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[JSC] An error whose captured stack has no frames gets its stack from the host's hook - #829
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WalkthroughThe Bun hook tracks when it returns no stack for an empty trace. The host-computed error-info path accepts an unmaterialized stack trace with no frames. If error info is not materialized, the caller continues to parser-position handling. ChangesError stack materialization
Priority: ➖ Normal Merge Risk: 🔵 Low · up to
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| Title check | |
The title clearly and concisely describes the main change: host-provided stacks for errors with captured traces that contain no frames. |
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The description is detailed and directly explains the bug, implementation, behavior changes, limitations, test results, and related issues. It does not use the repository template format or include an… |
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Explanation
The description is detailed and directly explains the bug, implementation, behavior changes, limitations, test results, and related issues. It does not use the repository template format or include an explicit Reviewed by line and changed-file list, but the required technical context is substantially present.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
The hostComputes change in Error.cpp closes the unguarded-hook-call path I raised on the previous push: an empty-trace parse error now reaches the hook only under the catch scope. Beyond the inline finding, I also checked the GC-end path (reconcileWeakReferencesAtGCEnd -> computeErrorInfo) for the newly reachable empty-trace case — it only fires on a trace with an unmarked frame, so an empty trace never takes it, and the clear()/nullptr of an empty vector with m_errorInfoMaterialized set leaves later property reads finding simply no own line/column, which is the intended outcome.
Extended reasoning...
The change touches two Bun-only (USE(BUN_JSC_ADDITIONS)) gates in Source/JavaScriptCore/runtime/Error.cpp and Source/JavaScriptCore/runtime/ErrorInstance.cpp so an ErrorInstance with a present-but-empty stack trace is handed to the host's onComputeErrorInfoJSValue hook, and makes line/column/sourceURL materialization conditional on frames or a parser-supplied line. No security-sensitive surface is involved. The one confirmed inline finding (a host with only the non-JSValue onComputeErrorInfo hook loses parser line/sourceURL for empty-trace parse errors because hostComputes early-returns while the JSValue-only branch in materializeErrorInfoIfNeeded is skipped) is a real inconsistency between the two gates, so a human should weigh whether that hook configuration matters for Bun; the previous run's concern is addressed by the new hostComputes condition.
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I measured this PR's preview build ( 1. It extends a Bun bug to every unhandled rejection with an Error, in the warn modes. // bun --unhandled-rejections=warn --trace-warnings x.mjs
Promise.reject(new Error("boom"));
The first warning still prints the reason's stack one time, so stderr loses the second copy. oven-sh/bun#45775 fixes it in Bun: the warning drops its trace before the put. That PR does not depend on this one, and the rows of its test are the same on this build. It changes the 2. An assigned Error.stackTraceLimit = NaN;
const e = new Error("m");
e.stack = "x";
Object.keys(e); // ["stack"] on main and on this build, [] in NodeThe same for a limit of 3. One flow loses an async function readMissing() {
await Readable.toWeb(createReadStream("/nonexistent")).getReader().read();
}main: |
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All three reproduce here on the same preview build, and are in the tables now.
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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 nit, I also checked the new if (stack) gate at ErrorInstance.cpp:508: stack stays the empty JSValue when m_stackPropertyProvidedByCapturedStackTrace is true (the hook is not called), so the put is still skipped in that case, and a hook that returns the empty value no longer reaches putDirect with it. The GC-end path (reconcileWeakReferencesAtGCEnd -> computeErrorInfo) cannot consume a frameless trace first, since hasUnmarkedFrame is false for an empty vector. With hostComputes at Error.cpp:258-259 now matching the materialize condition, an empty-trace parse error reaches the hook under the catch scope at Error.cpp:274-276.
Extended reasoning...
