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…n-Error value throws Printing an uncaught non-Error value (the reportError([1,2,3]) branch of print_error_instance_body) discarded the formatter's result. When the value's toString/Symbol.toPrimitive/getter throws during formatting, the secondary exception stayed pending on the VM: bun test aborted when the next test callback was invoked, and at module scope the rest of evaluation was severed with a phantom second uncaught error. Propagate the JSError so print_error_from_maybe_private_data clears it, which is what the printer did before the port.
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Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 0 remain after this review. WalkthroughChangesThe VM now propagates failures from error-instance formatting. Regression tests cover hostile values in test execution, error reporting, and HTTP server error handling. ChangesError formatting
Suggested reviewers: Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to The change clears exceptions raised while formatting hostile values, allowing later tests and server requests to continue. No actionable merge-blocking issue remains; merge after normal checks. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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LGTM — minimal restoration of the pre-port ? propagation so the caller's existing clear_exception() path fires again.
What was reviewed
- Traced the propagation path:
print_error_instance_body→print_error_instance_js→print_error_from_maybe_private_data, which already handlesCrateError::JSErrorby callingglobal().clear_exception()(VirtualMachine.rs:4925-4926). - Checked the sibling
let _ = formatter.format(...)in the Error-property loop above — it already clears the exception inline viahas_exception()/clear_exception(), so it doesn't share this defect. - Both new tests drain pipes concurrently, assert
signalCode === null, and pin the observable failure (runner completes all 3 tests;reportErrordoesn't sever module evaluation).
Extended reasoning...
Overview
One-line functional change in src/jsc/VirtualMachine.rs: the non-Error branch of print_error_instance_body (the "reportError([1,2,3]) should still show something" path) previously discarded the Result from formatter.format(...) with let _ =. The PR changes this to ?, propagating a JSError up to print_error_from_maybe_private_data, whose existing error arm already calls self.global().clear_exception(). Two regression tests are added — one exercising the bun test runner abort and one exercising reportError severing module evaluation.
Security risks
None. This is error-printer control flow; no new user-controlled data reaches allocation, paths, or protocol handling. The change makes the printer more robust against hostile toString/Symbol.toPrimitive on reported values, which is a hardening.
Level of scrutiny
Low-to-medium. The diff is a targeted revert of a Zig→Rust port regression (try formatter.format(...) became let _ = ...). The caller's cleanup path is unchanged and I confirmed it exists at VirtualMachine.rs:4917-4926. Tag::get_advanced(...)? two lines above already uses ? in the same result type, so the propagation is type-correct by construction. I checked for other let _ = formatter.format sites in the file that might share the bug — the only other one (the Error own-property loop) explicitly checks has_exception() and clears it inline, so it is not affected.
Other factors
- Tests follow harness conventions:
tempDir,bunEnv/bunExe,await using, concurrent pipe drain,signalCodenull assertion beforeexitCode, and assert on the actual observable ("3 fail", "after\n", absence of "error: 1"). - The PR description documents
USE_SYSTEM_BUN=1failing andbun bdpassing for both test files, plus a broader console/inspect suite run. - Early return via
?skips the trailing\nandprint_stack_tracefor this value, which matches the pre-regressiontrysemantics the PR is restoring.
…ror output With --console-depth 0 the property walk of a very deep value throws a stack overflow RangeError. The non-Error branch discarded the Err and left the exception pending, so reportError() threw it into the script and bun test aborted with a pending exception. Propagate it so print_error_from_maybe_private_data clears it, as it did before the port (the same line as #36921). Tests: depth 0 prints every level, the deep value case continues, and an uncaught Error's properties, causes and AggregateError members print as before.
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This leak also has a network-reachable face. A Measured on release builds, one POST of a 200000-deep array, loopback:
So this diff is what keeps the server process alive once the formatter reports a stack limit instead of crashing. Full matrix in #35288. |
# Conflicts: # test/js/bun/test/bun-test.test.ts
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Updated this PR to current main in 3d188d6 (it was 1,269 commits behind).
