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fetch: park an unread body stream instead of buffering it without bound - #39590
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… is collected A response body stream that nothing reads (res.body touched and left alone, or its reader released) made the fetch switch to BufferAll on the next chunk: the rest of the body was read into the ByteStream with no bound. The flip existed because the tasklet rooted the stream (a Strong on it, and a counted ref that also rooted the source wrapper), so an abandoned stream could only go away once the body ended. The tasklet now holds only the counted ref on the native source, and that ref no longer roots the wrapper (SourceContext::NATIVE_REF_ROOTS_WRAPPER, off for ByteStream). The body is delivered through the source. When the wrapper is collected with no native consumer attached, ByteStream::finalize_detach tells the producer, and the tasklet cancels the fetch from a queued task, the same path as reader.cancel(). While the stream is alive but unread, the transport stays paused. The event loop ref follows the consumers: a pull that goes pending or a sink that is wired refs it, a delivered chunk that nothing waits for unrefs it, and creating the stream no longer refs it. poll_ref becomes a JsCell because the drain signal re-enters the tasklet through a shared ref.
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In `@src/runtime/webcore/fetch/FetchTasklet.rs`:
- Around line 1749-1762: Update on_response_stream_collected to remain
pointer-based: access global_this through the raw this pointer without creating
a &FetchTasklet reference, and invoke
ThreadSafeRefCount::<FetchTasklet>::ref_(this.as_ptr()) directly. Preserve the
shutdown check and CollectedResponseStreamHop enqueue flow.
In `@test/js/web/fetch/fetch-backpressure.test.ts`:
- Around line 686-699: Start draining proc.stderr concurrently before entering
the proc.stdout for-await loop, retain its resulting promise, and await that
promise in the final assertion instead of beginning the stderr read afterward.
Keep process exit observation concurrent as required and preserve the existing
combined stderr, exitCode, and ordered lines assertions.
In `@test/js/web/fetch/fetch-stream-cancel-leak.test.ts`:
- Around line 247-253: Scale the collection deadline in the fetch cancellation
test around the aborted polling loop using the harness build flags isASAN and
isDebug, increasing the 3000 ms baseline for debug and ASAN builds while
preserving the existing deadline for normal builds and the N - aborted
assertion.
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src/runtime/webcore/ByteStream.rssrc/runtime/webcore/ReadableStream.rssrc/runtime/webcore/fetch/FetchTasklet.rssrc/runtime/webcore/streams.rstest/js/web/fetch/fetch-backpressure.test.tstest/js/web/fetch/fetch-stream-cancel-leak.test.ts
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@robobun test/js/bun/io/bun-write.test.js - SIGSEGV on 🐧 13 aarch64 |
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I reviewed this PR and didn't find any bugs. Because it reworks GC-rooting, refcount ownership, and event-loop keep-alive semantics for the fetch body stream (removing the Strong root, adding a finalizer-driven cancel hop, and changing when poll_ref is held), a human look is still worthwhile.
What was reviewed:
- Refcount balance on the
CollectedResponseStreamHop—ManagedTask::new_ownedinstalls acleanupthat drops the box, and the box'sDropcallsFetchTasklet::deref, so the ref taken inon_response_stream_collectedis released whether the task runs or the VM releases the queue unrun. on_consumer_attachedno longer needs theBufferAll → Pausedtransition since the only path that setBufferAllon a stream (drop_backpressure_if_unobserved) is removed; the othersignal_consumer_attachedcallers (server request body, S3) hit no-opSourceHandle::startarms and are unaffected.- Terminal-delivery ordering in
on_body_received: producer is unhooked beforeon_data, anddecrement_countruns after, so a delivery into a source whose wrapper is already gone lands in memory the counted ref still pins. on_wrapper_finalizedcorrectly stays silent when a nativesinkorbuffer_actionis attached, since those consumers do not hold the wrapper.
Extended reasoning...
