fetch, S3: collect an abandoned body stream whose body stalls under the high-water mark - #43743
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…stalls under the mark The producer's hold on a response body stream rooted the stream's source wrapper except while parked, and a fetch parks only once 256 KiB wait unread. A body that stalls or trickles under the mark never parks, so a stream that was touched and dropped was never collected: its connection and its request slot stayed taken until the peer gave up. 256 of them and every later fetch() pended. The hold now roots the wrapper only for a consumer that takes the bytes without holding the stream (a native sink, a whole-body read). Any other reader holds the stream from JS, so once nothing can read it, it is collected and its fetch or S3 download is aborted, parked or not. Parking keeps its other meaning: pause the transport and release the loop.
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Updated 9:29 PM PT - Sep 21st, 2026
❌ @robobun, your commit 7d1739a has 1 failures in
🧪 To try this PR locally: bunx bun-pr 43743That installs a local version of the PR into your bun-43743 --bun |
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Status: ready for review. CI: On build 119531, How I reproduced it. 256 status probes against a raw TCP origin that sends a response head and half a chunk, then nothing. Then 5 full collections, then one more import net from "node:net";
let open = 0;
const srv = net.createServer(s => {
open++;
s.on("error", () => {});
s.on("close", () => open--);
s.once("data", d =>
/GET \/ok/.test(d)
? s.end("HTTP/1.1 200 OK\r\nContent-Length: 2\r\nConnection: close\r\n\r\nok")
: s.write("HTTP/1.1 200 OK\r\nContent-Type: text/event-stream\r\nTransfer-Encoding: chunked\r\n\r\n64\r\n" + Buffer.alloc(50, "x")),
);
});
await new Promise(r => srv.listen(0, "127.0.0.1", r));
const url = `http://127.0.0.1:${srv.address().port}`;
async function probe() {
const res = await fetch(url + "/events");
if (!res.body) throw new Error("no body");
return res.status;
}
for (let i = 0; i < 256; i++) await probe();
for (let i = 0; i < 5; i++) {
Bun.gc(true);
await Bun.sleep(100);
}
console.log("connections the peer still holds:", open);
console.log("next fetch():", await Promise.race([fetch(url + "/ok").then(r => r.text()), Bun.sleep(3000).then(() => "PENDING after 3 s")]));
process.exit(0);
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Navigate logical layers of code changes, visualize relationships, and explore their blast radius. No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info⚙️ Run configurationConfiguration used: Repository: oven-sh/bun/.coderabbit.yaml Review profile: ASSERTIVE Plan: Essentials Run ID: 📒 Files selected for processing (1)
Included review availability: Your plan provides up to 10 included reviews per hour; 1 remains after this review. WalkthroughThe change synchronizes stream wrapper rooting with active sink or buffer actions. Fetch and S3 delivery paths use instance delivery state. Regression tests cover garbage-collection aborts and consumer retention during trickled responses. ChangesStream wrapper rooting and collection
Suggested reviewers: Priority: ⬆️ High Merge Risk: ⚪ Minimal · up to Abandoned stalled fetch and S3 streams can be collected and aborted while active consumers retain complete bodies. The supplied regression coverage addresses these behaviors, leaving no merge-blocking risk beyond normal CI validation. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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In `@test/js/web/fetch/fetch-backpressure.test.ts`:
- Around line 1727-1797: Replace the parameterized test loops over shapes and
consumers with describe.each(...) blocks, preserving each entry’s human-readable
name and the existing asynchronous test bodies and serial-suite behavior.
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src/runtime/webcore/ByteStream.rssrc/runtime/webcore/ReadableStream.rstest/js/web/fetch/fetch-backpressure.test.ts
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| if bytes.sink.get().is_some() || bytes.buffer_action.get().is_some() { | ||
| Source::root_wrapper(source); | ||
| } else { | ||
| Source::unroot_wrapper(source); |
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🟡 (optional) High-throughput fetch clients that touch res.body and then drop the Response now lose a keep-alive connection per request whenever a GC lands before the short body finishes. The unroot at ByteStream.rs:119 makes the wrapper collectable as soon as JS drops it; the sweep then reaches on_body_stream_collected (FetchTasklet.rs:1767) and abort_transport closes the HTTP/1.1 socket the pool would have reused after draining. On the base the wrapper stayed rooted under the mark, the body drained, and the connection returned to the pool. Fix: for a collected stream whose remaining body is small (under the high-water mark and content-length known), drain to the mark and return the connection to the pool instead of aborting, as undici does; abort only when the remaining body exceeds the drain budget.
