[robobun]

Problem

• element.onEndTag(fn) makes fn a GC root (ProtectedJSValue in EndTagHandler, src/runtime/api/html_rewriter.rs). If fn reaches the output Response and the rewrite stops early, 200 of 200 Responses stay.
• Main also uses the streams of a collected rewrite, in four places. cancel_from_output() and abandon_suspension() close a dead JS input (SEGV in JSReadStreamIntoSinkOperation::result(), or ASSERTION FAILED: status() == Status::Pending). abandon_suspension() writes to a freed output (heap-use-after-free in ByteStream::on_data). Bun.ModuleGraph.dispose() cancels a stream source that is dead, or that is freed under the call (a segfault on a release build with no options). The leak hid the first.

Fix

• onEndTag callbacks live in a JS array in endTagHandlers, a new visited slot of the transform cell. The lol-html handler keeps an index.
• cancel_from_output() cuts the output's edge to the transform cell last, as fail() and finish() already do.
• The pipe's reference to its own cell is a JsRef instead of a bare JSValue, so a dead, unswept cell reads as None.
• The two entries that nothing reachable makes, the abandon_suspension() task and NewSource's on_abort (dispose()), hold the wrapper that they read for the call, and leave a collected one to its finalizer.
• Verified: 23 new tests (test/js/workerd/html-rewriter-leak.test.ts, module-graph-gc.test.ts). 11 fail on a debug build of main, 13 on release ASAN. Other suites: Notes. Self-reviewed: 3 concerns raised, 3 addressed.

Background

• gcProtect makes a value a GC root, so a cycle through it is never garbage. A visited slot is an edge that the collector traces.
• The transform cell (HTMLRewriterTransform) keeps the input stream of one transform() alive. The native pipe holds that stream as a raw pointer.
• Considered the cell in the PipePin guard of every entry point (an earlier version of this PR), and a Strong on the input (a root per rewrite). Notes say why not.

Downsides

• Element grows from 40 to 48 bytes, RewriterPipe from 272 to 304, the release binary by 8,960 bytes.
• A handler without onEndTag() costs the same within noise (150 ns per element, base-to-base difference up to 1.2 ns). With it, 41 to 48 ns less.
• Two leaks without onEndTag() remain (Notes).

Notes

Report. There is no GitHub issue. #43210 measured the leak and left it for a follow-up, and its review asked for it: #43210 (comment). This is the last ProtectedJSValue in html_rewriter.rs.

Why a visited slot. REVIEW.md ("Root or copy every JSValue held beyond the current call") asks for WriteBarrier members declared in .classes.ts, and keeps protect for a justified self-keepalive. #43210 applied that to the handlers slot. The third shape below also needs it: a rewrite whose input never ends reaches no terminal state, so only collection of the transform cell can free it, and a root prevents that collection.

Repro of the leak. MODE is ok, throw or cancel. The </div> end tag never arrives.

const { heapStats } = require("bun:jsc");
let fired = 0;
const fr = new FinalizationRegistry(() => fired++);
const N = 200;
const mode = process.env.MODE ?? "throw";
async function once() {
  const holder = {};
  const rw = new HTMLRewriter()
    .on("div", { element(el) { el.onEndTag(() => holder.res); } })
    .on("p", { element() { if (mode === "throw") throw new Error("boom"); } });
  let ctrl;
  const res = rw.transform(new Response(new ReadableStream({ start(c) { ctrl = c; } })));
  holder.res = res;
  fr.register(res, 0);
  const chunk = new TextEncoder().encode("<div><p>x</p>");
  if (mode === "cancel") {
    const reader = res.body.getReader();
    ctrl.enqueue(chunk);
    await reader.read();
    await reader.cancel();
  } else {
    ctrl.enqueue(chunk);
    ctrl.close();
    try { await res.text(); } catch {}
  }
}
for (let i = 0; i < N; i++) await once();
for (let i = 0; i < 6; i++) { Bun.gc(true); await Bun.sleep(20); }
console.log(mode, "retained", N - fired, "of", N, "protected Function", heapStats().protectedObjectTypeCounts.Function ?? 0);

Measurements of the leak (debug builds, retained Responses and protected Functions of 200, graphs of 15):

shape	main (367d939)	this PR
rewrite completes	0, 0	0, 0
a later handler throws, the body is read	200, 200	0, 0
the output reader cancels	200, 200	0, 0
the input never ends, all dropped, nothing pending on the output	200, 200	0, 0
disposed Bun.ModuleGraph whose code left such a rewrite	15 graphs	1 graph
the same graph code without onEndTag() (control)	1 graph	1 graph

Release builds of the merge base 37471e5 and of this PR give the same result for throw and cancel: 200 of 200, and 0 of 200. On main the heap snapshot of the new ModuleGraph test names the retainer: root(ProtectedValues) Function -> JSLexicalEnvironment -> JSModuleEnvironment -> JSLexicalEnvironment -Variable:moduleGraph-> ModuleGraph.

