Bump WebKit (oven-sh/WebKit#475 preview): a property lookup stops at a lazy property whose builder threw

[robobun]

Problem

• After one read of Bun.sql whose builder throws, every megamorphic access site returns undefined for Bun.sql (false for in) and never runs the builder again (release Bun 1.4.0). On debug builds the same read aborts: under BUN_JSC_validateExceptionChecks=1 with a Proxy prototype behind Bun (unchecked as of this scope: getNonIndexPropertySlot @ JSObjectInlines.h:286), or in Structure::storedPrototype when the builder transitions Bun first (Fix stale-structure abort when a lazy Bun property builder throws mid-lookup #37001).
• Cause: setUpStaticFunctionSlot (WebKit Lookup.cpp:73) reports the throw as a miss and every lookup loop walks on. The megamorphic paths in JITOperations.cpp then record the miss for Bun's structure, which the throw did not change.
• reifyAllStaticProperties also runs the builders of {...Bun} without checking between them, which keeps this test file in test/no-validate-exceptions.txt.

Fix

• Stop a property lookup after a static-table PropertyCallback builder throws WebKit#475, pinned as its preview. Each lookup loop checks for an exception after the own-property step on a static-table object and stops. reifyAllStaticProperties checks after each builder. Land that PR first, then set WEBKIT_VERSION to the merged sha.
• Correct because only a static table can make that step run a builder, so other objects are unchanged, and it is the check the loops already make after an overridden getOwnPropertySlot. Nothing runs after the throw, so nothing records the miss or reads the stale structure.
• Tests: four new tests in test/js/bun/util/BunObject.test.ts, the process.env workaround above them removed, the file un-quarantined. On the current pin the four fail with the symptoms above and the file aborts under the validator. Also ran bun/util, bun/jsc, globals under the validator.
• Supersedes the engine half of Fix stale-structure abort when a lazy Bun property builder throws mid-lookup #37001 (JSObject::getPropertySlot: reload the structure before the prototype step WebKit#390) and carries its test.

Background

• Bun properties are static-table entries with a builder. The first read runs it and stores the result. In this fork a builder can throw. It then stores nothing, and the next read runs it again.
• The own-property step returns only hit or miss. Six loops call it and walk to the prototype on a miss. Spread and Object.assign reify every entry in one loop instead.
• The megamorphic cache is a VM-wide table keyed by structure and property name. A recorded miss is valid only while the property cannot appear without a structure change.
• The validator is a debug mode: a caller must check every scope that ran. The ASAN lane runs tests with it, except the quarantined files.

Notes

Megamorphic repro on release Bun 1.4.0 (read trained on 8 or more shapes; with 4 shapes or BUN_JSC_useJIT=0 the retry works):

const RealSymbol = Symbol;
globalThis.Symbol = NaN;
function read(o) { return o.sql; }
// ... call read() with 8+ differently shaped objects a few thousand times ...
try { read(Bun); } catch {}          // TypeError from the builder
globalThis.Symbol = RealSymbol;
read(Bun);                           // undefined; a fresh `Bun.sql` is a function

The six loops: JSObject::getPropertySlot, JSObject::getNonIndexPropertySlot, and the get_by_id, get_by_val, in_by_id, in_by_val megamorphic helpers (the with_this forms share them). The parent-class-table loop in getOwnStaticPropertySlot gets the same stop. The in helpers record into a separate has-cache, so in sites are poisoned independently of get sites.

Relation to #37001 / oven-sh/WebKit#390: that change reloads the structure before the prototype step. With oven-sh/WebKit#475 a throwing builder returns before that step, so the assertion it fixes is unreachable and its test (the Error proxy test here, reworded) passes on #475 alone. Both PRs pin a different preview on the same line, so whichever lands second has to re-pin anyway. Suggested order: land oven-sh/WebKit#475 (with or after #390), re-pin here, close #37001. #39436 and #38821 name #37001 as their engine prerequisite; this landing is the one they need.

Validator findings while testing (both in the WebKit PR): a builder that succeeds through getNonIndexPropertySlot was reported as unchecked by that loop's own scope, so the check there is made on a hit as well. reifyAllStaticProperties (spread, Object.assign, Object.entries, delete) runs builders back to back, so ({...Bun}) aborted with defaultBunSQLObject unchecked as of defaultBunSQLObject; it now uses a TopExceptionScope and checks after each builder. That is what un-quarantines this file. Of the other quarantined files, resolve/import-meta.test.js and resolve/resolve.test.ts already pass under the validator on the current pin, and node/module/*.test.js abort on unchecked scopes in NodeModuleModule.cpp; both are unrelated to this change and left alone.

Not done: the self-review also suggested skipping the store in reifyStaticProperty when a builder returns a value with an exception already pending. A builder that throws returns empty and stores nothing today, which is the case these tests rely on. The other case is a builder run with an exception already pending, which is a caller bug, and skipping the store there would make it run again later. Left as is.

Test notes: Symbol stays broken for both accesses of each megamorphic site because the three sql properties share one module, so once any read loads it the other builders stop throwing. Each site has its own property (sql, postgres, SQL, $) because the record is per name. On Windows the $ builder reifies Bun.inspect before it throws, so there that site is the transition-then-throw case through the megamorphic path. The removed process.env pre-read worked around the same assertion on Windows; the assertion is in the prototype step that is no longer reached.