Why this was flagged
Trigger: any code that reads res.body (e.g. checks res.body !== null, calls getReader() and releases it, or passes the body to a helper that never reads it) and then drops every reference before the body completes. Common in status probes, HEAD-like GETs, and libraries that access .body eagerly. Rate: once per such fetch; a GC during a body still arriving over several packets is routine under load. Mechanism: hold() at ByteStream.rs:101 now calls sync_wrapper_root, which takes the unroot branch at ByteStream.rs:119 because no sink or buffer_action exists; ReadableStream.rs:1197-1202 downgrades this_jsvalue to Weak. On sweep NewSource finalize runs consumer_collected (streams.rs:1117) -> on_body_stream_collected (FetchTasklet.rs:1767) -> abandon_response_body (FetchTasklet.rs:1931), which calls abort_transport, closing the socket. On the base branch the wrapper stayed Strong (increment_count at ReadableStream.rs:1169-1171) until buffered_len reached BODY_HIGH_WATER_MARK, so a short body completed and the keep-alive connection was reused. The PR calls this an accepted downside, but at…
Verification: nit — acknowledged in diff: the PR description's "Downsides" paragraph states exactly this ("A touched stream dropped while a short body still arrives is aborted if a collection lands first. On HTTP/1.1 that closes a connection the pool would have reused"), and the bound is accurate. Trigger: JS accesses res.body (or getReader() without reading/cancelling), drops every reference, and a GC…
) ### Problem - A Worker that ends while `Bun.write(path, response)` pipes a native body (a `fetch()` body, a child's stdout) frees the sink, then uses it. ASAN: `heap-use-after-free` in `FileSink::finalize` (`src/runtime/webcore/FileSink.rs:1052`) under `~JSReadableFileSinkController`. Release builds are silent. - Since #42114 a pipe's controller cell holds no ref on the sink. In the VM's last sweep its destructor ran `FileSink::finalize`, which releases the keep-alive ref, then a wrapper's ref. For a native source the keep-alive ref is the last one. - The freed sink's `Drop` also detaches the dying cell: debug JSC asserts first (`ASSERTION FAILED: decontaminate()`). ### Fix - The controller destructor calls a new `JsSinkType::controller_finalize` hook, which defaults to `finalize` for the other sinks. - FileSink's hook drops its root on the dying cell untouched and releases only the refs that shutdown strands. A new flag tracks the JS pump promise's ref. - Verified: the new test in `test/js/web/workers/worker-terminate-lifetime.test.ts` fails with main's `src/` and passes with the fix, on Linux and Windows. ### Background - A stream pipe (`Bun.write(file, stream)`, a `Bun.spawn` stdin) creates a controller cell, which the sink roots until the pipe ends. - The keep-alive ref holds a sink until its writer closes, an event that never arrives at VM teardown. - The last sweep (`Heap::lastChanceToFinalize`) destroys every cell, rooted or not, in no fixed order. ### Downsides - A pump that ended before its pipe drained still leaks the sink at worker teardown, and a late-reading child never sees EOF. Same on main. - Windows: `Bun.spawn({ stdin: jsStream })` in a terminated worker still leaks one FileSink. Same on main. <details><summary>Notes</summary> **Related.** #40213 (open, a refactor of `FileSink.rs`) adds the same `controller_finalize` hook. #42108 (open) covers a different pair: a JS pump whose `pull()` meets the termination. **Report and repro.** A fuzz ledger row reported this on main a65a98f (no GitHub issue). Its script: a local `Bun.serve` streams 40 MB, each of 10 workers runs `fetch(url).then(r => Bun.write(path, r))`, and the parent calls `terminate()` 5 ms after the first message. Debug ASAN build of main: 0 of 4 runs survive (`ASSERTION FAILED: decontaminate()` from `JSSinkController__setPipe` <- `PipeCell::clear_slots` <- `FileSink::release_pipe` <- `Drop` <- `clear_keep_alive_ref` <- `finalize` <- `~JSReadableFileSinkController` <- `PreciseAllocation::lastChanceToFinalize`). With the fix: 3 of 3 survive. **The ASAN report on a debug build.** The debug JSC assertion fires before the read of freed memory. An experiment build of main that drops the `PipeCell` root before the release (so `Drop` does not touch the cell) shows the reported frames: `heap-use-after-free READ of size 1` in `FileSink::finalize` <- `FileSink__finalize` <- `~JSReadableFileSinkController`, freed by `FileSink::deref` <- `clear_keep_alive_ref` (`FileSink.rs:621`) <- `finalize`, allocated by `FileSink::init` <- `pipe_readable_stream_to_blob` (`Blob.rs:1531`). **Refs at the last sweep.** `pipe_readable_stream_to_blob` drops `init`'s ref on return. A native pipe (ByteStream or FileReader source) then holds only the keep-alive ref. A JS pump holds the ref taken before `promise.then(...)