The dead input, case 1: cancel_from_output(). An earlier version of this PR failed on the x64 ASAN lane: the new test "when the output reader is cancelled" aborted with ASSERTION FAILED: status() == Status::Pending. The cause is on main. cancel_from_output() starts with detach_output(), which clears the owner slot of the output stream. If script holds only the reader, that slot was the last GC path to the transform cell. The next step allocates the abort reason, so a collection can finish there. detach_input_source() then closes the sink controller of the JS input through SourceHandle::JSController, a raw pointer to a cell that only the transform cell kept alive. On main the protected onEndTag callback kept such a rewrite alive forever, so the window was closed for exactly the rewrites that the leak tests make.

Main has the bug with no onEndTag() call. This script requests a collection before each reader.cancel():

const encoder = new TextEncoder();
const N = 200;
let cancelled = 0;
for (let i = 0; i < N; i++) {
  let controller;
  const input = new ReadableStream({ start: c => void (controller = c), cancel() { cancelled++; } });
  const reader = new HTMLRewriter().on("div", { element() {} }).transform(new Response(input)).body.getReader();
  controller.enqueue(encoder.encode("<div><p>x</p>"));
  controller = undefined;
  if ((await reader.read()).done) throw new Error("done early");
  Bun.gc(false);
  await reader.cancel();
}
Bun.gc(true);
console.log(JSON.stringify({ rewrites: N, cancelled }));

release ASAN build	main	this PR
plain run, 3 runs	cancelled is 195, 194, 200 of 200	200 of 200
BUN_JSC_collectContinuously=1	SEGV on unknown address 0x000000000010	200 of 200

The stack of the SEGV: JSC::ClassInfo::isSubClassOf < JSReadStreamIntoSinkOperation::result() < Bun::WebStreams::rsisFinish < jsWebStreamsHandler_onReadStreamIntoSinkClose < JSC::runInternalMicrotask. The failure needs a collection that finishes between detach_output() and the close of the input. It did not occur on a debug build or on a release build without ASAN. The regression test runs the script under BUN_JSC_collectContinuously=1 on release builds (not on Windows, where that option is very slow), so it fails on main only on the release ASAN lanes.

The fix for case 1. detach_output() moves to the end of cancel_from_output(), next to release_input_roots(). The reader that cancels then reaches the cell through the owner slot until the input is closed. This is the order of fail() and finish(), and the rule that the doc comment of release_input_roots() already states for the input's edges. Alternatives:

• Keep the cell on the stack in the PipePin of write, end_from_stream, resume, cancel_from_output and fail (an earlier version of this PR). Each of those is entered by a peer whose own cell has an internal edge to the transform cell (owner, sinkOwner, the Response's transform slot, the context of a promise reaction). The guard mattered where the pipe itself cut that edge too early (this case), and where the first caller of the chain held nothing (cases 3 and 4). It does nothing for a cell that is already dead at the entry, so that version still crashes in case 4. It also made EnsureStillAlive a struct field. Everywhere else in the tree it is a local.
• A Strong on the controller in SourceHandle::JSController. That is one more root per rewrite with a JS input, and a root is what made the leak.

The dead input, case 2: abandon_suspension(). A review of this PR found it. It is on main too. A handler returns a promise that never settles, and script drops everything. The collector then frees the promise, and the destructor of its native context queues abandon_suspension. That task asked cell.is_cell() to learn if the transform cell is alive. The pipe's cell field was a bare JSValue that the cell's finalizer clears, and the finalizer runs at the sweep. A cell that is dead but not swept yet still passes is_cell(), so the task called fail(), and fail() closed the dead sink controller. That field is a hand-written weak reference. JsRef is the one the rest of the runtime uses for a native object's own wrapper (the stream sources next to this pipe do), and since #39334 its try_get() gives None for a dead, unswept cell, which is the answer of JSC::Weak::get(). The field is now a JsCell<JsRef>, and every read of it goes through try_get(). The task clears the input and output handles without a call into them when that is None, as it already did for a swept cell.

