[robobun]

Problem

• A read of RegExp.lastMatch or another legacy static property can end the process after a test() that matched: SIGSEGV at 0x4, a null JSArray* in RegExpGlobalData::getBackref.
• test() records only the position of the match. The property read runs the match again for the captures. That second run can fail, and RegExpCachedResult::lastResult() (runtime/RegExpCachedResult.cpp:81) then uses a null array.

Fix

• The first commit is upstream WebKit/WebKit@191803bb2304 (324533@main, bug 328417), cherry-picked unchanged. lastResult() throws an out of memory error when the second run fails.
• The second commit makes the new test skip the fork's two RegExp modes. It is the change that Upgrade to upstream WebKit e8f704815e94 #923 has for this test.
• Verified: JSTests/stress/regexp-legacy-static-properties-rematch-failure.js passes in 17 modes. The two programs in Notes end with SIGSEGV on main and pass here.

Background

• Upgrade to upstream WebKit e8f704815e94 #923, the upgrade to upstream e8f704815e94, carries the same commit. Bun cannot take that upgrade yet. Its bump Upgrade WebKit to e8f704815e94 bun#47112 fails tests of the ahead-of-time compiler in steps that block a merge (Notes). After Upgrade to upstream WebKit e8f704815e94 #923 merges, this pull request adds nothing.
• RegExp.lastMatch, RegExp.$1 and the other legacy static properties are computed from the last match when they are read.
• regexp-flat-program and regexp-image-mode are test modes of the fork. In them the second run does not run out of stack.

Downsides

• None found. The four new lines are on the path where createRegExpMatchesArray() returned null. That path ended in a null dereference before, and ends in a RangeError now.

Notes

The two programs

var s = "a".repeat(10000) + "c";
var re = new RegExp("(?:(?!(?:a|b)*d)a)*c" + "(x)?".repeat(200));
re.test(s);         // true
RegExp.lastMatch;   // main: SIGSEGV. Here: RangeError: Out of memory

var N = 3000000, s = "a".repeat(N) + "b", re = new RegExp("(?:a){" + N + "}b");
re.test(s);
function rec() { try { return rec(); } catch (e) { return RegExp.lastMatch.length; } }
rec();              // main: SIGSEGV. Here: 3000001

jsc	first program	second program
x64 lto-aot lane of main (fcc1ea0973)	SIGSEGV	SIGSEGV
local Release build of main (23d7f1dcef), Linux x64, without the ahead-of-time compiler	SIGSEGV, 3 of 3	SIGSEGV, 3 of 3
the same build with the first commit	RangeError: Out of memory, 3 of 3	3000001, 3 of 3

The test in the fork's modes

With the first commit alone, run-javascriptcore-tests --filter=regexp-legacy-static-properties-rematch-failure runs the test in 18 modes of the local build. 17 pass. regexp-flat-program fails with expected RangeError but got undefined: a flat program makes the second match within --maxRegExpStackSize=65536. regexp-image-mode exists only in a build with the ahead-of-time compiler, and #923 found the same there. With the second commit the 17 modes pass and the two modes are skipped. JSTests/BUN-TEST-DIFFERENCES.md lists the test in section B, and the fork now changes 128 upstream tests (main has 127).

Other RegExp tests

Local Release build of this branch, Linux x64, without the ahead-of-time compiler: run-javascriptcore-tests --filter="regex|regexp|yarr", 7258 runs from all collections, no failure.

What the merge of #923 has to do after this

runtime/RegExpCachedResult.cpp and the test are the same here and on #923, and git merges them without a conflict. A trial merge of the first version of this branch with #923 at bcd362b31e had two conflicts, both the count of changed upstream tests in CLAUDE.md and JSTests/BUN-TEST-DIFFERENCES.md. Now those files say 128 here and 129 on #923 (b67a0b1a3a). 129 is right after that merge.

Rebased on main at 8c6c963e3c

After this branch was cut, main moved from 23d7f1dcef to 8c6c963e3c, and its count of changed upstream tests went from 124 to 127. That count conflicted with the second commit. The branch now has the same two commits on 8c6c963e3c. The upstream commit has the same patch as before (git patch-id gives the same value), and the second commit says 128. The head before the rebase was c50875535b: its CI was green, and the local results in these notes are from that version.