`, and sometimes the keep-alive ref. `Bun.spawn` stdin adds the Subprocess's `Writable::Pipe` ref: the old trailing `deref` took that ref away from the Subprocess. **Cells, one worker each, debug builds.** The body never ends, and the worker reports once the pipe is live, so no cell depends on timing. | cell | Linux main | Linux fix | Windows main | Windows fix | | --- | --- | --- | --- | --- | | `Bun.write(path, response)` x `terminate()` | assert | pass | assert | pass | | same x `process.exit()` in the worker | assert | pass | assert | pass | | same x uncaught throw in the worker | assert | pass | assert | pass | | `Bun.file(path).write(response)` | assert | pass | assert | pass | | `Bun.write(path, new Response(response.body))` | assert | pass | assert | pass | | `Bun.write(path, new Response(child.stdout))` | assert | pass | pass | pass | | `Bun.write(path, new Response(jsStream))` | assert | pass | assert | pass | | `Bun.spawn({ stdin: response.body })` | assert | pass | pass | pass | | `Bun.spawn({ stdin: jsStream })` | assert | pass | leaks 1 | leaks 1 | Windows closes a worker's pipe handles in the teardown stop phase, so its pipe-backed sinks end before the last sweep. The last row leaks one FileSink on Windows with and without this change (the pump promise's ref, on the `on_close` path). The test skips that cell on Windows. A separate Windows-only crash (a worker terminated while a child with `stdin: "pipe"` is alive) also reproduces on main. Both are reported separately. **The leak assertion.** The test also expects `fileSinkInternals.liveCount()` to return to its baseline. An experiment build whose `controller_finalize` releases nothing fails it with `leakedFileSinks: 9`. **Not changed.** A sink that has both a JS wrapper and a live native pipe (`proc.stdin` read while a stream feeds stdin) can still be freed by the wrapper's `finalize` in the last sweep with the controller attached. `Drop` then detaches a cell whose structure can be dead. That is the old behavior, and it needs the wrapper to be swept first. Probed 3 runs each with `Bun.spawn({ stdin: body })` plus `proc.stdin` in a terminated worker, with a fetch body and with a JS stream: no assertion and no leak, because the Subprocess holds its own ref. **Also not changed, filed separately.** A pump that ends through the controller's `end()` before the pipe drains leaks the sink. `__endImpl` nulls `m_sinkPtr` before `endWithSink`, so the destructor's `if (m_sinkPtr)` skips both hooks, and the keep-alive ref that the `Pending` arm of `end_from_js` took is never released. `Bun.spawn({ stdin: directStream })` in a terminated worker leaks one FileSink, and a child that reads its stdin late never sees EOF. The same on the baked canary, so it is not from this change, and it survives it: the route is not the destructor this PR fixes. **Left open.** `FlushPendingTask::release_unrun` releases the task's ref without reading `run_pending_later.has`, while the `has`-gated release here does read it. Both predate this change, and the two new callers of the helper are idempotent against each other through the same `has.replace(false)`. Which side owns that ref needs its own change: `run_pending` clears `has` with a task still queued, so `run_from_js_thread` has to release unconditionally, and gating `release_unrun` on the flag would strand the ref. **Suites run on the Linux debug ASAN build with the fix:** `worker-terminate-lifetime.test.ts`, `bun-write.test.js`, `spawn-stdin-readable-stream.test.ts`, `spawn-stdin-readable-stream-edge-cases.test.ts`, `spawn-stdin-pipe-fd-leak.test.ts`, `serve-direct-readable-stream.test.ts`, `html-rewriter.test.js`, `html-rewriter-leak.test.ts`, `fetch-backpressure.test.ts`, `direct-readable-stream.test.tsx`. The new test also passed 5 of 5 with `detect_leaks=1` and `BUN_DESTRUCT_VM_ON_EXIT=1`, and 5 of 5 on Windows. **Local failures that do not come from this change:** the `dns.lookup()` LeakSanitizer report in `worker-terminate-lifetime.test.ts` (a 16-byte `node_fs_binding::Binding`), 5 s timeouts in `bun-write.test.js` ("on large files" alone, four more only under the concurrent full-file run), a 5 s timeout in `spawn-stdin-readable-stream-edge-cases.test.ts` (three sequential debug children take 1.8 s each), and a 5 s timeout in `spawn-stdin-readable-stream.test.ts` ("does not leak native FileSink when ReadableStream is used as stdin"), which times out the same way on a debug ASAN build of main without this change. **After the rebase onto b7ea95a:** rebuilt the debug ASAN build and reran the new test plus its neighbours in `worker-terminate-lifetime.test.ts` (Blob stdin, streaming-request-body fetches, HTMLRewriter async handlers, async-iterable bodies): 5 of 5 pass. The same test fails on a debug build of main's `src/` with `ASSERTION FAILED: decontaminate()`, and passes silently on a release build of main. **Against main 4ada08b.** #43743 landed after this branch was pushed and changes when a fetch body's stream wrapper is rooted. A body with a native sink attached stays rooted, so the fetch cells of the new test are not affected: on a local merge of this branch with that commit the new test passes 5 of 5 on the debug ASAN build. The branch still merges cleanly. </details>