Two more faces of the same defect, both clean with this PR: const a = [1];
Object.defineProperty(a, 1, { get() { throw new Error("boom"); }, enumerable: true });
reportError(a); // main: throws "boom" to the caller
Bun.serve({ port: 0, fetch() { throw a; } }); // main, debug build: panic "a task returned Ok with a JS exception pending" on the first requestThis PR also matters for the open stack overflow fixes in the error printer. #34884 and #36602 stop the SIGSEGV for a deeply nested array, but a stack overflow Build note: after this merge the branch includes #42851 (LLVM 23.1.1), so a local |
The runner test now rejects with each boxed primitive, a RegExp, a String subclass and from an afterEach hook, asserts the exact normalized output, and checks that the test after them really runs its body. The printer tests cover reportError, an unhandled rejection, and an AggregateError member.
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Updated 8:13 PM PT - Sep 30th, 2026
✅ @robobun, your commit 8529c9db844db04378cbb2e3aaf4574879a4dab3 passed in 🧪 To try this PR locally: bunx bun-pr 36921That installs a local version of the PR into your bun-36921 --bun |
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Inline comments:
In `@test/js/bun/util/reportError.test.ts`:
- Around line 132-148: Replace the parameterized test.each invocation for the
“error printer clears an exception thrown while formatting the value” cases with
describe.each, and move the current async test body into a nested
test.concurrent while preserving the existing scripts, expectations, and case
name interpolation.
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src/jsc/VirtualMachine.rstest/js/bun/test/bun-test.test.tstest/js/bun/util/reportError.test.ts
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LGTM — the latest commit addresses the earlier test nits (full normalized snapshots, test.concurrent, and the Promise.reject / AggregateError-member variants), and the native change is a four-line ? propagation.
What was reviewed:
- Traced the propagated
JsErrorfrom the non-Error branch ofprint_error_instance_bodyup throughprint_error_instance_jstoprint_error_from_maybe_private_data, which clears the exception onCrateError::JSError; the AggregateError member path reaches the same handler viaprint_errorlike_object. - Checked the other caller,
print_externally_remapped_zig_exception: it passesJSValue::ZERO, so the changed branch is never taken there. - The unchanged sibling
let _ = formatter.formatin the own-property loop already handles its exception viahas_exception/clear_exceptionand intentionally continues to the next property, so it is not the same defect. - Snapshots show a clean
errorline followed by the next(fail)entry with no partial formatter output, andsignalCodeis asserted null as a hang/abort guard.
Extended reasoning...
Overview
The PR changes one site in src/jsc/VirtualMachine.rs (print_error_instance_body, the else if error_instance != JSValue::ZERO branch used for non-Error thrown values): the JsResult from formatter.format::<true/false>(...) was discarded with let _ = and is now propagated with ?. Two test files gain coverage: test/js/bun/test/bun-test.test.ts spawns bun test on a fixture that rejects with boxed String/Number/Boolean, RegExp, and a String subclass whose toString/Symbol.toPrimitive throw, plus a hostile afterEach and a trailing test, snapshotting full stdout/stderr; test/js/bun/util/reportError.test.ts adds a test.concurrent.each covering reportError(hostile), a top-level Promise.reject(hostile), and Bun.inspect(new AggregateError([hostile, new Error(...)])), asserting combined {stdout, stderr, exitCode, signalCode}.
Security risks
None. The change affects only how an already-uncaught value is rendered to stderr; it does not touch input parsing, auth, crypto, or file/network paths. The exception being cleared is the printer's own secondary exception, and the clear happens in the existing print_error_from_maybe_private_data handler that already did this for every other JSError from the printer.
Level of scrutiny
Moderate, focused on verifying the mechanism rather than trusting the description. I confirmed that Formatter::format returns JsResult<()> so ? maps to CrateError::JSError, that print_error_instance_js returns the body's result unchanged after freeing the exception holder, and that print_error_from_maybe_private_data calls self.global().clear_exception() on exactly that error variant. For Bun.inspect(AggregateError) the member is printed through print_errorlike_object → print_error_from_maybe_private_data, so the same clear applies and the second member is printed, which the test asserts. The only other caller of the body, print_externally_remapped_zig_exception, passes JSValue::ZERO and never enters the changed branch. The behavioral difference from the old discard (skipping the trailing newline, stack trace, and cause loop for a value whose formatting threw) matches the pre-port try formatter.format(...) and is what the snapshots pin.