const N = 50;
const encoder = new TextEncoder();
const tick = () => new Promise(resolve => setImmediate(resolve));
let cancelled = 0;
for (let i = 0; i < N; i++) {
  let controller;
  const input = new ReadableStream({ start: c => void (controller = c), cancel: () => void cancelled++ });
  new HTMLRewriter().on("p", { element: () => new Promise(() => {}) }).transform(new Response(input));
  controller.enqueue(encoder.encode("<p>x</p>"));
}
for (let i = 0; i < 5; i++) await tick();
for (let round = 0; round < 10; round++) {
  Bun.gc(false);
  const junk = [];
  for (let j = 0; j < 500; j++) junk.push({ j });
  await tick();
}
process.stdout.write(JSON.stringify({ rewrites: N, cancelled }));

Run it as a file. A release build of main exits with panic(main thread): Segmentation fault at address 0x0 in 10 of 10 runs. A debug build of main stops at ASSERTION FAILED: decontaminate(). A release ASAN build of main reports the same SEGV as case 1, with this stack: JSReadStreamIntoSinkOperation::result() < rsisFinish < pumpOnClose < sinkControllerOnClose < SourceHandle::cancel < RewriterPipe::fail < RewriterPipe::abandon_suspension. This PR prints {"rewrites":50,"cancelled":0} on all three builds. Bun 1.4.2 does not crash on this script. #37108 fixed the opposite direction, a controller destructor that reached a freed pipe.

The freed output, case 3: abandon_suspension() again. It is on main too. The handler's promise is collected while script still holds the Response, so the task is queued for a reachable rewrite. Script drops the Response before the task runs. The cell is then unreachable, but no collection has found that out, so it reads as alive. fail() allocates (the error, the abort of the input), and a collection in there sweeps the cell and both streams. fail() then writes the error through output, a raw pointer to the freed ByteStream. This entry is a task that a destructor queued, so nothing on its stack reaches the cell. It keeps the cell that it read in a local EnsureStillAlive until it returns.

const encoder = new TextEncoder();
const tick = () => new Promise(resolve => setImmediate(resolve));
const responses = [], promises = [];
function start() {
  let controller;
  const input = new ReadableStream({ start: c => void (controller = c) });
  const response = new HTMLRewriter()
    .on("p", { element() { const p = new Promise(() => {}); promises.push(p); return p; } })
    .transform(new Response(input));
  response.body;
  responses.push(response);
  controller.enqueue(encoder.encode("<p>x</p>"));
}
for (let round = 0; round < 20; round++) {
  for (let i = 0; i < 4; i++) start();
  await tick();
  promises.length = 0;
  Bun.gc(true);
  responses.length = 0;
  await tick();
}

With BUN_JSC_slowPathAllocsBetweenGCs=25 (a full collection at every 25th slow-path allocation) a release ASAN build of main reports the heap-use-after-free in 10 of 10 runs, and also for each of 7, 13, 20, 33, 50, 64, 80, 100 and 150. This PR exits with 0. A debug build and a release build without ASAN of main do not report it.

The dead or freed source, case 4: Bun.ModuleGraph.dispose(). It is on main too, and needs no onEndTag(). dispose() cancels the source of every stream that the graph's script was given (NewSource's on_abort, #42590), from native code. The JS wrapper owns the source, and nothing on that stack reaches a wrapper that script has dropped. AbortHandleOwner has a KeepAlive type for what keeps the owner alive while on_abort runs. NewSource declared none.

• A collection that finishes inside cancel() frees the source under the call, together with the rewrite that feeds it.
• After a collection that finished before, the wrapper is dead and waits for its sweep. cancel() then tells the rewrite, which closes its dead input.

NewSource now keeps its wrapper (this_jsvalue.try_get()) alive for the call. For a wrapper that is collected and not swept yet it does nothing: the peers are collected too, and the finalizer releases the source. A source whose wrapper is finalized while native refs remain (a child's pipe that nobody reads) is cancelled as before.

