error printer: keep async and <anonymous> frame names once error.stack has been read

[robobun]

Problem

• Once error.stack has been read (or assigned), Bun.inspect(err), console.error(err) and the unhandled rejection output print at mid (...) for a frame that error.stack shows as at async mid (...), and print a bare at /app/main.js:9:9 for a frame it shows as at async <anonymous> (...) or at <anonymous> (...). Before .stack is read the same error prints these frames the way error.stack does, so the printed trace depends on whether a logger or error reporter touched .stack first. Reproduces on 1.4.0 and on main (repro and output below).
• Cause: once .stack is materialized JSC drops the structured frames, so fromErrorInstance (src/jsc/bindings/ZigException.cpp) re-parses the string with V8StackTraceIterator. Its callback copied is_async but only set code_type for new X (Constructor) and global code (Global) lines; every other frame was left as ZigStackFrameCodeNone. NameFormatter (src/jsc/ZigStackFrame.rs) renders the async prefix and the <anonymous> placeholder only in its FUNCTION arm; the fallback arm prints the name as-is, and parseFrame turns <anonymous> into an empty name, so those frames printed as a bare location. The structured path marks the same frames ZigStackFrameCodeFunction (populateStackFrameMetadata), which is why it prints them correctly. The is_async of parsed frames has been set since Enable async stack traces #22517 but never reached the formatter.

Fix

• parseFrame records whether the line had a name part (name (url) or <anonymous> (url), as opposed to a bare url), and the callback marks such frames ZigStackFrameCodeFunction; new X and global code lines keep their Constructor / Global types as before.
• This gives the parsed frames the code_type the structured path gives function frames, so the same formatter arm renders both and the output no longer depends on whether .stack was materialized. error.stack only prints a name part (including <anonymous>) for frames that had a callee, and prints module top-level and native frames as a bare location, so "had a name part" is exactly the set of frames the structured path marks Function.
• Output for named non-async frames is unchanged (FUNCTION and the fallback arm both print the bare name); the only lines that change are the ones gaining async or <anonymous>, and both now match error.stack and the pre-.stack printout. Bare-location lines (at unknown today, and the lines error printer: keep parsing error.stack past frames without a function name #38308 starts accepting) stay None and still print as a bare location, so nothing gains an invented <anonymous>.
• Verified with the new describe block in test/js/bun/test/stack.test.ts: an assigned stack string covering async named, async anonymous, anonymous, new, plain, global code and bare unknown frames (with and without colors); a real async chain whose printed frame names have to be the same before .stack is read, in .stack, and after; and a spawned process checking console.error and the unhandled rejection output. All three fail on main and pass with the fix.
• One existing test changes: test/cli/inspect/inspect.test.ts "error.stack doesnt lose frames" compares Bun.inspect output with and without reading error.stack and had snapshotted the bare-location rendering of the last frame for the error.stack case, comparing the two outputs minus that frame. The two outputs are now identical, so its snapshot is updated and it compares them in full.
• Also ran test/js/bun/test, test/cli/test and the inspect / reportError / vm printer tests against the debug build; the only failures are unrelated to frame printing and happen without this change too (a [2.46s] timing suffix, ANSI expectations without a TTY, the --parallel worker scaling timing, the deep-nesting pretty_format SIGSEGV that test_runner: add StackCheck to pretty_format to stop SIGSEGV on deeply nested diff/snapshot values #34885 addresses, and the debug-only at require (51:24) frame in inspect-error.test.js).
• Open PRs in the same function, all independent of this one: error printer: keep parsing error.stack past frames without a function name #38308 (accept bare-location lines in parseFrame), error printer: remap frames when the original source is unavailable, and not twice after error.stack #38296 (positions of re-parsed frames remapped twice), error: include the top-level-await caller in async stack traces #35685 (async module frames; it changes the formatter's fallback arm for nameless async frames, which this change does not touch).

