WebAssembly.Exception.getArg: an index that is not an unsigned long is a TypeError (WebKit bump for oven-sh/WebKit#613)

[robobun]

Problem

• WebAssembly.Exception.prototype.getArg(tag, index) throws RangeError: Expect an integer argument in the range: [0, 2^32 - 1] when index is -1, 2 ** 32, NaN, undefined or anything else that is not an unsigned long. Node 26 and Chromium throw TypeError. The spec IDL is [EnforceRange] unsigned long index ([js-api] Add missing EnforceRange to getArg argument. WebAssembly/exception-handling#257), and a failed [EnforceRange] conversion is a TypeError.
• The cause is in JSC: WebAssemblyExceptionPrototype.cpp asks its conversion helper for a RangeError. Upstream WebKit 282520@main did that to pass a WPT test that predates exception-handling#257 (V8 and Firefox mark that WPT subtest as an expected failure).

Fix

• Pin WebKit to autobuild-preview-pr-613-07a1e624, the preview build of [JSC] WebAssembly.Exception.prototype.getArg: an index that is not an unsigned long is a TypeError, not a RangeError WebKit#613, which makes getArg throw TypeError for a failed index conversion and keeps RangeError for an index past the payload.
• The pin is a preview tag, not a merge commit. Do not merge before [JSC] WebAssembly.Exception.prototype.getArg: an index that is not an unsigned long is a TypeError, not a RangeError WebKit#613 lands. I move the pin to the merged commit then. The preview also carries [JSC] Share one ScriptFetchParameters per type instead of allocating one per module request WebKit#561, Bun support for ARM based OS #566 and Fixes typo in src/c.zig #568, which are on WebKit main past the current pin.
• Verified: test/js/bun/wasm/js-api.test.ts (new, 19 tests). bun 1.4.3 fails the 13 getArg error-class cases, bun bd test with this pin (debug + ASAN) passes all 19. test/js/bun/wasm/wasi.test.js and test/js/web/workers/structured-clone.test.ts pass with the pin.

Background

• WebIDL [EnforceRange] unsigned long: NaN and the infinities throw TypeError, then the integer part must be in [0, 2^32 - 1] or it throws TypeError. So getArg(tag, -0.5) and getArg(tag, 0.9) read argument 0.
• The same test file pins the WebAssembly.Memory buffer rules that bun already follows and that the same differential run questioned: a non-shared memory detaches and replaces its ArrayBuffer on every grow, even grow(0), while a shared memory hands out a new SharedArrayBuffer only when its length changed. V8 main (crrev.com/c/7660970), SpiderMonkey and [threads][test][js-api] Add tests for growing shared memory WebAssembly/threads#248 agree. Node 26 ships an older V8 that replaces the SharedArrayBuffer on grow(0).

Notes

• [JSC] WebAssembly.Exception.prototype.getArg: an index that is not an unsigned long is a TypeError, not a RangeError WebKit#613 has the JSC-side verification: the new JSTests/wasm/js-api/exception-getarg-index-enforce-range.js in all 13 wasm modes, the full JSTests/wasm.yaml run (17940 runs), and a Debug+ASan run with exception-check validation.
• Source for the Node and Chromium comparison: a WebAssembly JS-API differential run against node v26.3.0 and Chromium 148.

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[policy-decision:webkit] gate passed · iteration 0 · 2 files touched

passes on PR (with fix)

Test-only change.

Debug/ASAN (expected pass):
$ bun bd test 'test/js/bun/wasm/js-api.test.ts'
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test test/js/bun/wasm/js-api.test.ts
bun test v1.4.3 (f42e98025)

test/js/bun/wasm/js-api.test.ts:
(pass) WebAssembly.Exception.prototype.getArg > index -1 is a TypeError [8.56ms]
(pass) WebAssembly.Exception.prototype.getArg > index -1.5 is a TypeError [0.67ms]
(pass) WebAssembly.Exception.prototype.getArg > index 4294967296 is a TypeError [0.46ms]
(pass) WebAssembly.Exception.prototype.getArg > index 4294967296.5 is a TypeError [0.36ms]
(pass) WebAssembly.Exception.prototype.getArg > index 9007199254740992 is a TypeError [0.42ms]
(pass) WebAssembly.Exception.prototype.getArg > index 1.7976931348623157e+308 is a TypeError [0.62ms]
(pass) WebAssembly.Exception.prototype.getArg > index NaN is a TypeError [0.38ms]
(pass) WebAssembly.Exception.prototype.getArg > index Infinity is a TypeError [0.36ms]
(pass) WebAssembly.Exception.prototype.getArg > index -Infinity is a TypeError [0.56ms]
(pass) WebAssembly.Exception.prototype.getArg > index undefined is a TypeError [0.32ms]
(pass) WebAssembly.Exception.prototype.getArg > index "x" is a TypeError [0.32ms]
(pass) WebAssembly.Exception.prototype.getArg > index {} is a TypeError [0.62ms]
(pass) WebAssembly.Exception.prototype.getArg > an index that converts is truncated toward zero [3.33ms]
(pass) WebAssembly.Exception.prototype.getArg > a valid index past the payload is a RangeError [8.79ms]
(pass) WebAssembly.Memory grow and the buffer object > non-shared memory, Memory.prototype.grow: grow(0) detaches and replaces the buffer [9.88ms]
(pass) WebAssembly.Memory grow and the buffer object > shared memory, Memory.prototype.grow: grow(0) keeps the buffer, a real grow replaces it [5.98ms]
(pass) WebAssembly.Memory grow and the buffer object > non-shared memory, memory.grow instruction: grow(0) detaches and replaces the buffer [13.25ms]
(pass) WebAssembly.Memory grow and the buffer object > shared memory, memory.grow instruction: grow(0) keeps
... (truncated)
Exit: 0

diff hotspot

scripts/build/deps/webkit.ts    |   2 +-
 test/js/bun/wasm/js-api.test.ts | 149 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 150 insertions(+), 1 deletion(-)

gate history · 1 passed · 0 rejected · iteration 0

evidence per changed file

file                             reads  edits  tests
scripts/build/deps/webkit.ts         0      0     13
test/js/bun/wasm/js-api.test.ts      2      1     12