const TICKS = 0; // or 1
const encoder = new TextEncoder();
const tick = () => new Promise(resolve => setImmediate(resolve));
const rewrite = input => new HTMLRewriter().on("div", { element() {} }).transform(input);
function start(chained) {
  for (let i = 0; i < 20; i++) {
    const input = new ReadableStream({ start: c => void c.enqueue(encoder.encode("<div>x")), cancel() {} });
    const output = rewrite(new Response(input));
    (chained ? rewrite(output) : output).body;
  }
}
for (const chained of [false, true]) {
  for (let i = 0; i < 20; i++) {
    const graph = new Bun.ModuleGraph();
    graph.run(() => start(chained));
    await tick();
    Bun.gc(false);
    for (let t = 0; t < TICKS; t++) await tick();
    graph.dispose();
  }
}

Run it as a file, with no options. 5 runs for each value of TICKS:

build	TICKS = 0	TICKS = 1
main, release	segfault, 5 of 5	segfault or abort, 5 of 5
main, debug	ASSERTION FAILED: decontaminate(), 5 of 5	the same, 5 of 5
main, release ASAN	decontaminate() or SEGV, 5 of 5	SEGV, 5 of 5
the version of this PR with the cell in PipePin, release ASAN	ASSERTION FAILED: result, 5 of 5	SEGV or decontaminate(), 5 of 5
this PR, release ASAN	passes, 5 of 5	passes, 5 of 5

Each of the changes is needed. Release ASAN builds, the repros of cases 1 to 3:

build	case 1 (slowPathAllocsBetweenGCs=1, 10 rewrites)	case 2	case 3
main	cancelled is 0 of 10	SEGV	heap-use-after-free
this PR with only the JsRef	cancelled is 0 of 10	passes	heap-use-after-free
this PR	10 of 10	passes	passes

Why Element has a back reference. on() handlers find their list through the pipe whose lol-html call is on the stack. onEndTag() cannot: after an await in an async handler no lol-html call is on the stack, and inside a handler of a nested, synchronous rewrite the pipe on the stack is the inner one. Both cases work on main and have a test here. handler_callback gives the pipe to the wrapper when it makes it. Element::invalidate() clears it together with the lol-html pointer. That happens when the handler returns, or when the pipe drops the wrapper it parked, so the reference never outlives the pipe.

Replacement. A second onEndTag() on the same element replaces the first callback, in one handler or across two on() handlers. That is the behavior of main (handlers.clear() then push). lol-html gives every matching handler the same Element, and a suspension moves the user data with the parked copy. So the first call pushes the one lol-html handler and records its slot as the element's user data, and a later call only stores the new callback into that slot.

A parked element that outlives its transform cell. A handler returns a promise, the element is parked, then the promise and the cell are collected together. Until the queued abandon_suspension task runs, script can still call onEndTag() on the parked element. The cell is then swept, or dead and not swept yet. In both states try_get() is None, and hold_end_tag_callback stores nothing. It also stores nothing for a parked element whose rewrite was cancelled or failed. In all of these the end tag can never come. The last test in the file covers the collected case.

Return value. Unchanged: the element while it is attached, null once it is detached.

Sizes. size_of in the release profile, merge base then this PR: Element 40, 48 (one per element handler call, freed when the handler returns). RewriterPipe 272, 304 (one per transform(), alignment 16). EndTagHandler 24, 16. The transform cell gets one more WriteBarrier (8 bytes). The first onEndTag() call on an element now boxes a 4-byte index as the lol-html user data, and no longer adds an entry to the VM's table of protected values. size of the linux-x64 release binaries: text 80,681,674, then 80,690,634. Data and bss do not change.

Speed. Release builds of the merge base and of this PR, linux-x64, pinned to one core. One process rewrites a document of 20,000 elements with one element handler, and reports the best of 40 runs in nanoseconds per element. 25 rounds run the processes in turn: base, PR, base again. The table gives the minimum of the rounds, and the median in parentheses. The third column is the same base binary, which shows the noise.

case	base	this PR	base again
<p>x</p> x 20,000, handler only	150.1 (154.9)	150.9 (155.7)	151.3 (157.7)
<p>x</p> x 20,000, handler calls onEndTag()	298.3 (306.2)	257.6 (266.8)	298.0 (306.7)
<div> nested 50 deep x 400, handler only	150.1 (154.2)	150.5 (154.4)	150.1 (154.5)
<div> nested 50 deep x 400, handler calls onEndTag()	322.1 (331.4)	274.6 (280.6)	326.8 (332.7)

try_get() is one call into C++ per handler call. The version of this PR without it measured 151.0, 259.8, 151.9 and 273.8 in the same rounds.