Overview
This PR removes the ReadableStreamStrong GC root that FetchTasklet held on its response body stream and replaces it with a design where only consumers root the stream. The tasklet keeps only a counted native ref on the NewSource<ByteStream> and delivers into it directly. A new SourceContext::NATIVE_REF_ROOTS_WRAPPER const (false for ByteStream) prevents increment_count from upgrading the wrapper backref to Strong. When the wrapper is collected, ByteStream::finalize_detach → SourceHandle::consumer_collected → FetchTasklet::on_response_stream_collected queues a ManagedTask that runs on_stream_cancelled outside the GC sweep. drop_backpressure_if_unobserved is removed; a stream nobody reads now leaves the transport paused. poll_ref becomes JsCell<KeepAlive> and is ref'd/unref'd by consumer signals (on_consumer_attached, on_stream_drained, update_keep_alive_for_body_consumer) rather than at stream creation.
Files touched: FetchTasklet.rs (core logic), ByteStream.rs (finalize hook, pull-goes-pending signal), ReadableStream.rs (NATIVE_REF_ROOTS_WRAPPER gate in increment_count), streams.rs (consumer_collected dispatch), plus two test files with 8 new tests.
Security risks
None identified. This is internal lifetime/backpressure plumbing; no untrusted input parsing, auth, or crypto is touched. The unbounded-buffering behavior being fixed was itself a resource-exhaustion vector.
Level of scrutiny
High. This is exactly the class of change the review guidelines flag as most-blocked: it removes a GC root, changes refcount ownership across a GC-finalizer boundary, adds code that runs inside a sweep and schedules deferred teardown, and redefines when the process stays alive. Each of those is individually memory-safety-sensitive; together they are an architectural change to how a fetch body stream's lifetime is managed. The reasoning in the PR description is thorough and the code is carefully commented, but the interaction surface (re-entrancy through JsCell, finalizer ordering, the residue case where a pending pull roots a dropped stream, the on_response_finalize path deciding from response_stream_source instead of the removed Strong) is large enough that a maintainer familiar with the #29831/#38457 history should confirm the design.
Other factors
- The tests are well-structured: they cover both directions (paused body doesn't hold the process; a pending read does), the resume path after a later reader attaches, and GC-driven abort in three abandonment shapes. The PR description reports they fail on stock bun and pass with the change, and that the wider fetch/http/stream suites pass.
- I verified the
ManagedTask::new_ownedcleanup path drops the boxed hop (which derefs the tasklet) so the ref is balanced when the VM tears down the queue unrun. - I checked that the other
signal_consumer_attachedcall sites (RequestContext, S3 client,wire_native_sink) route throughSourceHandle::startarms that are either the fetch arm or no-ops, so the semantic change toon_consumer_attached(now only refs the loop) does not break them. - The
ReadableStreamStrongimport is still used for the request-body variant atFetchTasklet.rs:181, so the removed field's type is not dead code.
…source across a delivery wire_native_sink stores the sink's cell in the source wrapper's sinkOwner slot, so the wrapper is what keeps a natively wired sink (HTMLRewriter) alive. With the producer ref no longer rooting the wrapper, a transform whose JS handles were dropped was collected while the fetch still delivered into it (use after free in RewriterPipe::write). NATIVE_REF_ROOTS_WRAPPER becomes a context method: ByteStream answers true while a sink is wired, and wire_native_sink re-evaluates it when it wires one. decrement_count still drops the root once only the wrapper's ref remains. A non-terminal delivery now holds its own ref on the source. The sink's handlers can cancel the body from inside on_data, which releases the producer ref, and a collection in the same script then freed the source under on_data; before, the stream handle that on_body_received kept on its stack happened to root it. on_response_stream_collected no longer forms a reference to the tasklet. The tests drain stderr concurrently and scale the collection deadline for slow builds; a new one drops every handle to a transform over a fetch body and collects while the body arrives.