[robobun]

^{Updated 3:54 AM PT - Sep 9th, 2026}

✅ @robobun, your commit 93663f762b78ad90d2433caa4304eb3abad7efb7 passed in Build #113548! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 42135

That installs a local version of the PR into your bun-42135 executable, so you can run:

bun-42135 --bun

[robobun]

Status: ready for review once oven-sh/WebKit#613 merges (the pin then moves to the merged commit).

Reproduction on bun 1.4.3:

const tag = new WebAssembly.Tag({ parameters: ["i32"] });
const ex = new WebAssembly.Exception(tag, [5]);
for (const idx of [2 ** 32, -1, NaN, undefined, 1, 0]) {
  try { console.log(idx, "->", ex.getArg(tag, idx)); }
  catch (e) { console.log(idx, "->", e.constructor.name); }
}
// bun 1.4.3: RangeError x4, RangeError, 5
// node 26 / this PR: TypeError x4, RangeError, 5

Verification: USE_SYSTEM_BUN=1 bun test test/js/bun/wasm/js-api.test.ts fails 13 of 19, bun bd test test/js/bun/wasm/js-api.test.ts with this pin passes 19 of 19.

[coderabbitai]

Walkthrough

The change updates the WebKit build version to an autobuild preview identifier and adds WebAssembly JavaScript API tests for exception argument conversion and shared and non-shared memory growth behavior.

Changes

WebKit build configuration

Layer / File(s)	Summary
Update WebKit build version scripts/build/deps/webkit.ts	WEBKIT_VERSION now uses the autobuild-preview-pr-613-07a1e624 release identifier.

WebAssembly JavaScript API tests

Layer / File(s)	Summary
Add WebAssembly test helpers test/js/bun/wasm/js-api.test.ts	Helpers construct WebAssembly binaries and report thrown error class names.
Test exception and memory APIs test/js/bun/wasm/js-api.test.ts	Tests cover WebAssembly.Exception.prototype.getArg conversion and error precedence. Memory tests cover growth, buffer detachment, replacement, identity, aliasing, and independence for shared and non-shared memories.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 93663

The current WebKit preview pin is usable for this change. Replace it with the merged commit when available to preserve future clean-build reliability.

🚥 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 identifies the corrected WebAssembly.Exception.getArg index behavior and the related WebKit dependency update.
Description check	✅ Passed	The description explains the problem, fix, verification results, affected WebKit preview pin, and test coverage. It does not use the exact template headings, but it provides the required content and i…

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@scripts/build/deps/webkit.ts`:
- Line 6: Update the WEBKIT_VERSION constant after WebKit#613 merges, replacing
the preview tag with the immutable merged commit SHA. Leave the current release
tag unchanged until that merge is complete.

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📥 Commits

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📒 Files selected for processing (2)

• scripts/build/deps/webkit.ts
• test/js/bun/wasm/js-api.test.ts

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[claude]

🔴 WEBKIT_VERSION is pinned to an ephemeral autobuild-preview-pr-* tag; GitHub deletes that release the moment oven-sh/WebKit#613 merges or closes, so any bun build after that point 404s on the prebuilt tarball for every platform (download.ts:323 has the dedicated error for exactly this). The PR description already says "do not merge before #613 lands" — this comment is the merge-blocker tracking that. Fix: before merge, swap this to the merged oven-sh/WebKit main SHA (40-hex) and confirm the autobuild-<sha> release has artifacts for every os×arch×{debug,lto,asan} flavor prebuiltSuffix() produces.

Extended reasoning...

prebuiltUrl() (webkit.ts:73) builds https://github.com/oven-sh/WebKit/releases/download/autobuild-preview-pr-613-07a1e624/.... Per download.ts:314-331, preview releases exist only while the WebKit PR is open; once #613 merges the release is deleted and every fresh build (CI and local cold cache) hits HTTP 404 in prebuiltDownloadError. On base the pin is a permanent 40-hex SHA whose autobuild-<sha> release is never deleted. The repo's dependency-bump policy ("Never merge a pin to an ephemeral artifact") and .claude/commands/upgrade-webkit.md:34 both require bumping to the merge-commit SHA before landing the bun PR. Nothing else in the repo breaks on the tag form: sync-webkit-source.ts resolves tags via git rev-parse, generate-protocol.ts and prebuiltDestDir() both handle the autobuild- prefix, and test/internal/source-lints/webkit-prebuilt-url.test.ts:122 accepts autobuild-*.

Verification: normal — acknowledged in diff: the PR description states "The pin is a preview tag, not a merge commit. Do not merge before oven-sh/WebKit#613 lands. I move the pin to the merged commit then." That note is accurate but does not resolve the hazard; the merge-blocker still needs to be tracked on the line. scripts/build/deps/webkit.ts:6 changes WEBKIT_VERSION from the 40-hex SHA… | normal —…

[robobun]

Agreed, and intended: this thread stays open as the merge blocker. When oven-sh/WebKit#613 merges I push the 40-hex merged SHA here, check that its autobuild-<sha> release carries every os, arch and debug/lto/asan tarball, and resolve this thread then.