Why ahead of #923

#923 is green and has no conflict with main. The Bun side holds it up. oven-sh/bun#47112 runs Bun's CI on the preview build of #923. Its Buildkite build 134232 (Oct 28) fails six test files in test-aot, the step of the ahead-of-time compiler's tests that blocks a merge:

• Four files: a compiled program shows another function name in a stack frame after { ...fn } than plain Bun does. Upgrade WebKit to e8f704815e94 bun#47112 names upstream WebKit/WebKit@e04a6f9faecf as the cause, a fast path of object spread for functions. Upgrade to upstream WebKit e8f704815e94 #923 has a change for this cause since 780dc90982.
• One file: Array.from of an unknown value keeps 443 functions in a trimmed program where it kept 3. That pull request names upstream WebKit/WebKit@37aa2b5bd94d, which changed how the JavaScript builtins iterate.
• One file: a hello world built with --trim maps 25,739,552 bytes, and the bound of the test is 25,700,000.

The next run of that pull request, Buildkite build 134977 (Oct 29), was not finished when this was written. So far eight of its jobs failed in steps that block a merge. The failures are in four files of test/aot/engine/ir-*.test.ts, one test under test/aot/parallel/arrays/ and test/bundler/compile-aot-size.test.ts.

Once #923 is on main, every later commit of main has that merge, so every later pin in Bun meets these failures until they are repaired.

oven-sh/bun#47160 runs Bun's CI on the preview build of this pull request. Its Buildkite build 134386 passed with the preview of c50875535b, and test-aot passed on every lane. Bun's main pins 8c6c963e3c since oven-sh/bun#47281, the commit this branch now starts from. oven-sh/bun#47129, the Bun side of #927, landed before it. So after a merge, Bun needs a one-line change of the pin and nothing else.

The crash is reachable from a plain script, and this fix does not depend on the rest of the upgrade.

To keep the upstream commit as it is, land this as a merge commit or with a rebase, not as a squash.

[robobun]

Status: closed without a merge. #923 is merged (a7b59e6), and it has the same upstream commit (WebKit/WebKit@191803bb2304) with the same change to its test. main has both files as they were on this branch.

• For Bun: the fix comes with the upgrade to that merge commit (Upgrade WebKit to e8f704815e94 bun#47112).
• The preview releases of this pull request go away now that it is closed.

[coderabbitai]

No actionable comments were generated in the recent review.

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Reviewing files that changed from the base of the PR and between c508755 and eb29690.

Files selected for processing: 4

• CLAUDE.md
• JSTests/BUN-TEST-DIFFERENCES.md
• JSTests/stress/regexp-legacy-static-properties-rematch-failure.js
• Source/JavaScriptCore/runtime/RegExpCachedResult.cpp

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Walkthrough

The change handles a null result from RegExp match-array creation. It adds a stress test for legacy RegExp properties after a rematch failure. The test skips two modes, and the test-difference documents update the changed-test count.

Changes

RegExp rematch failure handling

Layer / File(s)	Summary
Handle failed match-array creation Source/JavaScriptCore/runtime/RegExpCachedResult.cpp	lastResult now throws an out-of-memory error and returns nullptr when match-array creation returns null.
Test rematch failure and track modes JSTests/stress/regexp-legacy-static-properties-rematch-failure.js, JSTests/BUN-TEST-DIFFERENCES.md, CLAUDE.md	The stress test checks for RangeError when reading or setting legacy RegExp properties after a successful match, then checks their values after a later match. The test-difference document records skips for two modes, and both documents update the changed-test count.

Possibly related PRs

• oven-sh/WebKit#343: Changes the same lazy RegExp rematch failure path and updates RegExpCachedResult::lastResult for match-array creation failures.

Priority: Normal

Merge Risk: Low · up to eb296

The regression test’s coverage remains uncertain; confirm its behavior in supported modes before relying on it as protection for this fix.