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…ventLoopStats Adds tasks, immediateTasks, concurrentTasksEmpty, loopActive and eventLoopAlive next to the existing counters, so a test that finds a process unexpectedly alive can say why. No-Verification-Needed: testing-only binding
No-Verification-Needed: dead-store removal, no behavior change
No-Verification-Needed: test-only change
… transport on_start_streaming flipped Paused->AutoPause and scheduled the resume before it took scheduled_response_buffer under the mutex. A chunk the HTTP thread appended (and paused for) in that window was drained into the new stream here, its progress task then found the buffer empty and returned early, and nothing was left to undo the pause: the body sat with one chunk buffered and the process stayed alive. Drain first, then flip and resume. Reproduced locally as 2/60 hangs of `void res.body` against a flooding server; 0/60 after.
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c7df647 removes the proxy abort part of this PR. The Verified with a debug build on c7df647: The two PRs are independent. Either one can land first. |
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@robobun don't do that. revert it back. |
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Reverted in 31d2cc3. The tree is identical to 8f3d32d again: the #36700 carries the same |
This was referenced Aug 19, 2026
robobun
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…sponse is collected A Response whose body nothing touches used to pause the transport behind the first packet of the body, which pinned its connection until the Response was collected. The collection then switched the transport to BodyReceiveMode::Ignore and resumed it, so that the connection could be pooled: the whole remaining body was downloaded to be thrown away, however long it was. callback() now keeps receiving such a body up to UNOBSERVED_BODY_HIGH_WATER_MARK, the bound #39590 gave a body stream that nothing reads, and pauses there. A short body completes on its own and frees its connection at once. A body that is still underway when its Response is collected is longer than that, or its origin has stalled, so on_response_finalize aborts the fetch, as on_body_stream_collected does for a collected stream. The resume-to-drain path in ignore_remaining_response_body has no caller left and goes away.
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…sponse is collected A Response whose body nothing touches used to pause the transport behind the first packet of the body, which pinned its connection until the Response was collected. The collection then switched the transport to BodyReceiveMode::Ignore and resumed it, so that the connection could be pooled: the whole remaining body was downloaded to be thrown away, however long it was. callback() now keeps receiving such a body up to UNOBSERVED_BODY_HIGH_WATER_MARK, the bound #39590 gave a body stream that nothing reads, and pauses there. A short body completes on its own and frees its connection at once. A body that is still underway when its Response is collected is longer than that, or its origin has stalled, so on_response_finalize aborts the fetch, as on_body_stream_collected does for a collected stream. The resume-to-drain path in ignore_remaining_response_body has no caller left and goes away.
Jarred-Sumner
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Aug 24, 2026
…sponse is collected A Response whose body nothing touches used to pause the transport behind the first packet of the body, which pinned its connection until the Response was collected. The collection then switched the transport to BodyReceiveMode::Ignore and resumed it, so that the connection could be pooled: the whole remaining body was downloaded to be thrown away, however long it was. callback() now keeps receiving such a body up to UNOBSERVED_BODY_HIGH_WATER_MARK, the bound #39590 gave a body stream that nothing reads, and pauses there. A short body completes on its own and frees its connection at once. A body that is still underway when its Response is collected is longer than that, or its origin has stalled, so on_response_finalize aborts the fetch, as on_body_stream_collected does for a collected stream. The resume-to-drain path in ignore_remaining_response_body has no caller left and goes away.