Other factors
The bug hunt ran to a dry streak with no findings. My earlier inline nits (substring-only assertions, missing test.concurrent, uncovered Promise.reject/AggregateError faces) are addressed by commit 3c4b22c. The sibling let _ = formatter.format in the own-property loop is intentionally different: it clears the exception itself under allow_side_effects and continues to the next property, so it is not the same class of defect. No CODEOWNERS entry covers the changed files, no third-party objections are outstanding in the timeline, and I did not build or run the tests locally (no debug build present); the PR's ASAN and release gate output shows the new tests failing on main and passing with the fix, and the snapshot contents are consistent with the code path I traced.
StatusReady for review. The branch is merged with main 5a183c1. How I reproduced it, on 1.4.0 and on canary 1.4.3-canary.1+367d939d9:
import { inspect } from "node:util";
const doc = { [inspect.custom]() { throw new Error("inspect"); } };
const server = Bun.serve({
port: 0,
development: false,
routes: {
"/ok": () => new Response("ok"),
"/bad": () => { throw { status: 400, doc }; },
},
});
console.log(server.port);The second
import { test } from "bun:test";
test("d", async () => { throw Object.assign(new String("q"), { toString(){ throw 1 }, [Symbol.toPrimitive](){ throw 1 } }); });
test("e", () => { console.log("e body ran"); throw new Error("m"); });The output has The tests in |
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One more face of this defect, on the current release canary (1.4.3-canary.1+c6b7fcb5b). The process no longer aborts there. Each later test in the file is reported as passed instead: import { test, expect } from "bun:test";
test("d", async () => {
throw Object.assign(new String("q"), { toString() { throw 1; }, [Symbol.toPrimitive]() { throw 1; } });
});
test("e: a failing assertion", () => { expect(1).toBe(2); });
test("f: a throw", () => { throw new Error("m"); });
test("g: a passing test", () => { expect(1).toBe(1); });Controls on the same binary: With this branch merged with main 38acc20 (debug ASAN build): |
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A note on overlap with #43238 (not a change request for the rest of this PR). #43238 touches the same non-Error branch of If #43238 lands first, the hunk here that turns those two lines into |
…rinter-pending-exception
…on throws Bun.serve prints a value that a route throws when there is no error() handler, and a value that a response body stream throws. The new block in serve.test.ts checks that the exception of the value's inspect hook does not stay pending. The next new timer callback runs. The next requests reach their handlers, also when error() throws such a value. After a direct stream body throws, error() does not get that exception for the next request. bun-test.test.ts: after a retried test rejects with such a value, the next test runs and the run does not exit 0.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I traced where the new ? at src/jsc/VirtualMachine.rs:7195/7197 can land: the only entry into print_error_instance_js is print_error_from_maybe_private_data (src/jsc/VirtualMachine.rs:6096), which clears the exception on JSError, and the recursive cause/AggregateError chain (line 7239) propagates via result? to that same handler. The other external caller, the bake dev-server's print_externally_remapped_zig_exception (src/runtime/bake/dev_server/error_report_request.rs:328), passes JSValue::ZERO, so the changed branch is skipped there and its let _ = is unaffected.
Extended reasoning...