The diff is two small hunks under USE(BUN_JSC_ADDITIONS) in Error.cpp and ErrorInstance.cpp that let the host's onComputeErrorInfoJSValue hook run for an ErrorInstance whose captured stack trace exists but has no frames, gate line/column/sourceURL on hasFrames or a parser-supplied line, and put stack only when the hook returned a non-empty value. It touches no security-sensitive surface. The latest commit (328a5ec) changed the stack-put gate, which I examined and found consistent with the earlier skip condition; the one reported finding is a structuredClone nit posted inline, so a human look remains worthwhile for the Bun-side hook contract this relies on.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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Source/JavaScriptCore/runtime/ErrorInstance.cpp— nit: A user who structuredClones an error with no frames now gets a clone with ownline: 0andcolumn: 0that the original does not have. The clone comes back through CloneDeserializerBase.h:709 with a non-null stack string, so finishCreation sets m_hasErrorInfo at ErrorInstance.cpp:262, and ErrorInstance.cpp:523-524 then puts line and column unconditionally. Fix: give the computeErrorInfo path the same position guard the hook path got at ErrorInstance.cpp:500, under USE(BUN_JSC_ADDITIONS), so a deserialized error with no line keeps the shape of its source. The PR's own table lists this row as a regression.Why this was flagged
Trigger: Error.stackTraceLimit = 0 with Bun's JSValue hook installed, then structuredClone(e) or postMessage(e). The serializer at CloneSerializerBase.h:629 reads the original's own properties via extractErrorInformationFromErrorInstance: line and column are absent, since ErrorInstance.cpp:500 now skips them for a frameless error, so 0 is written; stack is now the hook's "Error: m" instead of absent. The deserializer at CloneDeserializerBase.h:709 calls ErrorInstance::create with LineColumn {0, 0} and that stack string; finishCreation at ErrorInstance.cpp:262 sets m_hasErrorInfo = !m_stackString.isNull(), which is now true. On first access materializeErrorInfoIfNeeded skips the hook branch (m_stackTrace is null), computeErrorInfo is a no-op, and ErrorInstance.cpp:517-524 puts line: 0 and column: 0 as own DontEnum properties. On the base branch the original had no stack, the serialized stack string was null, m_hasErrorInfo stayed false and nothing was put, so 'line' in structuredClone(e) was false, matching the original.
Verification: On the original, ErrorInstance.cpp:500 skips putting
line/column, while line 508-509 putsstack. CloneDeserializerBase.h:709 calls ErrorInstance::create with the non-null stack string; line 262m_hasErrorInfois true, and lines 517-524 putline: 0andcolumn: 0unconditionally. On the base branch the serializer wrote a null stack, and the clone gained no line/column.
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Inline comments:
Review comments at @Source/JavaScriptCore/runtime/ErrorInstance.cpp:
- Line 508: In the `if (stack)` branch, store the hook’s returned value with
`ErrorInstance::put()` instead of `putDirect()` so the rejection reason’s stack
is preserved.
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…ys as if the hook had not been asked
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Closing together with oven-sh/bun#45883, which was its only user. Asking the host for the stack of an error without frames at the first touch marks the error as materialized, and that changes what a later |
What
In Bun, an error whose captured stack trace has zero frames has no
stackproperty at all. V8 gives it its header,"Error: message".The second line is at the default limit.
secure-json-parsesets the limit to 0 aroundJSON.parse, so code that then doeserror.stack.split("\n")throws aTypeErrorin Bun.Why this is a change to the fork and not to Bun
stackis the frames and nothing else. With no frames there is nothing to make, socomputeErrorInfo()is gated onm_stackTrace && !m_stackTrace->isEmpty()(7dcc589, 2018, https://bugs.webkit.org/show_bug.cgi?id=185996). That is correct for JSC's format, and this PR leaves it alone.VM::onComputeErrorInfoJSValue()lets the host make thestackvalue. Bun's hook writes V8's format, which begins withname: message. In that format a stack with no frames is not empty.USE(BUN_JSC_ADDITIONS)block inmaterializeErrorInfoIfNeeded()that calls the hook (3d38420, 2024, "Support setting error stack JSValue directly") copied upstream's!isEmpty()gate. So the host is never asked.formatStackTracetestsstackTrace.isEmpty()for aSyntaxError, anderrorInstanceLazyStackCustomGetterpasses anemptyTraceof its own.The change
The host was asked for a stack only if the trace had a frame. That condition was written in two places, and both go:
