util/inspect-error.test.js`:
- Line 280: Replace the parameterized test declaration using test.each(cases)
with describe.each(cases), and place the existing single test inside the
describe callback while preserving its setup, header, and assertions.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Inline comments:
In `@src/jsc/bindings/ZigException.cpp`:
- Around line 524-525: Update the exception cleanup around Bun::toStringRef so
that when message coercion fails and the exception is cleared, except.message is
replaced with err->sanitizedMessageString(global) rather than retaining the dead
value. Add a regression test using an object whose toString throws.

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[claude]

Code review found no new issues

No new issues were found in this update; 1 finding from earlier reviews is still open above.

[robobun]

Another audit found the same bug on current main (d316760), with one more sink: a `bun test` run. A test that throws an Error whose own `message` property is a throwing getter dies in the debug build with `panic: a task returned Ok with a JS exception pending` (`src/jsc/event_loop.rs:779`). The rest of the file never runs.

I applied this PR's `ZigException.cpp` change on top of main and rebuilt the debug binary. Both cases now print `Error: ` with the frames and continue:

• `bun -e 'const e = new Error("x"); Object.defineProperty(e, "message", { get() { throw new Error("mg"); } }); throw e;'` exits 1 with no assertion.
• The `bun test` file above reports `(fail)` for that test and `(pass)` for the next one, exit code 1.

The diff still applies cleanly on main. The only CI failure was `h2-conformance.test.ts` on macOS aarch64, which this diff does not touch.

[claude]

Code review found no new issues

No new issues were found in this update; 1 finding from earlier reviews is still open above.

[claude]

Code review found no new issues

No new issues were found in this update; 1 finding from earlier reviews is still open above.

Still open from earlier reviews (1):

• 🔴 test/js/bun/util/inspect-error.test.js:317 — The new subprocess tests spawn with plain bunEnv and never set BUN_JSC_validateExceptionChecks=1, so the "not a plain s…

[claude]

LGTM — the earlier feedback is addressed: both new describe.concurrent blocks now spawn with { ...bunEnv, BUN_JSC_validateExceptionChecks: "1" }, so the non-string-message cases will observe the debug-build abort when the fix is reverted.

What was reviewed:

• The message read in fromErrorInstance now checks scope.exception() before using the result, clears non-termination exceptions, and falls back to sanitizedMessageString — matching the pattern already used for stack, name, syscall, code in the same function.
• Termination is still respected (clearExceptionExceptTermination() returning false → early return on both the get and the stringify).
• Tests drain stdout/stderr/exited concurrently, assert output before exit code, use describe.concurrent for the independent subprocess spawns, and cover the sink matrix (console.error, Bun.inspect, uncaught throw, unhandled rejection, both worker kinds) plus the non-string variants.

Extended reasoning...

Overview

The PR fixes fromErrorInstance() in src/jsc/bindings/ZigException.cpp so that when reading error.message via getIfPropertyExists (a real [[Get]] that runs getters and Proxy traps), any exception thrown by the getter is cleared before the printer proceeds to the next property read. Previously the result was truth-tested before the exception check, and on the RETURN_IF_EXCEPTION path the pending exception was left for the next VMInquiry read (code) to trip an assertNoExceptionExceptTermination abort in debug builds. The new code checks scope.exception() immediately, clears it (except termination), nulls the value, and only stringifies when a value was actually obtained — with a second clear after the stringify. Two new test suites in test/js/bun/util/inspect-error.test.js exercise six sinks and five non-string message shapes under BUN_JSC_validateExceptionChecks=1.

Security risks

None. This is defensive exception handling in the error-printing path; no untrusted-input parsing, auth, crypto, or filesystem effects are touched. The change makes the printer more robust against hostile Error subclasses (throwing getters, Proxy traps) rather than less.

Level of scrutiny

Moderate. JSC exception discipline is a frequent source of blocked PRs per REVIEW.md ("Exception checks after every call that can enter JS"), but this change is ~15 lines, mirrors the exact clearExceptionExceptTermination() pattern already used by every neighboring read in the same function (name, syscall, code, path, stack), and the PR description documents auditing sibling sites. The function runs under a TopExceptionScope, so swallowing rather than propagating is the intended contract here — REVIEW.md's "Never clearException()" is about hiding failures on paths that should propagate, which does not apply to a printer whose job is best-effort formatting of an already-uncaught error.

Other factors

My one prior open finding — that the new subprocess tests spawned with plain bunEnv and would pass with the fix reverted — has been addressed in the latest push: both blocks now use a shared validateExceptionChecksEnv that sets BUN_JSC_validateExceptionChecks: "1". Tests follow the harness conventions from CLAUDE.md/REVIEW.md (concurrent subprocess spawns, Promise.all pipe drain, output asserted before exit code, normalizeBunSnapshot, -e for single-file spawns). The two updated inline snapshots are purely mechanical (the file's own import line changed). No CODEOWNERS entry covers these paths, and the hunt exited on dry_streak with no findings.

[claude]

LGTM — the latest push addresses the earlier note (both new describe blocks now spawn with BUN_JSC_validateExceptionChecks=1), and the C++ change is a small, self-contained fix that mirrors the existing stack read a few lines below.