Fail-before was taken on a debug ASAN build of this branch with WEBKIT_VERSION set to 0f966e81 (main's pin): receiver Bun aborts at getNonIndexPropertySlot @ JSObjectInlines.h:286, the function receiver at ProxyObject::getOwnPropertySlotCommon, the transition test at StructureInlinesLight.h(56) storedPrototype, the megamorphic test prints ["TypeError","undefined"] twice and ["TypeError",false] twice, and the whole file under BUN_JSC_validateExceptionChecks=1 aborts in hasNonReifiedStatic. Pass-after: 8 of 8 plain and under the validator against the published preview tarball on Linux x64 debug ASAN (and before that against a locally built JSC from the branch), and 8 of 8 plain and under the validator on a Windows x64 debug build. The same Windows build with the pin set to 0f966e81 fails 5 tests, including the test whose process.env pre-read is removed here (its child aborts on the structure assertion), so the Windows quirk that pre-read worked around is real on the old pin and gone with the new one. With either pin, fuzzy-wuzzy.test.ts aborts on a debug build (_http_outgoing $assert, or, with a Redis server reachable, debug_assert!(self.is_subscriber()) in js_valkey.rs:1320 when new Bun.RedisClient().punsubscribe() gets its reply), and the large DOMJIT variants and the process.env.USER check fail in this container. The DOMJIT timings are the same with either pin (7.0 s against 7.1 s for 200k new TextEncoder().encode() calls on the same host). When bun/util runs as one batch in one process, exotic-global-mutable-prototype.test.ts fails after BunObject.test.ts because the first test in that file sets globalThis.a (unchanged by this PR); the same happens with the stock pin, and CI runs each file in its own process.

Rebases: main moved its pin thirteen times while this was open, to b7f217b4 (#39829, oven-sh/WebKit#477), aea1f010 (#35343, oven-sh/WebKit#330), c148a12d (#40201, oven-sh/WebKit#494), cb61607f (#40276, the 8c4fd56347 upstream merge), 1cb96a7b (#40417, oven-sh/WebKit#513), 76882271 (#40507), 2da33d53 (#40570, oven-sh/WebKit#519), 72597399 (#40270, oven-sh/WebKit#521), 0bb01ed5 (#40643), f5deafe0 (#40674), 1817c3c3 (#40681, the 6b879687ee upstream merge), c4ddc0cf (#40677, oven-sh/WebKit#527) and ceb9f90f (#40767, oven-sh/WebKit#530 and #531). oven-sh/WebKit#475 was rebased onto each (current head 94c5a2d5, the same four-file diff, applied without changes each time; the 6b879687ee merge touched JITOperations.cpp only in operationPolymorphicCall, none of the lookup loops). Two of those heads (17a4e95f, build 106127, and b3532b38, build 106406) passed every CI job and this PR pins the matching preview on top of main, squashed to one commit. The only conflict each time was the WEBKIT_VERSION line, plus #40065's rework of test/no-validate-exceptions.txt, which still lists BunObject.test.ts for the builder reason this change removes. Re-verified against each new preview on Linux x64 debug ASAN: this file 8 of 8 plain and under the validator, the related files above (178 tests), #39829's ffi-ptr-non-view-cell-arg stress fixture and node/buffer.test.js (678 pass) for the new bases.

Landing detail: the preview release is deleted when oven-sh/WebKit#475 closes, so the pin must be swapped to the merged sha (and the comment above it removed) before this merges.

---

[decide:webkit] gate passed · iteration 14 · 3 files touched

passes on PR (with fix)

Test-only change.

Debug/ASAN (expected pass):
$ bun bd test 'test/js/bun/util/BunObject.test.ts'
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test test/js/bun/util/BunObject.test.ts
bun test v1.4.1 (65362b53b)

test/js/bun/util/BunObject.test.ts:
(pass) hasNonReifiedStatic [254.08ms]
(pass) require('bun') [5.30ms]
Module {
  $: [Function: BunShell2],
  Archive: [class Archive],
  ArrayBufferSink: [class ArrayBufferSink],
  CSRF: {
    generate: [Function: generate],
    verify: [Function: verify],
  },
  Cookie: [class Cookie],
  CookieMap: [class CookieMap],
  CryptoHasher: [class CryptoHasher],
  FFI: {
    viewSource: [Function: viewSource],
    dlopen: [Function: dlopen],
    callback: [Function: callback],
    linkSymbols: [Function: linkSymbols],
    toBuffer: [Function: toBuffer],
    toArrayBuffer: [Function: toArrayBuffer],
    closeCallback: [Function: closeCallback],
    cfunction: [Function: cfunction],
    CString: [class CString],
    ptr: [Function: ptr],
    read: {
      u8: [Function: u8],
      u16: [Function: u16],
      u32: [Function: u32],
      ptr: [Function: ptr],
      i8: [Function: i8],
      i16: [Function: i16],
      i32: [Function: i32],
      i64: [Function: i64],
      u64: [Function: u64],
      intptr: [Function: intptr],
      f32: [Function: f32],
      f64: [Function: f64],
    },
  },
  FileSystemRouter: [class FileSystemRouter],
  Glob: [class Glob],
  Image: [class Image],
  JSON5: {
    parse: [Function: parse],
    stringify: [Function: stringify],
  },
  JSONC: {
    parse: [Function: parse],
  },
  JSONL: JSONL {
    parse: [Function: parse],
    parseChunk: [Function: parseChunk],
  },
  MD4: [class MD4],
  MD5: [class MD5],
  RedisClient: [class RedisClient],
  S3Client: [class S3Client],
  SHA1: [class SHA1],
  SHA224: [class SHA224],
  SHA256: [class SHA256],
  SHA384: [class SHA384],
  SHA512: [class SHA512],
  SHA512_256: [class SHA512_256],
  SQL: [Function: SQL2],
  TOML: {
    parse: [Function: parse],
    stringify: [Function: stringify],
  },
  Terminal: [class Te
... (truncated)
Exit: 0