materializeErrorInfoIfNeeded():fn && m_stackTrace && !m_stackTrace->isEmpty()becomesfn && m_stackTrace.addErrorInfo():hostComputesloses its!stackTrace()->isEmpty(). This is the branch that lets the position the host computed stand, and that drops what the host's formatter throws. Without this change a parse error with no frames would get the host's line and then have it overwritten by the parser's. And aprepareStackTracethat throws would replace theSyntaxErrorthatimport()rejects with.The hook may return nothing, which says that it has put a
stackof its own:if (!m_stackPropertyProvidedByCapturedStackTrace) putDirect(stack)becomesif (stack) putDirect(stack).stackis only assigned under the old condition, so the old condition is still in it. The reason isError.prepareStackTrace: V8 calls it whenstackis read, and this function runs at the first touch of the error's own properties (Object.hasOwn,Object.freeze,delete). For an error with no frames Bun now puts its lazy accessor here and calls no JavaScript.line,columnandsourceURLare put only when the error has a position, that is a frame or a line from the parser:hasFrames || m_lineColumn.line.addErrorInfo()'s one caller is the parser, which always passes a line. Sonew Error()under a limit of 0 does not gainline: 0, and a parse error keeps itsline, with or without a source URL.A trace that is null still makes nothing. That is what
getStackTrace()returns whenError.stackTraceLimitis not a number (or isNaN).What it cannot change
fnis null andhostComputesis false. Thejscshell installs none, so no JSTest can reach the new paths, and none changes.reconcileWeakReferencesAtGCEnd->computeErrorInfo) only runs for a trace with an unmarked frame. An empty trace has none, so what such an error gets does not depend on when a collection runs.Every route to a trace that is present and empty
getStackTrace()under a limit that clamps to 0 (0, negative)new Error(), any error the engine throwsgetStackTrace()with no JavaScript frame on the stackfs, DNS,fetch,WebAssembly.compileErrorInstance::captureStackTrace(), which usesstackTraceLimit().value_or(0)Error.appendStackTrace, when the destination was made under a limit that is not a numberBehaviour
One behaviour on one API per case. "bug" means different from Node 26.7. For what only Bun has (
line,column,sourceURL,originalLine,Bun.inspect,Error.appendStackTrace, async frames) the notes under each table say what is taken as right. Of 85 cases, 39 differ from main's.stackbun#45883 against this branch. It pins this branch's preview build and has what Bun changes with it.bug -> bug
undefined"header""header + frames"["undefined",0]["Error: m",0]["H",1]fs.promises.readFile: the promise is awaited at the top level of a module. Node's frames are async ones, of its ownfscode and of the module.stack. A hook that is installed after that is not called for it.bug -> non-bug
undefined"Error: m""Error: m"undefined"Error: m""Error: m"undefined"TypeError: m""TypeError: m"undefined"SyntaxError: m""SyntaxError: m"falsetruetrueundefined[ 'm', [] ][ 'm', [] ]undefined"Error: m""Error: m"undefined"header""header"undefined"header""header"undefined"header""header""undefined""string""string"undefined"2"12truefalsefalse[ 'stack' ][][]011011["",0]["configurable,enumerable,get,set",0]["configurable,enumerable,get,set",0]["",0]["configurable,enumerable,get,set",0]["configurable,enumerable,get,set",0][",0,,0,true",0]["H,1,H,1,true",1]["H,1,H,1,true",1]["undefined",0]["H",1]["H",1]["message",0]["message,stack",0]["message,stack",0]["false",0]["true",0]["true",0][",",0]["H,H",1]["H,H",1]["undefined",0]["Error: m",0]["Error: m",0]["undefined",0]["H",1]["H",1][",",0]["RangeError,RangeError",2]["RangeError,RangeError",2]"undefined""UnhandledPromiseRejectionWarning: Error: boom""UnhandledPromiseRejectionWarning: Error: boom"322122266[result, calls of the hook]. The hook returns"H".at fake (/x/fake.js:1:5)inBun.inspect(e), and inutil.inspect(e)for Node.e.lineof 1 is the off-by-one noted in Bun'sformatStackTrace.non-bug -> bug
None.
non-bug -> non-bug
All six return something else.
undefined0""undefinedfalsetrue"header + frames""header""header"at x.js:1at <parse> (x.js:1:1)header + framesheadercolumn,sourceURL,originalLine: a parse error with no frames now has the own properties that one with frames has:line,column,stack, thensourceURLif it is not empty andoriginalLineif the line was mapped.new Functionandvm.Scriptget the samecolumn.vm.Script's report: both name the position.node:vmputs the source line in front of thestack, and the printer reads the frame out of that string.at async readMissing. Bun adds it when the rejection reaches an async function, and by design only to an error whosestackwas not read yet (Bun__attachAsyncStackFromPromise).node:streamreadserr.stackwhen it destroys a stream (internal/streams/destroy.ts, Node's workaround for a V8 leak). That read now counts for an error with no frames.Error.appendStackTraceadds nothing to an error whosestackwas read. Reading now counts for an error with no frames. The function is not inbun-typesand nothing in Bun calls it.Tests
The hook exists only in Bun, so the tests are in oven-sh/bun#45883. Against 87ee10d, 11 tests of
test/js/node/v8/capture-stack-trace.test.jsfail. Against this branch the file passes (75), undervalidateExceptionCheckstoo.