The production change is a two-line ? propagation in the non-Error branch of the shared error-printer body in src/jsc/VirtualMachine.rs, plus regression tests in three test files; it touches no security-sensitive surface. Inline findings were reported and additional verified findings went unposted, so approval is not on the table; this note only records the caller-chain check that was ruled out.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
-
🟣
src/jsc/VirtualMachine.rs— pre-existing: callers of Bun.inspect/console.log on an AggregateError still lose every member after an Error member whose own property throws while it is formatted, and get no exception. The sibling own-property loop returns Ok(()) at src/jsc/VirtualMachine.rs:7153-7154 with the exception still pending when allow_side_effects is false, so the for_each at :5992 stops and :5996 clears it. Fix: make that exit surface the pending exception as Err(CrateError::JSError), as the changed branch at :7195 now does, so print_error_from_maybe_private_data clears it and the remaining members print. The PR text says this loop has no such defect; that only holds for allow_side_effects == true.Why this was flagged
Input:
Bun.inspect(new AggregateError([Object.assign(new Error("a"), { p: Object.assign(new String("q"), { toString() { throw 1 }, [Symbol.toPrimitive]() { throw 1 } }) }), new Error("second")])), or console.log of the same. ConsoleObject.rs:3880 calls print_errorlike_object with allow_side_effects = false; the member is an ErrorInstance, so print_error_instance_body takes the own-property loop. formatter.format at src/jsc/VirtualMachine.rs:7143-7147 runs the String object's ToString, which throws; the result is discarded. Because allow_side_effects is false, :7153-7154 returns Ok(()) with the exception still pending. Back in agg_iter the JSC forEachInIterable sees a pending exception after the callback and stops; errors.for_each returns Err and :5996 clears it, so "second" is never printed and nothing is thrown to the caller. The base branch behaves the same; the PR fixes exactly this member-dropping for non-Error members via the?at :7195/:7197 but leaves the Error-member sibling.Verification: pre-existing (the diff touches only the sibling arm at :7194-7198). Trigger: Bun.inspect/console.log of an AggregateError whose Error member has an own property that throws on ToString. With allow_side_effects = false, src/jsc/VirtualMachine.rs:7153-7154 returns Ok(()) leaving the exception pending; the for_each returns Err, and :5996 silently clears it: every later member is never printed.
… one-line comments The block about a thrown value whose inspection throws moves out of serve.test.ts into the file for errors that a Bun.serve handler throws. serve.test.ts is the same as on main again. The comments above the new tests state the invariant in one line.
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Two notes on the last review (2026-09-30).
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2 participants
Problem
Bun.serveroute with noerror()handler throws a non-Error value whose inspection throws. Since 1.4.0 the next request gets a 500 and skips its handler, or the process aborts:panic(main thread): abort() called.bun test: after a test rejects with such a value, every later test in the file reports(pass)and never runs. Withretrythe run exits 0.print_error_instance_body(src/jsc/VirtualMachine.rs:7194) discards the formatter result withlet _ =for a non-Error value. The exception stays pending.Fix
?. The caller,print_error_from_maybe_private_data, already clears the exception onJSError.tryin the Zig code).test/js/bun/http/bun-serve-propagate-errors.test.ts(4 tests),test/js/bun/test/bun-test.test.ts(2),test/js/bun/util/reportError.test.ts(3). All fail on a debug build of main.Background
console.logformatter, which runs user code such as a nested[util.inspect.custom].Err(JsError)means an exception is pending on the VM. JavaScript must not run until native code clears it.?keeps one clearing site, in the caller. One value formatter for console, Bun.inspect, the error printer and bun:test #44268 carries the same?.Downsides
throw reqwith such a value inreq.params. TheRequestprinter returns success with the exception pending (Bun.serve: fix crash and cross-request 500 when inspecting a non-Response return value throws #43058 changes it).Notes
Bun.serve. The server prints a value that a route throws when there is noerror()handler. Run this server, then send/ok,/bad,/okfrom another process:/ok500 Something went wrong!, the handler does not run (3 of 3)error() { throw { status: 500, doc }; }: on 1.4.0 every request after/badgets that 500 (3 of 3). With this PR they get 200.setTimeout(() => {}, 1)to the/badroute: on 1.4.0 the process aborts when the timer fires (3 of 3). With this PR the timer runs and the server keeps serving.Setholds the value, and forthrow { req }or aMapthat holdsreqwith such a value inreq.params.error()is set. With a direct stream whosepullthrows it, on 1.4.0 the next request does not reach its handler:error()gets the exception that the hook threw for the first request, and the client gets that 500 (3 of 3). With this PR the next request gets 200.throw reqandPromise.reject(req)with such a value inreq.paramsare not covered. TheRequestprinter (console_print_runtime_object_inner,src/runtime/jsc_hooks.rs) drops the exception and returns success, so there is no error to propagate. With this PR alone they still abort (debug build with ASAN of main bf42a52). With this PR and Bun.serve: fix crash and cross-request 500 when inspecting a non-Response return value throws #43058 they are clean on that build. On 1.3.14throw reqends the process with an uncaughterror: inspect, exit 1.throw [req]hitsASSERTION FAILED: Unexpected exception observedon a debug build with this PR alone, for the same reason.bun testwithretry.1.4.0 and canary 367d939:
(pass) must fail,2 pass,0 fail, exit 0, andBODY RANis never printed (3 of 3). 1.3.14 and main bf42a52 with this PR:1 pass,1 fail, exit 1.bun test,reportError, unhandled rejections. The value in the first report was a boxed primitive or RegExp whose owntoStringthrows. Debug builds panic:a task returned Ok with a JS exception pending. Older canaries aborted in the runner:ASSERTION FAILED: Unexpected exception observed ... ExceptionScope::assertNoException()inInterpreter::executeCallImpl.reportError(value)and an unhandled rejection stop module evaluation and print a phantomerror: 1. The formatter renders boxed primitives and RegExp objects with specToString, which calls a user-definedtoStringorSymbol.toPrimitive. The related console and inspect suites pass (list below).Other open PRs that change this line. #39637 and #44268 carry the same
?. #43238 keepslet _ =, clears the exception after the newline, and still prints the stack frames of the throw. In a review of #43238 a maintainer prefers that way: #43238 (comment). Only one of them has to land it. The other way is 4 lines in this branch (if allow_side_effects && global_ref.has_exception() { global_ref.clear_exception(); }after the newline, withlet _ =kept). I built it on this branch: the tests here pass with it when each report of such a value expects one more blank line, except the AggregateError case, becauseconsole.logandBun.inspectrun the printer withallow_side_effects == false.Repro from the report:
dfails,eruns and fails.error,(fail) d, thenpanic(main thread): abort() called, exit 134.(fail) d, then(pass) e. The body ofenever runs. Exit 1 only becausedfailed.(pass)lines, then thedebug_assert!atsrc/jsc/event_loop.rs:787.What the tests cover:
Bun.serve(bun-serve-propagate-errors.test.ts, the client is the test process): a route throws or rejects with an object, an array or aMapthat holds the value, or an object that holds itsRequestwith the value inreq.params, and the next new timer callback runs. A route throws such an object, with noerror()handler and with one that throws such an object too, and the next requests reach their handlers. A direct stream body throws such an object witherror()set, anderror()does not get that exception for the next request.retry(bun-test.test.ts): a retried test rejects with such an object, and the next test runs and fails (1 pass,1 fail, exit 1).new String,new Number,new Boolean, a RegExp, a String subclass, and a rejection from anafterEachhook. The exact normalized stdout and stderr are inline snapshots. The last test logs from its body, which is what catches the false(pass).reportError(value), an unhandled rejection, and an AggregateError member, each with exact stdout, normalized stderr, and exit code.Bun.inspect(new AggregateError([hostile, new Error("x")]))drops every member after the hostile one. The older canary threw the hostile exception into the caller. With the fix the remaining members print, as in 1.3.10.The Error own-property loop above the changed branch also discards the formatter result. When the printer reports an uncaught error it clears the exception inline, so it does not have this defect there. For
console.logandBun.inspectit returns with the exception pending, and an AggregateError then loses the members after that one. This PR does not change that loop.Behavior that this PR does not change: when a container such as
[1, hostile]throws in the middle of the print, the early return leaves the partial line ([ 1,) without a newline. 1.3.10 did the same. No test pins that line.Fail-before evidence (this branch at 8529c9d, merged with main 5a183c1): on a debug build with ASAN and the
src/of main, the 9 new tests fail and the 7 older tests in the three files pass. With thesrc/of this branch all 16 pass. The 9 new tests also fail on the release canary 367d939.The
Bun.servecases are inbun-serve-propagate-errors.test.ts, the file for errors that aBun.servehandler throws.serve.test.tshas no diff against main.Suites run with the fix (on 09-16, the first version of this PR):
test/js/bun/console/(86 pass, 1 skip),test/js/web/console/(10 pass),test/js/bun/util/inspect.test.js(81 pass),test/js/bun/util/inspect-error.test.js(39 pass),test/js/bun/test/bun_test.test.ts(7 pass),test/js/bun/test/test-test.test.ts(24 pass, 16 skip).[human-review] gate passed · iteration 1 · 4 files touched
fails on main (without fix)
passes on PR (with fix)
diff hotspot
gate history · 2 passed · 1 rejected · iteration 1
evidence per changed file