diff hotspot

scripts/build/deps/webkit.ts       |   7 +-
 test/js/bun/util/BunObject.test.ts | 145 +++++++++++++++++++++++++++++++++++--
 test/no-validate-exceptions.txt    |   1 -
 3 files changed, 144 insertions(+), 9 deletions(-)

gate history · 14 passed · 0 rejected · iteration 14

evidence per changed file

file                                reads  edits  tests
scripts/build/deps/webkit.ts            9     12      0
test/js/bun/util/BunObject.test.ts      4      5      0
test/no-validate-exceptions.txt         2      2      0

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---

Walkthrough

Changes

Lazy-property exception handling

Layer / File(s)	Summary
WebKit preview pin scripts/build/deps/webkit.ts	Updates the WebKit pin to autobuild-preview-pr-475-94c5a2d5 and documents its property-lookup and exception-scope changes.
Lazy-property validation test/js/bun/util/BunObject.test.ts, test/no-validate-exceptions.txt	Adds subprocess tests for reentrant builders, inherited receivers, megamorphic access, exception propagation, and retry behavior. Removes BunObject.test.ts from the ASAN exception-validation exclusion list.

Suggested reviewers: dylan-conway, jarred-sumner

Merge Risk: 🟡 Moderate · up to 07e34

This change fixes lazy Bun property lookups after a throwing builder and the targeted tests pass, but the current build still depends on a temporary WebKit preview that must be replaced with the merged immutable SHA before merge to avoid unavailable or non-reproducible builds. Owner awareness is also needed for validator-focused tests that may not prove the validation path is active.

🚥 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 WebKit dependency bump and the lazy-property lookup failure that the change addresses. It is specific, but longer than necessary.
Description check	✅ Passed	The description explains the problem, fix, scope, testing, and landing requirement. It does not use the exact template headings, but it provides the required information in equivalent sections.

---

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[robobun]

Status: the engine change is oven-sh/WebKit#475 (head 94c5a2d5, rebased onto ceb9f90f, main's current pin). This PR pins its preview build, adds the tests and takes BunObject.test.ts out of test/no-validate-exceptions.txt.

CI runs on the current head (07e34b96, the thirteenth rebase, pushed after the new preview was built and tested locally). The previous head (54b4953c, build 107653) passed 180 of 181 jobs, as did eb67c99c (build 107164) and b4304f4e (build 107220); the one failed job each time was macOS x64 test-bun on test/js/web/url/url.test.ts (special-scheme hosts use the Unicode 16 IDNA table), which fails on main builds too (a system ICU version difference on that agent) and is reported to main-break triage. None of these failures involve this change. The ASAN lane ran BunObject.test.ts with the exception validator, now that the file is out of no-validate-exceptions.txt, and passed. Two earlier heads of the same change passed every CI job (17a4e95f, build 106127, and b3532b38, build 106406).

The PR is ready to merge once oven-sh/WebKit#475 has landed and the pin has been swapped to the merged sha.

Reproduced with:

• Release Bun 1.4.0: a get_by_id site trained on 8 or more object shapes, then read(Bun) with the sql builder made to throw. The next read(Bun) returns undefined while a fresh Bun.sql returns the function. in, get_by_val and in_by_val sites behave the same way.
• Debug build on the pin at the time (0f966e81): Object.setPrototypeOf(Bun, new Proxy(Object.prototype, {})), then a throwing read of Bun.sql under BUN_JSC_validateExceptionChecks=1 aborts at getNonIndexPropertySlot @ JSObjectInlines.h:286. The same read through a function that inherits from Bun aborts at ProxyObject::getOwnPropertySlotCommon. A builder that reifies another Bun property and then throws aborts at StructureInlinesLight.h(56) (the Fix stale-structure abort when a lazy Bun property builder throws mid-lookup #37001 case). The whole test file aborts under the validator in its first test, which spreads Bun.

The four new tests in test/js/bun/util/BunObject.test.ts fail this way against 0f966e81, and the file passes plain and under the validator against the published preview on Linux x64 debug ASAN and on a Windows x64 debug build (details in the comment below).

[robobun]

^{Updated 8:08 AM PT - Aug 28th, 2026}

✅ @robobun, your commit 07e34b96964a3b0a66258d1d8e314f9c20d1120a passed in Build #107728! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 39703

That installs a local version of the PR into your bun-39703 executable, so you can run:

bun-39703 --bun

[robobun]

On the "$" in Bun note (review thread on line 152): the $ builder is kept on purpose, and no process.env pre-read is needed with this pin.

The assertion the sibling test works around is Structure::storedPrototype (StructureInlinesLight.h:56, object->structure() == this). In lookup code it is only reached from the prototype step of JSObject::getPropertySlot (JSObject.h), which used the Structure* read before the builder ran. That is what #37001 fixes with a reload. oven-sh/WebKit#475 returns before that step when the builder threw, so the step is not reached whether or not the builder transitioned Bun first. The two ways "$" in Bun can run in this test both end before it:

• megamorphic site (the expected case): inByValMegamorphic reloads the structure and then returns on the new exception check,
• not yet megamorphic: opInByVal -> JSObject::hasProperty -> getPropertySlot, which returns on the new check before storedPrototype.