Not changed. Each of these keeps a rewrite alive on main and on this PR with no onEndTag() call at all (100 rewrites each):

• A failed rewrite keeps the handler's error in the output body as a Strong until the body is read (fail()). If that error references the output Response (for example error.response = res) and nothing reads the body, 100 of 100 Responses stay. With the body read, 0 stay. An error that does not reference the Response retains nothing.
• A read() that is pending on the output while the JS input stream can never produce again keeps 100 of 100 Responses, through the protected promise and buffer of the pending pull. A pending .text() leaves 100 protected Promises (promise_value.protect(), src/runtime/webcore/Body.rs:453).
• A rewrite that fails or is cancelled keeps its lol-html rewriter (the parser arena) until the transform cell is collected. Only finish() frees it earlier. A parked wrapper can still point into it, so an earlier free needs its own change.

Tests. 23 new tests. On a debug build of main 11 fail: the three leak shapes, four of the five kept-Response shapes, the parked rewrite that dies with its handler promise, the two dispose() tests of case 4, and the ModuleGraph test. On a release ASAN build of main the cancel test under continuous collection and the test of case 3 fail too. The other tests guard the new storage and pass before the fix: the callback stays alive until its end tag with nothing else referencing it (GC churn in between, half of the outputs dropped), slot reuse under nesting, one end tag that closes several elements, callbacks that run after a handler cancelled the output earlier in the same chunk, replacement (one handler, two handlers, across a suspension), onEndTag() after an await, an outer element used in a nested rewrite, an indexed accessor on Array.prototype, the parked element whose rewrite was collected, and the kept-Response case of a rewrite that completes.

Suites run on the debug (ASAN) build: test/js/workerd/html-rewriter-leak.test.ts (49 pass, 1 skipped in debug), test/js/bun/module-graph/module-graph-gc.test.ts (33 pass), test/js/workerd/html-rewriter.test.js (186 pass), test/js/workerd/html-rewriter-end-error.test.ts, test/js/web/html/html-rewriter-doctype.test.ts, test/regression/issue/htmlrewriter-additional-bugs.test.ts, test/regression/issue/text-chunk-null-access.test.ts, test/js/bun/http/serve-stream-body-error.test.ts, and the HTMLRewriter cases of test/js/web/workers/worker-terminate-lifetime.test.ts and test/js/bun/module-graph/module-graph-isolation.test.ts. On the release ASAN build: the leak file with the CI environment of the ASAN lane (BUN_JSC_validateExceptionChecks=1, BUN_GARBAGE_COLLECTOR_LEVEL=1) and html-rewriter.test.js, 3 runs. Also run by hand, on the earlier version with the cell in PipePin: a nested document with Bun.gc(true) in handlers and callbacks under BUN_JSC_collectContinuously=1, and terminate() of workers that hold pending callbacks in each state.

A test of this file that is slow on main. "HTMLRewriter does not leak element/document handler allocations" passes 15 s on a loaded release ASAN build, on main and on this PR. #44359 and #44377 change that test.

Self-review. The review of the first commit asked for the ModuleGraph test, for the narrower wording of the measured shapes, and for the "Not changed" list. All three are in.

---

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📒 Files selected for processing (3)

• src/runtime/api/html_rewriter.rs
• src/runtime/webcore/ReadableStream.rs
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Walkthrough

HTMLRewriter stores pending onEndTag() callbacks in indexed transform-owned slots. Element wrappers associate slots with elements, and dispatch clears callback entries. Weak transform references and stream cancellation handling support cleanup. New tests cover callback lifetime, slot reuse, cancellation, nested rewrites, and module-graph collection.

Changes

End-tag callback lifetime

Layer / File(s)	Summary
Callback slot storage and dispatch src/runtime/api/html_rewriter.classes.ts, src/runtime/api/html_rewriter.rs	The transform stores callbacks in indexed slots, reuses freed slots, and clears callback entries during dispatch and rewrite release. Elements associate slots with end-tag handlers.
Weak transform references and cleanup src/runtime/api/html_rewriter.rs, src/runtime/webcore/ReadableStream.rs	The pipe uses a weak transform-cell reference and checks its availability before cell-dependent work. Cancellation ordering and stream abort handling account for wrapper availability.
Callback lifetime and collection tests test/js/workerd/html-rewriter-leak.test.ts, test/js/bun/module-graph/module-graph-gc.test.ts	Tests cover callback retention and release across rewrite completion, errors, cancellation, slot reuse, nested rewrites, parked elements, and module-graph disposal.

Suggested reviewers: jarred-sumner

Priority: ⬆️ High

Merge Risk: ⚪ Minimal · up to 5d80a

No actionable issue is established that would prevent merging after normal checks.