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Aug 25, 2026
…n.write(dest, response) streams to disk (#39690) ### What this changes `fetch()` response bodies now follow two rules, each implemented in one place: **1. Backpressure — one high-water mark.** Body bytes that no consumer has taken yet live in exactly two places: the HTTP→JS hop buffer (`scheduled_response_buffer`, HTTP thread) and the body `ReadableStream`'s internal buffer (JS thread). Whichever side reaches `BODY_HIGH_WATER_MARK` (256 KiB) flips the fetch `Flowing → Paused`; whoever takes bytes out flips it back and schedules the resume. The transport applies `Paused` after its next read (h1: stop polling the socket; h2: withhold `WINDOW_UPDATE`; h3: `want_read(false)`). `.text()`/`.json()`/`.arrayBuffer()`/`.bytes()`/`.blob()`, `Bun.write(file, res)` switch to `BufferAll` (never pause, pre-reserve `Content-Length`); `Bun.readableStreamTo*(res.body)` is never paused either. **2. Abandonment — one path.** When nothing can ever read the rest of a body — the `Response` was garbage-collected with nothing waiting on it, the parked body stream was collected, `reader.cancel()` / `res.body.cancel()`, or a null-body status (204/205/304/HEAD) that still has content on the wire — `abandon_response_body()` marks the fetch `Abandoned`, aborts the transport (h1: close the socket; h2/h3: reset the one stream, session stays pooled), releases the event-loop ref and the native response. Safe inside a GC sweep (no JS is touched). `BodyReceiveMode` is `Flowing | Paused | BufferAll | Abandoned`. Gone: pausing the transport after **every** chunk once a stream is attached (#29831), the separate 256 KiB rule for streams (#39590) vs. first-packet pause for untouched responses, the `Ignore` mode's "resume and download the rest into the void" path, `is_buffering_body`, and fetch's use of the `response_body_streaming` signal. Also (from the earlier commits on this PR): `.body` / `.textStream()` on a body that already failed now hand out *the* body stream and mark it used (`Body.rs`), and a `Response` collected while `Bun.write(file, res)` waits for it no longer drops the body (fixes #40278). ### `Bun.write(dest, response)` streams to disk `Bun.write(path, response | request | readableStream)` used to collect the whole body in memory (via `on_receive_value`) and write it at the end; a bare `ReadableStream` was stringified to `"[object ReadableStream]"`; a `new Response(jsStream)` never settled; and a `Response` collected mid-download left the write pending forever (#40278). Now: - A body that is a stream, or whose producer can stream (fetch, `Bun.serve` request bodies, `HTMLRewriter`), is piped into a `FileSink`. Native `ByteStream` sources are wired straight to the sink (`wire_native_sink`, no per-chunk JS); the fetch backpressure above applies, so a download to disk holds at most the high-water mark. **128 MiB `fetch → Bun.write`: peak RSS +161 MB on 1.4.0 → +13 MB here** (debug build). - `FileSink` gains `truncate`/`mkdirp` options (so `Bun.write` replaces the file and creates parent dirs, `writer()` is unchanged) and a completion promise for a piped stream that resolves with the bytes written or rejects with the error that ended the stream *or* the write (sync write error, deferred flush error, `end()` flush error are all recorded; on a write error the source is cancelled so the download stops). - The JS pump (`readStreamIntoSink`) treated a rejected `write()` as success and carried on; it now fails the pump with that error. String chunks are counted by UTF-8 length. - A body that has already fully arrived (also behind an untouched `.body` stream) is written as a blob. A used/locked/disturbed body rejects with `ERR_BODY_ALREADY_USED` instead of writing an empty file. - Types: `Bun.write`/`BunFile.write` accept `ReadableStream` and `Request`. ### S3 - **Upload** (`Bun.write(s3file, response)`, `s3file.write(stream)`, `s3file.writer()`): the multipart uploader's sink already returns backpressure when its part queue is full, so with the fetch change the origin is paced end to end (test: a 16 MiB fetch body through a one-part queue stops the origin while the part PUTs are held). These now resolve with the bytes written instead of `0` (#35671). - **Download** (`s3file.stream()`, `Bun.write(file, s3file)`, `new Response(s3file)`): had **no** receive backpressure — the S3 client wired `Signals` without `body_receive_mode` and its producer's `on_ready` was a no-op, so a slow or absent reader