So on Windows the $ site is the one access in the test in which the builder transitions Bun and then throws, which is the route #37001 is about, taken through the new check. That is why it stays. Windows CI on this PR will confirm it once the preview build exists.

The sibling test's pre-read is left as it is because #37001 removes it, and changing those lines here would conflict with that PR.

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Actionable comments posted: 2

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Inline comments:
In `@scripts/build/deps/webkit.ts`:
- Around line 6-11: Update the WEBKIT_VERSION constant from the unavailable
preview identifier to the merged PR 475 commit SHA, keeping the dependency pin
blocked until that merge is available; do not add a process.versions.webkit
assertion.

In `@test/js/bun/util/BunObject.test.ts`:
- Line 104: Import isDebug and isASAN from harness, then skip both
validator-dependent tests in BunObject.test.ts at the sites around lines 104 and
141 when !isDebug && !isASAN, providing a clear skip reason; leave other tests
unchanged.

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[robobun]

On the two review notes:

• Build-mode guard for the tests that set BUN_JSC_validateExceptionChecks=1: left as they are. On a release build the option is inert and the tests still check the behaviour that matters there (the builder's exception reaches the caller, phase 1 proves the re-entrant path ran, and the next read runs the builder and succeeds). On debug and ASAN builds the same runs also catch the unchecked scopes and the structure assertion, which is the part that failed before the pin. This is how the other tests that set this option are written (web-crypto.test.ts, macro-test.test.ts, worker-terminate-lifetime.test.ts): none of them is gated on the build mode.
• The preview pin: expected. The tag is published by Stop a property lookup after a static-table PropertyCallback builder throws WebKit#475's preview build (the release for the current head does not exist yet, which is why CI here is red). The PR description and the status comment above list the swap to the merged sha as the landing step.

[claude]

I reviewed 57c2e89 and didn't find any bugs. The earlier note about "$" in Bun on Windows is resolved — the follow-up commit removes the process.env; workaround from the sibling test too, and the rebuttal (the storedPrototype assertion lives in the prototype step that #475 no longer reaches after a throw) is sound. A human look is still warranted: this is a WebKit engine bump touching JSC's property-lookup hot paths, it's pinned to a preview tag that must be swapped for the merged sha, and there's a cross-repo landing order with oven-sh/WebKit#475 and #37001 to coordinate.

What was reviewed:

• The four new subprocess tests — pipes drained concurrently, exact JSON assertions, test.concurrent, each has a stated fail-before against pin 0f966e81.
• The removed process.env; workaround and updated comment — consistent with the fix returning before storedPrototype.
• The no-validate-exceptions.txt un-quarantine — matches the reifyAllStaticProperties half of the WebKit change; other entries left alone as stated.
• prebuiltDestDir / prebuiltUrl in webkit.ts already handle autobuild--prefixed version strings.

Extended reasoning...

Overview

This PR bumps WEBKIT_VERSION from 0f966e81 to autobuild-preview-pr-475-43f67cb4 (a preview build of oven-sh/WebKit#475), adds four regression tests to test/js/bun/util/BunObject.test.ts covering the throwing-lazy-builder bug class (transition-then-throw, Proxy-prototype walk, megamorphic miss-cache poisoning), removes the process.env; Windows workaround from the existing sibling test, and takes the file out of test/no-validate-exceptions.txt. The actual engine fix — exception checks after the own-property step in six lookup loops and per-builder in reifyAllStaticProperties — lives in the WebKit PR, not here.

Security risks

None identified. The change adds exception checks in JSC lookup paths (fail-earlier, not new capability) and the Bun-side diff is a version pin, tests, and a quarantine-list removal. No auth, crypto, or untrusted-input parsing is touched.

Level of scrutiny

High. WebKit is the JS engine; the touched paths (JSObject::getPropertySlot, getNonIndexPropertySlot, the four megamorphic JIT slow paths, reifyAllStaticProperties) are among the hottest and most correctness-critical in JSC. Per the repo's own guidance, dependency/vendor bumps and engine changes warrant maintainer review. Additionally, the pin is explicitly a preview tag that will be deleted when the upstream PR closes — the description itself says the pin must be swapped to the merged sha before this merges, so a human needs to sequence the landing regardless.

Other factors

• Prior review: my earlier inline nit about the $ site on Windows was answered and resolved; commit 57c2e89 went further and removed the sibling test's workaround, which is internally consistent with the claim that #475 returns before the storedPrototype step. Windows CI will be the arbiter once the preview release exists.
• Test quality: the new tests follow harness conventions (await using, Promise.all on stdout/stderr/exited, combined-object assertions, test.concurrent, per-test bunEnv spread). Each documents its failure mode against the old pin, and the megamorphic test includes an afterwards: "function" post-condition so it can't pass vacuously.
• Coordination: the description lays out an explicit landing order relative to oven-sh/WebKit#390 / #37001, and names #39436 and #38821 as downstream. That sequencing is a human decision.
• Cannot verify locally: the engine diff is in another repo and the preview release may not exist yet, so CI here is the only proof of correctness — another reason not to auto-approve.

[robobun]

One correction to the note above: CI cannot arbitrate the Windows part. The Windows lanes build the release profile, and both the storedPrototype assertion and the exception validator exist only in asserts builds. The only asserts lane in PR CI is linux x64 ASAN, and that lane does run this file with the validator now that it is out of no-validate-exceptions.txt. For the two Windows-specific claims (the $ site in the megamorphic test, and the removed process.env pre-read) I will run this file on a Windows x64 debug build against the preview once its Windows debug tarball exists, and add the result here.