🚥 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 summarizes the main changes: storing onEndTag callbacks in a visited slot and avoiding use of streams from a collected rewrite.
Description check	✅ Passed	The description explains the problem, the fix, and how the author verified it. It does not use the template’s exact headings, but its Problem, Fix, and Verified sections cover the required information…

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[robobun]

Status: merged as 403209c. Commits 5d80a54 and 382b36c reworked the lifetime part before the merge: the transform cell is a JsRef, cancel_from_output() cuts the output's edge last, and the two entries that nothing reachable makes hold their wrapper for the call. The PR description has the final design. The numbers below are from the version of Oct 1.

How to reproduce the leak (release build of main at 37471e5):

const { heapStats } = require("bun:jsc");
let fired = 0;
const fr = new FinalizationRegistry(() => fired++);
const N = 200;
const mode = process.env.MODE ?? "throw"; // "ok" | "throw" | "cancel"
async function once() {
  const holder = {};
  const rw = new HTMLRewriter()
    .on("div", { element(el) { el.onEndTag(() => holder.res); } })
    .on("p", { element() { if (mode === "throw") throw new Error("boom"); } });
  let ctrl;
  const res = rw.transform(new Response(new ReadableStream({ start(c) { ctrl = c; } })));
  holder.res = res;
  fr.register(res, 0);
  const chunk = new TextEncoder().encode("<div><p>x</p>"); // the </div> end tag never arrives
  if (mode === "cancel") {
    const reader = res.body.getReader();
    ctrl.enqueue(chunk);
    await reader.read();
    await reader.cancel();
  } else {
    ctrl.enqueue(chunk);
    ctrl.close();
    try { await res.text(); } catch {}
  }
}
for (let i = 0; i < N; i++) await once();
for (let i = 0; i < 6; i++) { Bun.gc(true); await Bun.sleep(20); }
console.log(mode, "retained", N - fired, "of", N, "protected Function", heapStats().protectedObjectTypeCounts.Function ?? 0);

• main before the merge: throw retained 200 of 200 protected Function 200, and the same for cancel. ok retains 0.
• With the fix: retained 0 of 200 protected Function 0 in each mode.

How to reproduce the two uses of a dead input (both scripts are in the PR description, under Notes):

• cancel_from_output(): the script requests a collection before each reader.cancel(). On a release ASAN build of main before the merge the input's cancel() runs 194 to 200 times of 200, and with BUN_JSC_collectContinuously=1 the run stops with SEGV in JSReadStreamIntoSinkOperation::result().
• abandon_suspension(): 50 rewrites whose handler returns a promise that never settles, all dropped. A release build of main before the merge exits with panic(main thread): Segmentation fault at address 0x0 in 10 of 10 runs.

Tests: the PR description has the final list. A deterministic variant of the cancel test is offered in #43379 (comment).

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline finding, I also checked the slot lifecycle in take_end_tag_callback: the slot is freed before the callback runs, so a re-entrant onEndTag() on a parked element of the same pipe can only reuse an index whose FnOnce has already been consumed, and the callback value itself is only a stack local across Z::init/to_js, which JSC's stack scan covers the same way the pre-existing Listed path relied on it. Also confirmed release_handlers() clears both the array and free_end_tag_slots together, so a stale index cannot survive into a later transform(). This is GC-lifetime code in native bindings, so a human look is still worthwhile.

Extended reasoning...

The diff replaces a ProtectedJSValue root with an index into a new visited endTagHandlers array slot on the transform cell, threads a BackRef<RewriterPipe> through Element, and uses lol-html UserData to dedupe repeated onEndTag() calls on the same element. I traced the index lifecycle (allocate in hold_end_tag_callback, consume-and-free in take_end_tag_callback, wholesale reset in release_handlers) and the liveness gate for parked elements (driving, phase == Done, is_cell/is_live_cell) and found them consistent with the described invariants; the four candidates the hunt ruled out (element struct reuse, free-list loss on OOM, GC during wrapper allocation, silent discard on dead cells) match my reading. The one confirmed finding is a dead defensive branch, not a correctness defect, but the change is subtle native GC-rooting work with a wide new test surface, so it should not be auto-approved.

[claude]

Re-reviewed after a342fa3 (which addresses the earlier inline note) and found no bugs, but since this rewires how onEndTag callbacks are rooted across parked wrappers, dead cells, and nested rewrites, a human look at the lifetime invariants is still worthwhile.