Pre-merge checks: 3 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Description check		The description follows the WebKit template only in part. It has a problem statement, a fix, and background. It gives reproduction steps, test results across modes, and a clear rationale for the chang…	Add the Bugzilla bug link (https://bugs.webkit.org/show_bug.cgi?id=#####) and the bug title in the template format. Add the 'Reviewed by' line, or state that review is pending. Add the per-file entries listed under the template's changed-pa…

Passed checks: 3 passed

Check name	Status	Explanation
Title check		The title names the main change: RegExp legacy static properties should not crash when the lazy re-match fails. It also states the change is a cherry-pick of upstream commit 191803b. The changese…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Title check

Explanation

The title names the main change: RegExp legacy static properties should not crash when the lazy re-match fails. It also states the change is a cherry-pick of upstream commit 191803b. The changeset matches this. RegExpCachedResult::lastResult now throws an out-of-memory error instead of using a null array, and the new stress test and skip entries match the title's subject.

Full details: Description check

Explanation

The description follows the WebKit template only in part. It has a problem statement, a fix, and background. It gives reproduction steps, test results across modes, and a clear rationale for the change. It does not follow the required template fields. It has no Bugzilla bug link (the template requires the bug URL and bug title). It has no 'Reviewed by' line. It has no per-file ChangeLog-style entries listing the changed paths and functions. The commit message format the template asks for is not present. The text is also very long for a PR description, and much of it covers Bun CI status and merge sequencing that is not part of the code change.

Resolution

Add the Bugzilla bug link (https://bugs.webkit.org/show_bug.cgi?id=#####) and the bug title in the template format. Add the 'Reviewed by' line, or state that review is pending. Add the per-file entries listed under the template's changed-path section, for example runtime/RegExpCachedResult.cpp (lastResult) and the new JSTests file. Keep the explanation of the fix in the commit message format. Move the Bun CI and merge-sequencing details out of the main description, or reduce them to a short note.

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[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at
@JSTests/stress/regexp-legacy-static-properties-rematch-failure.js:
- Line 26: Update the `regExp.test(string)` check to assert that the initial
match succeeds, then run the rematch assertions unconditionally rather than
skipping them when the test returns false.

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Files selected for processing: 4

• CLAUDE.md
• JSTests/BUN-TEST-DIFFERENCES.md
• JSTests/stress/regexp-legacy-static-properties-rematch-failure.js
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[claude]

I reviewed this PR and didn't find any bugs. Because it changes a runtime path in JavaScriptCore (a CODEOWNERS-covered area) and adds a test whose expectation depends on stack-usage behavior of specific modes, a human look would still be worthwhile.

What was reviewed:

• The 4-line null check in Source/JavaScriptCore/runtime/RegExpCachedResult.cpp matches createRegExpMatchesArray's contract (returns null on position == -1) and mirrors the null handling already in RegExpObjectInlines.h; m_reified stays false on the throw, so a later read retries rather than caching a null.
• The new test's $skipModes names (regexp-flat-program, regexp-image-mode) match the mode names in Tools/Scripts/run-jsc-stress-tests, and the fork's bookkeeping (BUN-TEST-DIFFERENCES.md row, 124 to 125 counts in both files) is consistent.
• Considered whether the test could fail in interpreter-only or other modes with "expected RangeError but got undefined"; the PR's own 17-mode run and upstream's unchanged test cover that, so no change was warranted here.

Extended reasoning...

The change is a verbatim cherry-pick of upstream WebKit 191803b adding a null check with throwOutOfMemoryError in RegExpCachedResult::lastResult, plus the upstream regression test with two fork-specific mode skips and the matching bookkeeping edits in CLAUDE.md and JSTests/BUN-TEST-DIFFERENCES.md. It touches no security-sensitive surface beyond converting a null dereference crash into a thrown RangeError. The code is small and correct against the callee's null-return contract, but /Source/JavaScriptCore and /JSTests are covered by .github/CODEOWNERS, and the test's correctness rests on stack-usage behavior across JIT modes that cannot be exercised in this checkout, which is why this is a defer rather than an approve.

[github-actions]

Preview build of eb29690: autobuild-preview-pr-929-eb296902

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

Closing: #923 is merged (a7b59e6) and has the same upstream commit (WebKit/WebKit@191803bb2304) with the same change to its test. main now has runtime/RegExpCachedResult.cpp and JSTests/stress/regexp-legacy-static-properties-rematch-failure.js as they are on this branch, so nothing is left for this pull request to add.