[robobun]

Windows x64 debug result, as promised above. Built this branch on Windows x64 with the debug profile (asserts build, bun-webkit-windows-amd64-debug tarball), once against the preview pin of this PR and once with WEBKIT_VERSION set back to 0f966e81, and ran test/js/bun/util/BunObject.test.ts with the same binary each time.

Preview autobuild-preview-pr-475-43f67cb4: 8 of 8 pass, plain and with BUN_JSC_validateExceptionChecks=1 set for the whole file.

Pin 0f966e81: 3 pass, 5 fail.

• "a lazy property whose builtin fails to load throws from the read" (the test whose process.env pre-read this PR removes): the child aborts (exit code 3, the Windows abort code). This is the structure assertion the pre-read worked around, so the quirk is real on this pin and the removal depends on the engine change.
• "reifies another property of Bun and then throws": child aborts (exit code 3).
• The two receiver tests: validator aborts at getNonIndexPropertySlot @ JSObjectInlines.h:286 and ProxyObject::getOwnPropertySlotCommon, as on Linux.
• The megamorphic test prints ["TypeError","undefined"] twice and ["TypeError",false] twice. The in_by_val site is the $ builder, which on Windows transitions Bun before it throws, so the miss is recorded against the new structure and the second access still hits it.

Linux x64 debug ASAN against the published tarball (not the local build used earlier): 8 of 8 plain and under the validator, plus the related bun/util, bun/jsc, globals, import-meta and namespace-prototype-pollution files (176 tests).

[robobun]

Rebased onto main after #39829 moved the pin to b7f217b4 (oven-sh/WebKit#477). oven-sh/WebKit#475 is rebased onto that as well (head 3438defc, the same four-file diff), and this PR now pins autobuild-preview-pr-475-3438defc, squashed to one commit. The only conflict was the WEBKIT_VERSION line. The test changes are byte for byte what was reviewed before.

Re-verified on Linux x64 debug ASAN against the new preview: this file 8 of 8 plain and under the validator, the related bun/util, bun/jsc, globals, import-meta and namespace-prototype-pollution files (176 tests), and #39829's ffi-ptr-non-view-cell-arg stress fixture, which confirms the preview carries the new base. CI is running against it now.

[claude]

I reviewed this PR and didn't find any bugs. Since it's a WebKit engine bump whose upstream change touches the core property-lookup loops (getPropertySlot, getNonIndexPropertySlot, the megamorphic JIT slow paths, reifyAllStaticProperties), and the pin is still a preview tag pending the oven-sh/WebKit#475 merge + sha swap, a human sign-off is still warranted.

What was reviewed:

• The four new subprocess tests: pipes drained concurrently, bunEnv spread correctly, assertions are exact JSON on {stdout, stderr, exitCode}, phase guard proves the trap ran.
• The removed process.env; pre-read and my earlier Windows $ concern — both verified against a Windows debug build in the thread.
• The no-validate-exceptions.txt removal is consistent with the reifyAllStaticProperties fix in the pinned WebKit change.

Extended reasoning...

Overview

This PR carries the Bun-side half of an engine fix that lives in oven-sh/WebKit#475: it updates WEBKIT_VERSION in scripts/build/deps/webkit.ts to a preview tag of that PR, adds four subprocess tests to test/js/bun/util/BunObject.test.ts covering the throwing-lazy-builder scenarios (transition-then-throw, prototype-walk with a Proxy behind Bun, function receiver via getNonIndexPropertySlot, and megamorphic get/in caching), removes the process.env; workaround from the existing test, and takes the file out of test/no-validate-exceptions.txt.

Security risks

None identified. The change is a dependency pin to an oven-sh–controlled WebKit release plus test additions; there is no new user-facing surface, parsing of untrusted input, or credential/auth logic in this diff.

Level of scrutiny

High. The behavioral change is entirely in the pinned WebKit build, and it modifies the property-lookup loops in JSObjectInlines.h / JITOperations.cpp and reifyAllStaticProperties — among the hottest and most correctness-critical paths in the engine. That code cannot be reviewed from this diff. Per the repo guidance on dependency bumps and the explicit landing gate in the description (preview tag is deleted when the WebKit PR closes; must swap to the merged sha), this needs human coordination rather than auto-approval.

Other factors

• The pin in the current diff is autobuild-preview-pr-475-3438defc, updated from the 43f67cb4 head referenced throughout the earlier thread — the WebKit PR head moved and this needs a green CI run against the new artifact.
• All prior review threads (my Windows $ concern, CodeRabbit's build-mode-gating and preview-pin notes) are resolved with detailed responses, including a Windows x64 debug fail-before/pass-after run.
• The tests themselves follow harness conventions cleanly (test.concurrent, await using, combined-object assertion, bunEnv spread) and each asserts observable behavior that holds on release builds, so BUN_JSC_validateExceptionChecks=1 being inert there does not make them vacuous.

[robobun]

The Windows failures in build 102256 (head fa0105ef) were caused by the commit message, not by the change. Every test process on the two Windows lanes exited with:

error: invalid JSC environment variable
    BUN_JSC_validateExceptionChecks=1. The WebKit change checks for the

That value is the rest of a line of the commit body, which after wrapping started with BUN_JSC_validateExceptionChecks=1.. On the Windows test lanes a commit body line of the form NAME=value ends up in the environment of the test processes, so bun rejected the option value at startup in every file on those lanes (this is also why BunObject.test.ts is in that list). The Linux, macOS and FreeBSD lanes were unaffected.