buffered the whole object (fake 64 MiB object, stalled reader: origin sent all 64 MiB on 1.4.0). Now `S3HttpDownloadStreamingTask` uses the same `BodyReceiveMode` and HWM rule; `S3DownloadStreamWrapper` does the JS-thread half, resumes on drain, parks an unread stream (loop released, wrapper collectable) and aborts on collection/cancel. Stalled reader → origin stops at ~8 MiB (socket buffers + HWM); `Bun.write(file, s3file)` resolves with the byte count (was `0`); a process holding an unread S3 stream exits. - The source-ref + parked bit + HWM decision that fetch and S3 share is one type, `byte_stream::ProducerHold`. - **Aborting the source of an S3 upload no longer commits a truncated object.** `ByteStream::on_cancel` left a wired native sink attached; the producer's later error was dropped as "already done" but flagged the last chunk, so the multipart sink saw EOF on its next drain and sent `CompleteMultipartUpload`. A cancelled stream now fails its native sink with an `AbortError` (pre-existing on main; test: abort mid-upload rejects and nothing is committed). ### What your server sees, before → after | Client code | Before (main) | After | |---|---|---| | `const r = await fetch(u); if (!r.ok) return;` — body **≤ 256 KiB**, `r` still reachable | Transfer stalls after the first packet; connection pinned until `r` is GC'd, then the rest is read and the connection pooled | Transfer completes immediately; connection back in the keep-alive pool immediately | | same, body **> 256 KiB** (or endless), `r` later GC'd | Stalls after first packet; on GC the **entire remaining body is downloaded and discarded** (a 1 GiB or infinite body is read in full), connection pooled | Stalls at ~256 KiB + kernel socket buffers; on GC: h1 connection closed (server sees `ECONNRESET`/`EPIPE`), h2/h3 `RST_STREAM(CANCEL)` on that stream only | | `for await (const c of r.body)` / `getReader()` loop that keeps up | Socket read-poll disabled after every read and re-enabled from the JS thread (2× `epoll_ctl` + timer reset + cross-thread wake per read) | No pausing at all while the reader keeps up; server sees a smoother, un-stuttered send | | reader stalls (slow consumer, `pipeTo` to a slow sink, `Bun.serve` proxy `return fetch(u)` to a slow client) | Paused after each chunk | Paused once 256 KiB is waiting; resumed when it drains. Server sees TCP/h2 flow control kick in the same way, slightly later | | `r.body` touched but never read, `r` kept | Paused at 256 KiB (#39590) | Same | | `reader.cancel()` / `AbortSignal` | Connection closed / stream reset | Same | | `205` (or other null-body status) framed **with** content | Extra bytes drained, connection pooled | Connection closed (RFC 9110 forbids content here) | | `r.text()`, `Bun.write(path, r)`, `Bun.readableStreamToText(r.body)` | Never paused | Never paused | ### RSS / memory - **Streaming consumer that keeps up:** unchanged (bytes pass straight through), minus the per-read syscalls. - **Stalled stream / untouched-but-reachable `Response`:** bounded at ≤ 256 KiB per side + one socket read (worst case ~512 KiB + one read if the JS thread is blocked while the stream already holds data). Before: an untouched `Response` held one packet (but pinned a connection per response); a touched-but-unread stream held 256 KiB. - **Abandoned + collected long body:** before, bandwidth and CPU for the whole remaining body (nothing retained, but it was all received, decompressed and thrown away); now nothing further is received. - Holding thousands of unread `Response` objects alive now costs up to 256 KiB each instead of one packet each — in exchange their connections are returned instead of pinned. Read or drop responses you don't need. ### Other observable changes - Chunk sizes seen by `getReader()` on a fast link are closer to wire/read granularity (more, smaller chunks) because the transport is no longer stop-and-wait per chunk. Total bytes and ordering are unchanged. - A `Response` collected while a *short* body is still mid-flight is aborted rather than finished-and-pooled. In practice a sub-256 KiB body that is `Flowing` completes in the same few milliseconds, well before a GC finalizer runs; we chose one rule over a "drain if small" special case (this is also what undici and Chromium do). - Bodies error earlier: a truncated/invalid short body is now detected as it arrives rather than when a reader first attaches (hence the `Body.rs` change so `.body`/`bodyUsed` behave on an already-failed body). ### How this compares | | Backpressure | Unread body, handle dropped | |---|---|---| | **undici (Node `fetch`)** | Pull-based: socket `pause()`d whenever the body stream's queue is non-empty (effectively per-chunk stop-and-wait, single thread) | `FinalizationRegistry` on `Response` → `body.cancel()` → request aborted → **socket destroyed**. undici docs tell you to always consume or cancel the body for this reason | | **libcurl** | No internal buffering; write callback per chunk, `CURL_WRITEFUNC_PAUSE` stops reading | Removing/cleaning up an easy handle mid-body = premature end → **h1 connection closed** (not reusable), h2 stream reset | | **Chromium** | Network service fills a 512 KiB data pipe; stops reading when full | Body/Response dropped → loader cancelled → **h1 connection closed** unless the body already completed, h2 `RST_STREAM` | | **Go `net/http`** | Reads on demand from `resp.Body` | `Body.Close()` with unread bytes → **connection not reused** | | **Bun (this PR)** | 256 KiB high-water mark across the two internal buffers; HTTP thread keeps reading while under it | ≤ 256 KiB bodies complete on their own and pool the connection; longer ones are aborted on GC / cancel (h1 close, h2/h3 stream reset) | So Bun ends up on the Chromium model (bounded pipe + cancel on drop), with the extra property that small bodies never need the consumer to show up for the connection to be reused. ### Tests No sleeps or quiescence polling in the tests this PR adds: waits are promises from the origin/bucket (first blocked write, Nth close/request, first part), `proc.exited`, `fs.watch`, or a `WeakRef` going empty with one full GC per event-loop turn. `test/js/web/fetch/fetch-backpressure.test.ts` (blocks "a Response whose body nothing touches", "body stream nothing is reading", "does not hold the process", "buffered consumers are not throttled", "peer … while receive is paused"), `fetch-backpressure.test.ts` block "S3 receive backpressure" (stalled reader pauses, Bun.write(file, s3file) byte count, unread stream doesn't hold the process, collected stream aborts — all fail on 1.4.0), `test/js/bun/io/bun-write.test.js` (block "Bun.write(path, response) streams the body to the file": streaming proof via bytes-on-disk before the origin finishes, collected Response, touched body, JS-stream body, Request body, bare ReadableStream, `/dev/full` rejections, used-body rejection), `body-mixin-errors.test.ts` (failed-before-read `.body` / `.textStream()`), `body.test.ts` (205 with content), `regression/issue/33227`, `fetch-response-finalizer-sweep`, `fetch-stream-cancel-leak`, `fetch-abort-stream-body`, `fetch-http2-client`, `fetch-keepalive`, `fetch-tcp-keepalive`, `body-stream` (9086), `stream-fast-path`, `body-clone`, `proxy.test.ts`, `filesink`, `spawn-stdin-readable-stream`, `streams.test.js`, `serve.test.ts` pass on the debug build. Pre-existing on this machine's debug build and unchanged by this PR: the four h3 "stalled …" cases and `fetch.stream` "multiple parts" brush the 5 s default under full-file concurrency (pass in isolation), `abort-signal-leak` (2×2500 aborted fetches take ~8 s in debug), `fetch-abort-socket-close-race` TLS case. Fixes #40278. Fixes #13237. Closes #40333. Closes #32906. Closes #31739. Closes #31689. Closes #38184. Closes #35671. <!-- robobun:evidence:begin --> --- **no test proof** · iteration 4 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/web/fetch/fetch-response-finalizer-sweep.test.ts, test/js/web/fetch/fetch-backpressure.test.ts, test/js/web/fetch/body.test.ts, test/js/bun/io/bun-write.test.js <!-- robobun:evidence:end --> --------- Co-authored-by: Jarred Sumner <jarred@jarredsumner.com> Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
tombii
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…n drain deadline drainReaderWithDeadline() called reader.releaseLock() immediately after a deadline win, discarding the still-pending reader.read() promise. Per WHATWG Streams, releaseLock() during a pending read only rejects that JS promise — it does not signal the underlying native source that the stream is abandoned. On Bun's native fetch this is the "touched then abandoned" shape that can trigger unbounded off-heap buffering (oven-sh/bun#39590), matching the flat-JS-heap, unbounded-RSS signature reported in #382. Now aborts first (so the losing read has a chance to actually settle via the torn-down connection), then waits for it to settle within a bounded grace window before releasing the lock, instead of racing ahead of it.