Background

• A JSC ErrorInstance keeps its captured JSC::StackFrames only until the stack property is first materialized (read or assigned); after that only the string is left. Bun's error printer (toZigException -> fromErrorInstance) therefore has two producers of ZigStackFrames: populateStackFrameMetadata for JSC frames, and the V8StackTraceIterator re-parse of the string.
• ZigStackFrame.code_type (None / Eval / Module / Function / Global / Wasm / Constructor) selects how NameFormatter renders the name: Function renders [async ]name or [async ]<anonymous>, Constructor renders new name, Global renders nothing, and the fallback renders the name as-is. print_stack_trace (src/jsc/VirtualMachine.rs) prints at NAME (LOCATION) when the formatter produced anything and at LOCATION otherwise, which is how a <anonymous> frame with code_type == None collapsed to a bare location.
• error.stack lines (FormatStackTraceForJS.cpp) look like at [async ]name (url:line:col), with <anonymous> substituted when a function or eval frame has no name, and a bare at url:line:col for frames without a callee (module top-level code, native frames).

Repro

async function inner() { await 1; throw new Error("X"); }
async function mid() { await inner(); }
(async () => { await mid(); })().catch(e => {
  console.log(Bun.inspect(e));   // before .stack is read
  e.stack;                       // e.g. a logger
  console.log(Bun.inspect(e));   // after
});

Frames of the two prints on bun 1.4.0 and main:

      at inner (/tmp/x.js:1:45)
      at async mid (/tmp/x.js:2:30)
      at async <anonymous> (/tmp/x.js:3:22)

      at inner (/tmp/x.js:1:45)
      at mid (/tmp/x.js:1:32)
      at /tmp/x.js:1:54

With this change the second print becomes:

      at inner (/tmp/x.js:1:45)
      at async mid (/tmp/x.js:1:32)
      at async <anonymous> (/tmp/x.js:1:54)

The line:column of the re-parsed frames being off is the separate double remap fixed by #38296; error.stack itself is unaffected either way.

[robobun]

^{Updated 1:05 AM PT - Aug 14th, 2026}

❌ @robobun, your commit 950510c has some failures in Build #95363 (All Failures)

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[robobun]

Status: fix pushed (950510c), review comments addressed, self-review in progress.

Reproduced on bun 1.4.0 and current main with test/js/bun/test/stack.test.ts ("printing the frames parsed back out of error.stack"): with USE_SYSTEM_BUN=1 the three new tests fail (at async outer prints as at outer, at async <anonymous> (...) prints as a bare location); with this branch's build they pass. The one existing test that encoded the old rendering, test/cli/inspect/inspect.test.ts "error.stack doesnt lose frames", is updated to expect identical output with and without reading error.stack; the rest of test/js/bun/test and test/cli/test is unchanged.

[coderabbitai]

Walkthrough

V8 stack parsing now records whether normalized frames have function names. Remapped exception stacks use this state to preserve function-frame classification. Tests cover async, anonymous, constructor, bare, global, console.error, and unhandled rejection stack output.

Changes

Stack frame classification

Layer / File(s)	Summary
Parse and remap function-formatted frames src/jsc/bindings/ZigException.cpp	The parser records non-empty normalized function names. Remapped frames use this flag for non-constructor, non-global function code.
Validate reconstructed stack output test/js/bun/test/stack.test.ts	Tests verify stack formatting after reading error.stack, including async frames in inspection, console.error, and unhandled rejection output.

Possibly related PRs

• oven-sh/bun#36602: Both changes modify V8 stack-frame parsing and remapping for anonymous, async, and top-level frames.
• oven-sh/bun#38308: Both changes modify V8 stack-frame reconstruction for unnamed and async frames.

Suggested reviewers: jarred-sumner

🚥 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 identifies the main change: preserving async and anonymous frame names after reading error.stack.
Description check	✅ Passed	The description explains the problem, cause, fix, scope, and verification, although it uses different headings than the repository template.