2 participants
Problem
fetch()body stream is never collected if its body stalls under the 256 KiB high-water mark. Its connection and request slot stay taken: after 256 of them every laterfetch()pends. Same fors3file.stream().ProducerHold(src/runtime/webcore/ByteStream.rs:68) rooted the stream's wrapper except while parked, and it parks only at the mark.Fix
sync_wrapper_root).Readable.fromWebhandle, a pending pull's promise. With none left it is collected and its transfer aborted.test/js/web/fetch/fetch-backpressure.test.ts. 5 collection tests fail on 1.4.3 and pass here. 10 consumer shapes still get their body through full collections.Background
ReadableStreamover a nativeByteStreamin a refcountedNewSource. JS readers reach it through that source's JS wrapper.ProducerHoldis the counted ref a fetch or S3 download keeps on theNewSource. Such a ref can root the wrapper. At the mark with nothing reading the producer parks: it pauses the transport and releases the loop.NewSource::finalizecallconsumer_collected, and the producer aborts its transport.Downsides
Notes
Repro: 256 status probes against a peer that sends a head and half a chunk, then nothing. Then 5 full collections, then one more
fetch().res.bodytouchedgetReader(), never readread(), thenreleaseLock()read(), reader droppedThe consumer tests drive each shape (reader loop,
pipeTo,tee,Readable.fromWeb,textStream,res.body.text(),res.text()afterres.body,HTMLRewriter,Bun.write, aBun.serveresponse) with a full collection before every piece of a 4 KiB body. They pass before and after: they guard the new rule against aborting a body that something still reads. A wider manual run (18 shapes, two collections per piece) also passes.The new tests are in a serial block. Under the 20-way concurrency of the rest of the file, a test that waits for collections took 2 to 5 s on the debug build (the 1 GiB drain tests keep the loop busy), and its own collections stalled the others.
The last two rows stay held on purpose while the peer is silent. The stream's controller has a pull in flight, and the promise of that pull is protected until bytes arrive for it, because a pending
reader.read()can be the only thing a suspended async function hangs from. The next bytes from the peer settle the pull, and the stream is then collected (the two "trickle peer" tests). A silent peer is bounded by the idle timeout. Node holds these two shapes as well.Self-review, checked directly:
PinnedBytespins the allocation,consumer_collectedonly releases the hold, and the delivery checksis_held()afterwards.sync_wrapper_rootinside a sweep: not reachable. The sweep path iswrapper_finalized,consumer_collected,release(). The root changes at hold, after a delivery, and from a consumer's drain or attach signal, all on the JS thread outside a collection.ProducerHoldand has its own test.park()andunpark()keep only the parked bit, which still drives the pause and the loop ref at the mark.FetchTasklet::on_response_finalizeis unchanged. It leaves a body that has a stream to the stream's own collection, because the stream can outlive its Response (const { body } = res).Not changed: a touched, unread stream under the mark still holds the event loop until it parks, ends, or is collected.
Suites run on the debug (ASAN) build, all passing:
fetch-backpressure(104),fetch-stream-cancel-leak,fetch-response-finalizer-sweep,fetch-abort-stream-body(75),fetch.stream(119; the 8 "multiple parts" cases brush the 5 s default under full-file concurrency on this build and pass alone),fetch-keepalive(49),body-stream(9086),body-clone,body-mixin-errors,regression/issue/33227,html-rewriter(186),html-rewriter-leak,streams(612),proxy(92),serve.test.ts -t "prox|stream|body"(84),serve-response-stream-sink-leak,node-fetch,s3-stream-cancel-leak,s3-stream-error-gc,s3-connection-close,s3-upload-stream-gc.Two failures that do not involve a response body:
bun-write"copyFileRange is not available, on large files" takes 4.9 s alone and 7 s in the full file on this build (5 s default timeout), andworker-terminate-lifetime"terminate() while dns.lookup() is in flight" reports a 16-byte LeakSanitizer leak from the c-ares teardown (24 of 25 pass).no test proof · iteration 1 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/web/fetch/fetch-backpressure.test.ts