Head 67d2aa5c is the same tree with the commit message reworded so that no line has that shape (git diff fa0105ef 67d2aa5c is empty). CI is running again. The underlying behaviour of the Windows lanes is worth knowing about for anyone who mentions an environment variable at the start of a line in a commit body.

[robobun]

Rebased onto main again: #35343 moved the pin to aea1f010 (oven-sh/WebKit#330) and #40065 reworked test/no-validate-exceptions.txt (it keeps BunObject.test.ts listed, with the PropertyCallback builders as the reason, which is what the WebKit change here addresses, so this PR still removes that line). oven-sh/WebKit#475 is rebased onto aea1f010 (head cb224872, the same four-file diff, no overlap with #330). The test changes are unchanged.

The new preview build is in progress. I will push once it is published and the branch has been rebuilt and tested against it, so that CI runs against a release that exists.

[robobun]

Pushed the rebase as eda85e47: one commit on top of main, pin autobuild-preview-pr-475-cb224872 (oven-sh/WebKit#475 on aea1f010), the test changes and the quarantine removal unchanged. The preview release exists, so CI runs against a real tarball this time.

Re-verified on Linux x64 debug ASAN against that preview: this file 8 of 8 plain and under BUN_JSC_validateExceptionChecks=1, the related bun/util, bun/jsc, globals, import-meta and namespace-prototype-pollution files (178 tests), node/buffer.test.js (678 pass, the new base's Buffer intrinsics) and #39829's ffi-ptr-non-view-cell-arg fixture.

[robobun]

Rebasing onto main a third time: #40201 moved the pin to c148a12d (oven-sh/WebKit#494) and #40229 trimmed test/no-validate-exceptions.txt (it still lists BunObject.test.ts). oven-sh/WebKit#475 is rebased onto c148a12d (head 594c6828, the same four-file diff, no overlap). The bun branch is rebuilt locally as one commit on main with the tests unchanged. I will push once the new preview release is published and the branch has been built and tested against it.

[robobun]

Pushed the rebase as be04fdf9: one commit on top of main, pin autobuild-preview-pr-475-594c6828 (oven-sh/WebKit#475 on c148a12d), the test changes and the quarantine removal unchanged. The preview release exists, so CI runs against a real tarball.

Re-verified on Linux x64 debug ASAN against that preview: this file 8 of 8 plain and under BUN_JSC_validateExceptionChecks=1, plus the related bun/util, bun/jsc, globals, import-meta and namespace-prototype-pollution files (178 tests).

[claude]

I reviewed the Bun-side changes and found no issues. My earlier concern about the $ site on Windows was answered and then verified against a Windows x64 debug build (8/8 pass on the preview, 5 fail on the old pin). Because the actual behavioral change lives in oven-sh/WebKit#475 (property-lookup exception handling across six loops and reifyAllStaticProperties), and the pin here is still a preview tag that must be swapped for the merged sha as part of the landing sequence, a maintainer should coordinate the merge.

What was reviewed:

• The four new subprocess tests: pipes drained concurrently, exact-output assertions, phase guard so the transition test can't go vacuous, each site uses a distinct property name so megamorphic records don't cross-contaminate.
• Removal of the process.env; pre-read and the no-validate-exceptions.txt entry — both depend on the engine change, and the Windows debug run against the old pin confirms the pre-read was load-bearing there.
• test.concurrent on the new spawns keeps the file fast; bunEnv is spread when adding BUN_JSC_validateExceptionChecks.

Extended reasoning...

Overview

This PR bumps WEBKIT_VERSION to a preview build of oven-sh/WebKit#475, adds four subprocess tests to test/js/bun/util/BunObject.test.ts covering the fixed behavior (throwing lazy-property builders through getPropertySlot/getNonIndexPropertySlot, the #37001 transition-then-throw case, and megamorphic get_by_id/get_by_val/in_by_id/in_by_val cache poisoning), removes a Windows process.env workaround from an existing test, and un-quarantines the file from test/no-validate-exceptions.txt. The engine fix itself is entirely in the WebKit fork.

Security risks

None identified. The change tightens exception propagation in JSC property lookups; it does not add new API surface, parse untrusted input, or touch auth/crypto paths.

Level of scrutiny

High. Although the Bun-repo diff is small (a version string, tests, and a quarantine removal), the pinned WebKit change alters property-lookup control flow in six lookup loops plus reifyAllStaticProperties — core engine behavior that every JS property access can reach. That change is not reviewable from this diff. The pin is also a preview tag (autobuild-preview-pr-475-594c6828) that the PR description says will be deleted when the upstream PR closes, so merging requires a coordinated swap to the merged sha. Both of these are maintainer-level decisions.

Other factors

• The tests themselves are well-constructed per the repo's review rules: subprocess isolation, concurrent pipe draining, exact JSON output assertions (not toContain), a phase sentinel that fails loudly if the sql module stops reading Error.prototype, and test.concurrent for the independent spawns.
• My earlier inline concern (the "$" in Bun site reifying Bun.inspect on Windows before throwing) was answered with a trace of where the assertion lives and then empirically verified on a Windows x64 debug build against both pins — the preview passes 8/8, the old pin fails 5 including the test whose workaround is removed.
• CI on head eda85e47 was 180/181 green (the one failure a known Windows --cpu-prof flake); the branch has since been rebased onto c148a12d and re-pinned, with a new build in progress.
• CodeRabbit's two findings (build-mode gating for validator tests; preview-pin ephemerality) were both addressed in-thread and withdrawn/acknowledged.
• No CODEOWNERS conflict apparent, but WebKit bumps in this repo have historically been landed by maintainers who can vouch for the upstream diff.