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[coderabbitai]

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📒 Files selected for processing (2)

• src/jsc/bindings/ZigException.cpp
• test/js/bun/test/stack.test.ts

[github-actions]

This PR may be a duplicate of:

1. error: include the top-level-await caller in async stack traces #35685 - its NameFormatter rewrite in src/jsc/ZigStackFrame.rs emits the async prefix whenever is_async is set regardless of code_type, which fixes the same user-visible defect (frames re-parsed out of error.stack printing at mid (...) instead of at async mid (...)) via the formatter side rather than the code_type side.

🤖 Generated with Claude Code

[robobun]

Not a duplicate of #35685, although the two touch the same symptom for one of the frame shapes.

#35685 fixes a different bug (#10483: the module top-level await frame is missing from async stack traces). Its NameFormatter change makes the fallback arm print async for frames that have no code type, which it needs for module frames. Applied to the frames this PR is about (the ones re-parsed out of error.stack, which are left with no code type), that change would print:

• at async mid (...) correctly for named async frames;
• at async (/app/x.js:4:3) for an at async <anonymous> (...) frame, since the parser turns <anonymous> into an empty name and the fallback arm then prints async with nothing after it;
• still a bare at /app/x.js:9:1 for an at <anonymous> (...) frame.

This PR instead gives the re-parsed frames the Function code type the structured path gives them, so the existing Function arm renders all three shapes exactly as error.stack and the pre-.stack printout do. The two changes are independent in the code as well: #35685 edits populateStackFrameMetadata and ZigStackFrame.rs, this PR edits parseFrame and the re-parse callback in fromErrorInstance, and the tests added here hold with or without #35685 (none of them produce a nameless async frame).

[claude]

LGTM — small, display-only fix that brings the re-parsed error.stack path's frame classification in line with the structured path.

What was reviewed:

• isFunction placement in parseFrame: set after stripping async /new and before clearing <anonymous>, so it captures exactly "line had a name part"; the unknown early-return leaves it false so bare frames stay None.
• Callback branch order (isConstructor → isGlobalCode → isFunction) keeps new X and global code frames on their existing types even though isFunction may also be true.
• Cross-checked NameFormatter in src/jsc/ZigStackFrame.rs: only the FUNCTION arm renders async and <anonymous>, confirming the fix routes the right frames there; V8StackTraceIterator has no other consumers.
• Prior review nits (stdout drain, long comment) are addressed in 8e1f917 / f02aa48.

Extended reasoning...

Overview

Adds a bool isFunction field to V8StackTraceIterator::StackFrame in src/jsc/bindings/ZigException.cpp, sets it in parseFrame when the parsed line had a name part (i.e. name (url) or <anonymous> (url) rather than a bare url), and adds an else if (frame.isFunction) branch in the fromErrorInstance callback to mark such frames ZigStackFrameCodeFunction. Total production change: ~6 lines. Also adds a ~100-line describe block to test/js/bun/test/stack.test.ts with three tests covering an assigned stack string (all frame kinds, with and without colors), a real async chain asserting Bun.inspect output is identical before and after .stack is read, and a spawned subprocess checking console.error and the unhandled-rejection printer.

Security risks

None. This only affects how stack frame names are formatted in error output — pure display logic. No parsing of untrusted input into anything but display strings, no allocation or lifetime changes, no auth/crypto/permissions.

Level of scrutiny

Low-to-moderate. The change is tiny and mechanical: it makes the string-re-parse path assign the same code_type that populateStackFrameMetadata already assigns for structured JSC frames, so both producers hit the same NameFormatter arm. I read NameFormatter::fmt in src/jsc/ZigStackFrame.rs to confirm the PR description's claim that only the FUNCTION arm honors is_async and substitutes <anonymous>, and that the fallback arm prints the bare name — both hold. I also confirmed V8StackTraceIterator is defined and consumed only in ZigException.cpp (the ChromeBackend.cpp hit is a comment reference).