What was reviewed:

• hold_end_tag_callback / take_end_tag_callback: put_index/get_direct_index map to putDirectIndex/getDirectIndex (own properties, no Array.prototype traps), JsResult propagated on every JS-entering call, and the four (slot, replaced) outcomes in Element::on_end_tag_ each leave the lol-html handler list and the array consistent.
• Element.pipe back-reference: only invalidate() clears it, and every parked-wrapper exit (SuspendedWrapper::drop, handler guard, Element::drop) goes through invalidate(), so the expect cannot outlive the pipe; the driving / is_live_cell gate in hold_end_tag_callback is the only path that reads the cell without drive_rewriter holding it.
• Tests: replacement across await and across two on() handlers exercises the user-data-survives-suspension claim behaviorally (the lol-html fork source is not present in this checkout to confirm it directly); leak assertions use heapStats counts and bounded setImmediate polls rather than sleeps.

Extended reasoning...

Overview

The change replaces the last ProtectedJSValue in src/runtime/api/html_rewriter.rs with a visited endTagHandlers slot declared in html_rewriter.classes.ts. RewriterPipe gains a free-slot list and two accessors; EndTagHandler now carries only an index; Element gains a back-reference to its pipe (set in init, cleared in invalidate); and on_end_tag_ uses lol-html UserData on the raw element to implement replace-in-place. Two test files add 18 tests covering the leak shapes, slot reuse, replacement, post-await and nested-rewrite registration, and the parked-element-after-collection case. The follow-up commit a342fa3 replaced the dead else { return Ok(NULL) } branch flagged in the prior review with an expect stating the invariant.

Security risks

None specific to this change. The JS array is a hidden internal, and both put_index and get_direct_index resolve to own-property JSC operations (putDirectIndex/getDirectIndex in src/jsc/bindings/bindings.cpp), so userland Array.prototype accessors cannot observe or intercept the stored callbacks; the two test.concurrent cases at the end of the leak test assert that directly. No untrusted lengths or indices flow in from outside the runtime.

Level of scrutiny

High. This is GC-rooting and lifetime code around raw lol-html pointers held across JS-entering calls. I traced: (1) the (slot, replaced) matrix in on_end_tag_ — first call pushes one boxed handler and sets user data; replacement stores into the existing slot; None from hold_end_tag_callback stores nothing, which is safe because that branch only fires when the rewrite is Done or the cell is dead/swept and no end tag can run; (2) take_end_tag_callback is only reachable under drive_rewriter, which pins the cell, and its Ok(None)/Err paths return Stop exactly as the sibling Listed arm does; (3) release_handlers resets the free list and both slots together and is skipped while driving, so a cancel mid-chunk still lets the rest of that chunk's end-tag callbacks run (covered by the "handler cancelled the output earlier in the chunk" test). What I could not confirm from source is the lol-html fork behavior that UserData travels with the parked owned copy on suspension — vendor/lolhtml is absent from this shallow checkout — though the replacement-across-await test exercises it behaviorally.

Other factors

The PR description is candid about what it does not fix (error-object cycles through fail(), pending reads on a dead input, the retained parser arena after cancel), and those are pre-existing. Minor non-blocking observation: hold_end_tag_callback and take_end_tag_callback reimplement the Some(list) if list.is_cell() filter that HandlerList::existing already provides; a shared helper would be marginally cleaner but is not worth a round-trip. Given no findings, an addressed prior comment, and non-trivial invariants that depend on a vendored fork's semantics, deferring to a human maintainer rather than approving is the appropriate outcome.

[robobun]

One fact for the point the second review could not check from source. The user data does travel with the parked copy. In the pinned fork (oven-sh/lol-html at 725ce499, LOLHTML_COMMIT in scripts/build/deps/lolhtml.ts), Element::into_suspended() moves both fields into the owned element:

https://github.com/oven-sh/lol-html/blob/725ce499aa9b71e38b7a2d0a9fbb6d7294a4079e/src/rewritable_units/element.rs#L103-L119

Element {
    start_tag: StartTagRef::Owned(start_tag),
    ...
    end_tag_handlers: self.end_tag_handlers,
    ...
    user_data: self.user_data,
}

handle_start_tag also makes one Element for a start tag and runs every matching handler on it (src/rewriter/handlers_dispatcher.rs, Element::new then do_for_each_active_and_deactivate_from). The test "when a later onEndTag() call replaces an earlier one" covers both facts: two handlers for one element, and a registration before and after an await.