[robobun]

Rebasing onto main again: #40276 upgraded WebKit to the 8c4fd56347 upstream merge (pin cb61607f). oven-sh/WebKit#475 is rebased onto that merge (head 7d1bf312). The upstream merge touches JITOperations.cpp and JSObject.cpp in other functions only, so the four-file diff applied without changes and is identical. The bun branch is rebuilt locally as one commit on main with the tests unchanged. I will push once the new preview release is published and the branch has been built and tested against it.

[robobun]

Pushed the rebase as 798be93d: one commit on top of main, pin autobuild-preview-pr-475-7d1bf312 (oven-sh/WebKit#475 on the 8c4fd56347 upstream merge, pin cb61607f), the test changes and the quarantine removal unchanged. The preview release exists and all 42 of its build lanes passed on the first attempt, so the rebase onto the upstream merge introduced no build problem.

Re-verified on Linux x64 debug ASAN against that preview: this file 8 of 8 plain and under BUN_JSC_validateExceptionChecks=1, plus the related bun/util, bun/jsc, globals, import-meta and namespace-prototype-pollution files (178 tests), and the megamorphic repro (second read of Bun.sql after a throwing first read returns the function).

[robobun]

Rebasing onto main again: #40417 moved the pin to 1cb96a7b (oven-sh/WebKit#513). oven-sh/WebKit#475 is rebased onto it (head 2501e6c0, the same four-file diff, no overlap). The bun branch is rebuilt locally as one commit on main with the tests unchanged. I will push once the new preview release is published and the branch has been built and tested against it.

[robobun]

The rebase onto 72597399 is ready locally but not pushed: the WebKit preview for the rebased head cannot be published right now because the windows-11-arm lane of every preview build in oven-sh/WebKit fails in its tool-install step within seconds (details and run ids on oven-sh/WebKit#475). This affects all PRs that use preview builds, not only this one. I will push the rebased branch once that lane works again and the release exists; until then the current head still pins the previous preview.

[robobun]

Update on the preview blocker: the windows-11-arm install step turned out to be intermittent rather than permanently broken (other preview runs passed that lane later in the day), so I re-ran the failed lane on oven-sh/WebKit#475's run for 898102d9. The other 42 lanes are already built. Once the lane passes and autobuild-preview-pr-475-898102d9 is published, I will push the rebased branch (ready locally, same tests, pin on main's current 72597399).

[robobun]

Still blocked on the WebKit preview pipeline: the Scoop installer that the windows-11-arm lane depends on broke upstream (ScoopInstaller/Install#136), so every preview run that uses the workflow on WebKit main fails that lane within seconds; 42 of 43 lanes for the rebased head are built. The fixes are oven-sh/WebKit#523 and oven-sh/WebKit#524 (either one). Once one of them is merged, the preview for 898102d9 can be published without further changes to the WebKit PR, and the rebased branch here (ready locally, tests unchanged, pin on main's current 72597399) gets pushed.

[robobun]

oven-sh/WebKit#523 (drop Scoop from the Windows arm64 job) has merged, which fixes the lane that blocked every preview build. I dispatched a new preview build for oven-sh/WebKit#475's head 898102d9 with the fixed workflow (no new commit needed). Once autobuild-preview-pr-475-898102d9 is published, the rebased branch here (ready locally, tests unchanged, pin on main's current 72597399) gets built, tested and pushed.

[robobun]

Pushed the rebase as 0454c0e8: one commit on top of main, pin autobuild-preview-pr-475-898102d9 (oven-sh/WebKit#475 on main's current 72597399), the test changes and the quarantine removal unchanged. The preview was published by a dispatched run with the fixed Windows arm64 workflow (oven-sh/WebKit#523); all 43 build lanes passed.

Re-verified on Linux x64 debug ASAN against that preview: this file 8 of 8 plain and under BUN_JSC_validateExceptionChecks=1, the related bun/util, bun/jsc, globals, import-meta and namespace-prototype-pollution files (178 tests), and the megamorphic repro.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

Rebasing onto main again: #40643 moved the pin to 0bb01ed5. oven-sh/WebKit#475 is rebased onto it (head 1e46058f, the same four-file diff, no overlap). The bun branch is rebuilt locally as one commit on main with the tests unchanged. I will push once the new preview release is published and the branch has been built and tested against it.

[robobun]

Pushed the rebase as eb67c99c: one commit on top of main, pin autobuild-preview-pr-475-1e46058f (oven-sh/WebKit#475 on main's current 0bb01ed5), the test changes and the quarantine removal unchanged. The preview release exists (all build lanes green on the first attempt), so CI runs against a real tarball.

Re-verified on Linux x64 debug ASAN against that preview: this file 8 of 8 plain and under BUN_JSC_validateExceptionChecks=1, the related bun/util, bun/jsc, globals, import-meta and namespace-prototype-pollution files (178 tests), and the megamorphic repro.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

Rebasing onto main again: #40674 moved the pin to f5deafe0. oven-sh/WebKit#475 is rebased onto it (head 49296ecc, the same four-file diff, no overlap). The bun branch is rebuilt locally as one commit on main with the tests unchanged. I will push once the new preview release is published and the branch has been built and tested against it.