Other factors

• Placement of the flag assignment is correct: after the global code check (which clears the name and sets isGlobalCode), after stripping async and new , and before the <anonymous>-to-empty normalization. So <anonymous> frames get isFunction = true and are rendered as <anonymous> by the FUNCTION arm; global code frames get isFunction = false and keep ZigStackFrameCodeGlobal.
• Branch precedence in the callback: isConstructor and isGlobalCode are checked first, so a new X frame (which also has isFunction = true) correctly stays Constructor. Bare-location / unknown frames take the early return before isFunction is set, so they remain None and print as a bare location — nothing gains an invented <anonymous>.
• Tests cover the variant matrix well (async named, async anonymous, anonymous, new, plain, global code, bare unknown; colors on and off; before/after .stack; Bun.inspect, console.error, and unhandled-rejection paths). The subprocess test now drains stdout concurrently and asserts it empty per the earlier review nit.
• All three prior review comments (CodeRabbit and my own on stdout draining, comment-cop on the long comment) are resolved and reflected in the current diff. No outstanding reviewer concerns.

[claude]

LGTM — the earlier stdout-drain nit was addressed in 8e1f917.

What was reviewed:

• parseFrame sets isFunction after stripping async /new and clearing global code, but before clearing <anonymous>, so anonymous frames become Function while global/bare-unknown frames stay None — matches populateStackFrameMetadata's classification for the structured path.
• Confirmed against NameFormatter in src/jsc/ZigStackFrame.rs: the FUNCTION and fallback arms render named non-async frames identically, so only async and <anonymous> lines change.
• inspect.test.ts update strengthens the assertion (full equality instead of trimming the differing frame); new tests drain both pipes and cover the constructor/global/bare-location cases that must not change.

Extended reasoning...

Overview

Six lines of native change in src/jsc/bindings/ZigException.cpp: a new bool isFunction on V8StackTraceIterator::StackFrame, set in parseFrame when the line had a name part, and consumed in fromErrorInstance's re-parse callback to mark such frames ZigStackFrameCodeFunction. This aligns the string-re-parse producer of ZigStackFrames with the structured producer (populateStackFrameMetadata), which already marks function frames FUNCTION. Three new tests in test/js/bun/test/stack.test.ts and one updated snapshot in test/cli/inspect/inspect.test.ts.

Security risks

None. Pure display-formatting logic in the error printer — no parsing of untrusted network/file input, no allocation or lifetime changes, no auth/crypto/permission surface. The added field is a plain bool defaulting to false, so the early-return unknown path and any parse-failure path leave it unset.

Level of scrutiny

Low-to-medium. The change is small and mechanical: it fills in a code_type that the structured path already sets, routing frames through an existing formatter arm. I traced the flag placement in parseFrame — it is set after global code is cleared (so global frames stay Global via isGlobalCode), after async /new are stripped (so isConstructor still takes precedence in the callback's if/else if chain), and before <anonymous> is cleared (so anonymous frames get isFunction = true and the FUNCTION arm renders <anonymous>). I also read NameFormatter::fmt in src/jsc/ZigStackFrame.rs and confirmed the FUNCTION arm and the fallback arm produce identical output for named non-async frames, so those are unaffected; the GLOBAL arm prints nothing and the CONSTRUCTOR arm prints new name, both unchanged.

Other factors

All three prior review comments (comment-cop, coderabbit, and my own stdout-pipe nit) are resolved in the current diff — the C++ comment is one line, and the subprocess test now drains stdout/stderr/exited concurrently and asserts stdout is empty. The inspect.test.ts change is a strict tightening: it removes the trailing-frame carve-out and asserts the two outputs are byte-identical, which is exactly the invariant this fix restores. The new tests cover the full frame-shape matrix (async named, async anonymous, plain anonymous, new, plain named, global code, bare unknown), both color modes, and both Bun.inspect and the console.error/unhandled-rejection paths. The PR description documents the interaction with #38308, #38296 and #35685; none conflict textually or semantically with this change.