What was reviewed:

• fromErrorInstance message read: checked that a throwing getter is cleared via clearExceptionExceptTermination (termination still returns early, matching every sibling exit in the function) and that the fallback to sanitizedMessageString is the same path the old else-branch took for an absent message.
• Sibling reads (syscall/code/path/fd/errno) go through VMInquiry and cannot run getters; the stack read already uses the same clear-then-use shape, so no other site in this function shares the bug.
• Tests: describe.concurrent.each is already used elsewhere in the suite; subprocess pipes are drained with Promise.all; tempDir cleanup is via using.

Extended reasoning...

The diff touches one block in src/jsc/bindings/ZigException.cpp (the message read in fromErrorInstance, under a TopExceptionScope) and adds subprocess tests in test/js/bun/util/inspect-error.test.js. It touches no security-sensitive surface; it only affects how the error printer handles an error whose message getter throws. The change is roughly ten lines, follows the exact exception-handling pattern already used for the stack read in the same function, and preserves the old else-branch fallback. The one finding from the prior review (tests not setting BUN_JSC_validateExceptionChecks=1) is addressed in the current code, the hunt ran dry, no CODEOWNERS entry covers the changed paths, and no third-party objection remains open.

[robobun]

Closing: the fix from this PR is on main.

• Hardening follow-up: fix regressions from #46021 and more crashes #46270 (a950fc5) applied the fromErrorInstance change of this PR as it is. The hunk on main is the same, line for line.
• Every case in the description passes on main (9b8347a). I ran them on a debug + ASAN build with BUN_JSC_validateExceptionChecks=1 and on the release canary (a0cc096). bun 1.4.3-canary (367d939) still fails them, so the cases can fail.
• The 13 tests of this PR pass on main. Main has a shorter form of one of them (the timer case, from Hardening follow-up: fix regressions from #46021 and more crashes #46270).

Case	1.4.3-canary (367d939)	main
green.test.js from the description	2 pass 0 fail, exit 0, BODY RAN is never printed	(pass) flaky (attempt 2), BODY RAN, (fail) must fail, 1 pass 1 fail, exit 1
run.mjs from the description	panic(main thread): abort() called	Error: , immediate ran, exit 1
A test file with beforeEach, afterEach and afterAll	No hook or body runs after the first throw. Each skipped test prints (pass).	Every hook and body runs.
10 kinds of hostile error through 6 sinks, one process each	3 of 60 abort	0 of 60 abort or assert
The bun test reporters: a thrown and a rejected error, toEqual, toThrow, toMatchInlineSnapshot	The 5 tests after the first throw print (pass). The last one never prints its console.log.	All 6 tests run and get a result.
Uncaught in a Worker (worker_threads and Web Worker: at the top level, in a timer, as a rejection)	Same as main	The parent gets the error event, then exit code 1.

Not on main from this PR: 12 tests. They are the four direct sinks, the retried bun test case, the two Worker cases, and the five non-string message cases.

Still open and outside this PR: #44491. bun test reports a callback as passed when other native code leaves an exception pending.

The two scripts that are not in the description

// hostile.mjs: VARIANT=<name> SINK=<name> BUN_JSC_validateExceptionChecks=1 bun hostile.mjs
class M extends Error { get message() { throw new RangeError("from message"); } }
const variants = {
  message: () => new M(),
  name: () => { class E extends Error { get name() { throw new RangeError("from name"); } } return new E("m"); },
  stack: () => { const e = new Error("m"); Object.defineProperty(e, "stack", { get() { throw new RangeError("from stack"); } }); return e; },
  all3: () => { class E extends Error { get message() { throw new RangeError("m"); } get name() { throw new RangeError("n"); } get stack() { throw new RangeError("s"); } } return new E(); },
  toStringThrows: () => { const e = new Error("m"); e.message = { toString() { throw new RangeError("ts"); } }; return e; },
  cause: () => new Error("outer", { cause: new M() }),
  aggregate: () => new AggregateError([new M(), new Error("ok")], "agg"),
  ownAccessor: () => { const e = new Error("x"); Object.defineProperty(e, "message", { get() { throw new Error("mg"); } }); return e; },
  nestedObject: () => ({ a: { b: new M() } }),
  nestedArray: () => [1, [new M()]],
};
const v = variants[process.env.VARIANT]();
const sinks = {
  "console.error": () => console.error(v),
  "Bun.inspect": () => console.error(Bun.inspect(v)),
  "%o": () => console.error("%o", v),
  "throw": () => { throw v; },
  "reject": () => { Promise.reject(v); },
  "timer": () => { setTimeout(() => { setImmediate(() => console.log("immediate")); throw v; }, 0); },
};
sinks[process.env.SINK]();
console.log("after");

On 1.4.3-canary the three runs that abort are message, all3 and ownAccessor through the timer sink.

// reporters.test.js: BUN_JSC_validateExceptionChecks=1 bun test ./reporters.test.js
import { test, expect } from "bun:test";
class M extends Error { get message() { throw new RangeError("from message"); } }
test("thrown hostile error", () => { throw new M(); });
test("rejected hostile error", async () => { await Promise.reject(new M()); });
test("toEqual", () => { expect(new M()).toEqual(new Error("x")); });
test("toThrow", () => { expect(() => { throw new M(); }).toThrow("x"); });
test("toMatchInlineSnapshot", () => { expect(new M()).toMatchInlineSnapshot(`"nope"`); });
test("last: body runs and fails an assertion", () => { console.log("LAST BODY RAN"); expect(1).toBe(2); });