[robobun]

Pushed the rebase as b4304f4e: one commit on top of main, pin autobuild-preview-pr-475-49296ecc (oven-sh/WebKit#475 on main's current f5deafe0), the test changes and the quarantine removal unchanged. The preview release exists (all build lanes green on the first attempt), so CI runs against a real tarball.

Re-verified on Linux x64 debug ASAN against that preview: this file 8 of 8 plain and under BUN_JSC_validateExceptionChecks=1, and the megamorphic repro. The related bun/util, bun/jsc, globals, import-meta and namespace-prototype-pollution files: 2324 pass, and the same two local-only failures as with the stock pin in this container, the large DOMJIT variants (same timings with either pin) and fuzzy-wuzzy.test.ts (a debug_assert in the Redis client when a non-subscriber's punsubscribe() reply arrives, reproduced on the stock pin). Fail-before re-checked on a build with the pin set to f5deafe0: the four new tests fail, the rest pass.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

Rebasing onto main again: #40681 moved the pin to 1817c3c3 (the 6b879687ee upstream merge). oven-sh/WebKit#475 is rebased onto it (head 960b8356, the same four-file diff, applied without edits; the upstream merge touched JITOperations.cpp only in operationPolymorphicCall, none of the lookup loops). The bun branch is rebuilt locally as one commit on main with the tests unchanged. I will push once the new preview release is published and the branch has been built and tested against it.

[robobun]

Pushed the rebase as 84019ca2: one commit on top of main, pin autobuild-preview-pr-475-960b8356 (oven-sh/WebKit#475 on main's current 1817c3c3), the test changes and the quarantine removal unchanged. The preview release exists (all build lanes green on the first attempt), so CI runs against a real tarball.

Re-verified on Linux x64 debug ASAN against that preview: this file 8 of 8 plain and under BUN_JSC_validateExceptionChecks=1, the megamorphic repro, and webkit-upgrade-6b879687ee.test.ts. The related bun/util, bun/jsc, globals, import-meta and namespace-prototype-pollution files: 2343 pass, and the same local-only failures as with the stock pin in this container (timeouts of the large DOMJIT variants and a few GC and leak tests under a load average above 80, plus exotic-global-mutable-prototype.test.ts, which fails in a one-process batch after BunObject.test.ts sets globalThis.a in its first test, a line this PR does not touch). Fail-before re-checked on a build with the pin set to 1817c3c3: the four new tests fail, the file aborts under the validator (unchecked as of this scope: defaultBunSQLObject @ BunObject.cpp:315), and the megamorphic repro returns undefined on the second read.

[robobun]

Main moved the pin once more while the previous rebase was being verified: #40677 set it to c4ddc0cf (oven-sh/WebKit#527, one commit on top of 1817c3c3, no overlap). oven-sh/WebKit#475 is rebased onto it (head 3945a1fc, the same four-file diff, applied without edits), and the bun branch is rebuilt locally as one commit on main with the tests unchanged. I will push once the new preview release is published and the branch has been built and tested against it.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

Pushed the rebase as 54b4953c: one commit on top of main, pin autobuild-preview-pr-475-3945a1fc (oven-sh/WebKit#475 on main's current c4ddc0cf), the test changes and the quarantine removal unchanged. The preview release exists (all build lanes green on the first attempt), so CI runs against a real tarball.

Re-verified on Linux x64 debug ASAN against that preview: this file 8 of 8 plain and under BUN_JSC_validateExceptionChecks=1, the megamorphic repro, and bun/jsc, globals, exotic-global-mutable-prototype and namespace-prototype-pollution (128 pass, the only failures the large DOMJIT variants that time out on this loaded host with either pin). Fail-before re-checked on a build with the pin set to c4ddc0cf: the four new tests fail, the file aborts under the validator, and the megamorphic repro returns undefined on the second read.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

Rebasing onto main again: #40767 moved the pin to ceb9f90f (oven-sh/WebKit#530 and #531, four commits on top of c4ddc0cf, no overlap with the lookup files). oven-sh/WebKit#475 is rebased onto it (head 94c5a2d5, the same four-file diff, applied without edits), and the bun branch is rebuilt locally as one commit on main with the tests unchanged. I will push once the new preview release is published and the branch has been built and tested against it.

[coderabbitai]

Note

GitHub couldn't provide a complete incremental comparison for this pull request, so CodeRabbit is performing a full review instead. This review may take a little longer.

[robobun]

Pushed the rebase as 07e34b96: one commit on top of main, pin autobuild-preview-pr-475-94c5a2d5 (oven-sh/WebKit#475 on main's current ceb9f90f), the test changes and the quarantine removal unchanged. The preview release exists (all build lanes green on the first attempt), so CI runs against a real tarball.

Re-verified on Linux x64 debug ASAN against that preview: this file 8 of 8 plain and under BUN_JSC_validateExceptionChecks=1, the megamorphic repro, and bun/jsc, globals, exotic-global-mutable-prototype and namespace-prototype-pollution (126 pass, the only failures the large DOMJIT variants that time out on this loaded host with either pin). Fail-before re-checked on a build with the pin set to ceb9f90f: the four new tests fail, the file aborts under the validator, and the megamorphic repro returns undefined on the second read.

[claude]

Code review found no issues

No high-confidence issues detected in this change.