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This was referenced Sep 1, 2026
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The "one read() after it parked" test from this PR went red on darwin aarch64 (builds 110683 and 110902, 5 of 20 bodies survived the 3 s wait). The survivors come from a stale stack slot, not from the fetch code. #41607 clears the slot in the test. |
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Problem
fetch()body stream that nothing reads (res.bodytouched but unread, or read once and thenreleaseLock()) was read to the end into memory with no bound:drop_backpressure_if_unobservedflipped the transport toBufferAllas soon as a chunk reached an unobserved stream. Against an endless body, RSS goes from ~30 MB to 1.8 GB in 2 s. Node stays flat.fetch()body piped natively into aBun.serveresponse (a proxy returning the upstreamResponse) was only unhooked, not cancelled, when the client left, so the upstream fetch sat paused on its connection for good.Fix
FetchTaskletkeeps receiving an unobserved body only until the stream holds 256 KiB nobody has taken (UNOBSERVED_BODY_HIGH_WATER_MARK), then parks it:Paused(memory bounded),NewSource::unroot_wrapper), so an abandoned stream can be collected. When it is,SourceContext::wrapper_finalized→SourceHandle::consumer_collectedtells the tasklet, which aborts the request from native state only (no JS in the sweep).A consumer that comes back — a reader draining the buffer,
res.text()/.blob(), or a native sink (Bun.write,HTMLRewriter, aBun.serveresponse) — unparks: re-ref, re-root, resume the transport. Bodies under the mark still complete unread, so their keep-alive connection is reused as before. Explicitcancel()still aborts immediately.RequestContext::on_abortnow cancels a natively piped body (ByteStream::detach_finished_sink) and releases the pipe's ref;end_chunkclearsbyte_streamfirst so that release cannot run twice.Behavior vs Node (undici), checked by hand
res/ released reader / idle readerread()Tests (all red on main)
fetch-backpressure.test.ts: unread / released bodies against a 1 GiB response leave the server blocked and a later reader resumes; a parked body picked up byres.text()or anHTMLRewritertransform drains fully; processes holding an unreadResponse, a released stream, or an idle reader exit; processes with a pendingread()(incl. one re-acquired after a release) stay alive until the body ends.fetch-stream-cancel-leak.test.ts: 20 abandoned bodies in four shapes (touched, read+released, pending read dropped, one partial read after parking) are collected and the server sees the aborts; a proxied upstream is aborted when the proxy's client leaves, reading or not, for bothnew Response(up.body)and returningupitself.html-rewriter.test.js: a transform over a fetch body with only its output promise held survives GC while the body arrives.Not changed here (follow-ups): moving the body keep-alive into
ByteStreamso S3 downloads get the same treatment; a bounded drain-and-discard for implicitly dropped bodies on h1 /RST_STREAMon h2+h3; splittingFetchTasklet's request/response state.Bun.write(file, res)afterres.bodywas touched hangs on 1.4 already and still does; separate bug.no test proof · iteration 2 · 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 test/js/workerd/html-rewriter.test.js