The shared helper for the is_cell() filter is a fair cleanup. I left it out to not restart CI for it.

[robobun]

^{Updated 7:48 PM PT - Oct 2nd, 2026}

@dylan-conway, your commit 382b36c is building: #123101

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
Review comments at @src/runtime/api/html_rewriter.rs:
- Around line 3139-3141: Update on_end_tag_ to handle a missing self.pipe
without panicking: replace the expect-based lookup with a fallible check and
return Ok(JSValue::NULL), matching the detached-element behavior.

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Reviewing files that changed from the base of the PR and between 4b02e10 and e2ecafc.

📒 Files selected for processing (4)

• src/runtime/api/html_rewriter.classes.ts
• src/runtime/api/html_rewriter.rs
• test/js/bun/module-graph/module-graph-gc.test.ts
• test/js/workerd/html-rewriter-leak.test.ts

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[claude]

I reviewed the latest push (4f03691 and f020af8 on top of the slot rework) and found no bugs; since this redesigns how handler callbacks are rooted across GC, suspension and cancellation in native code, a human look at the lifetime reasoning is still worthwhile.

What was reviewed:

• abandon_suspension now takes the dead-cell branch on !is_live_cell() (isPendingDestruction), which covers the dead-input point raised inline on the previous push; the writes fail() then makes to a not-yet-swept cell follow the pre-existing handlers pattern.
• PipePin keepalive: EnsureStillAlive is a no-op on a non-cell, so the swept-cell paths through fail()/end_from_stream are unchanged, and placing it in pin() covers every external entry at once.
• Slot lifecycle: hold_end_tag_callback touches the cell only while driving (where handler_list() already assumes a live cell) or after the Done/dead checks; take_end_tag_callback guards is_cell(); release_handlers clears the array and free list together; putDirectIndex/getDirectIndex never reach Array.prototype accessors.
• Replacement through lol-html element user data after a suspension could not be checked against the vendored fork (no checkout here); the b1/await/b2/b3 test asserting a single b3 run would fail if the parked copy dropped it.

Extended reasoning...

The change touches src/runtime/api/html_rewriter.rs and html_rewriter.classes.ts: EndTagHandler drops its ProtectedJSValue for an index into a new GC-visited endTagHandlers array slot on the transform cell, Element gains a BackRef to its pipe, PipePin adds an EnsureStillAlive on the cell, and abandon_suspension treats a pending-destruction cell as dead; about 150 runtime lines plus ~580 test lines. No injection, auth or data-exposure surface, but it is GC-lifetime and memory-safety code in the HTMLRewriter pipe, the category the repository guidance flags as most-blocked. The bug hunt ran dry and my own read found nothing, and the two new commits address the inline point from the prior push, but the complexity of the rooting/suspension/cancellation interplay and the inability to verify the lol-html fork's user-data handling locally rule out approving without a human look.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

A follow-up for the cancel test, in case you want it. I did not open a PR for it.

The test "cancelling the output cancels a JS stream input while a collection is in flight" can fail only on a release ASAN build of the old code. The shape of its script hides the bug on the other builds:

• It calls reader.cancel() in the continuation of an await. With that shape the old code cancels every input, also with BUN_JSC_slowPathAllocsBetweenGCs=1 (4 of 4 on a debug build and on a release build).
• From a macrotask (setImmediate(() => reader.cancel())) the old code loses the input. cancelled is 0 of 2 on a debug build and on a release build of main from before this PR, for BUN_JSC_slowPathAllocsBetweenGCs of 2, 3 and 5.

Commit 3c8f5ed (branch robobun/5ebe824b/endtag-more-tests, one commit on top of main at 8f6a13a) replaces that test with test.concurrent.each([2, 3, 5]) of the macrotask shape:

• It has no isDebug or isWindows branch, and it does not use BUN_JSC_collectContinuously.
• On a debug build of main at 8f6a13a the three cases pass. On a debug build of 519963e (before the merge) the three cases fail.
• The cases take an explicit 30 s timeout. One case needs 3 to 9 s on a debug build when the machine is loaded.

The commit also adds one test to "every end tag runs the callback of its own element": async onEndTag() callbacks that park the rewrite while the callbacks of the outer elements wait in the array. It passes before and after this PR. That path had no test.

To take it: git cherry-pick 3c8f5ed082. It touches only test/js/workerd/html-rewriter-leak.test.ts.