fetch/S3: one high-water-mark rule for response-body backpressure; Bun.write(dest, response) streams to disk

[robobun]

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 (s3: resolve streamed write/download with the byte count, not 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.

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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

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Walkthrough

Changes

Fetch response handling now uses explicit receive modes and high-water-mark backpressure. Collected or cancelled bodies abandon transport state. Error streams retain body state. Bun.write streams native bodies to files, propagates errors, honors file options, and reports byte counts. Tests cover garbage collection, connection reuse, body errors, and file writes.

Fetch response and file streaming

Layer / File(s)	Summary
Explicit response receive modes src/http/Signals.rs	BodyReceiveMode now uses explicit Flowing, Paused, BufferAll, and Abandoned states.
Fetch transport backpressure and finalization src/runtime/webcore/fetch/FetchTasklet.rs, src/http/h2_client/ClientSession.rs, src/runtime/webcore/Response.rs, test/js/web/fetch/fetch-response-finalizer-sweep.test.ts	FetchTasklet pauses at the high-water mark, resumes after buffered data is consumed, preserves active consumers, and abandons unconsumed bodies.
Errored response body state src/runtime/webcore/Body.rs, test/js/web/fetch/body-mixin-errors.test.ts	Errored streams remain available through .body. Failed reads mark the body as used.
Native response-to-file streaming src/runtime/webcore/Blob.rs, src/runtime/webcore/FileSink.rs	Bun.write streams eligible bodies directly to files, applies truncation, permissions, and directory creation options, propagates errors, and resolves with accepted byte counts.
Lifecycle validation test/js/web/fetch/fetch-backpressure.test.ts, test/js/web/fetch/body.test.ts, test/js/bun/io/bun-write.test.js, src/jsc/bindings/webcore/streams/BunStreamSource.cpp	Tests cover transport backpressure, peer resets, collected responses, framing, pooling, body-use errors, file errors, and streamed byte counts. Sink writes throw already-rejected promise reasons before backpressure handling.

Suggested reviewers: dylan-conway, alii

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	The PR includes substantial changes beyond #40278, including generalized fetch backpressure, BodyReceiveMode redesign, abandonment behavior for multiple consumers, null-body protocol handling, failed-…	Split the generalized backpressure, abandonment, protocol, and body-error changes into separately linked pull requests, or add linked issues that explicitly define these requirements as part of this PR.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address #40278 by preserving the active Bun.write response-body pipeline through finalization, propagating completion and errors, and adding regression coverage for garbage-collected respo…
Title check	✅ Passed	The title clearly identifies the two primary changes: unified response-body backpressure and streaming Bun.write(dest, response) to disk, including the fetch/S3 scope.
Description check	✅ Passed	The description explains the implementation, observable behavior, linked issues, verification coverage, and known test limitations. It uses different headings from the template but provides the requir…

Full details: Linked Issues check

Explanation

The changes address #40278 by preserving the active Bun.write response-body pipeline through finalization, propagating completion and errors, and adding regression coverage for garbage-collected responses.

Full details: Out of Scope Changes check

Explanation

The PR includes substantial changes beyond #40278, including generalized fetch backpressure, BodyReceiveMode redesign, abandonment behavior for multiple consumers, null-body protocol handling, failed-body stream semantics, and broad stream-to-file behavior.

Full details: Description check

Explanation

The description explains the implementation, observable behavior, linked issues, verification coverage, and known test limitations. It uses different headings from the template but provides the required change summary and verification details.

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[robobun]

Status: reproduced on main at 0a4e3b1 (debug build, includes #39590). A raw origin with Content-Length: 33554432 wrote all 33554432 bytes to a client that had dropped the Response unread and run Bun.gc(true), and the socket stayed open. On this branch the origin sees the socket close after the loopback buffers (about 3 MiB here), and a body under 256 KiB is taken in full while the Response is still alive, so its connection is reused without a collection.

Proof in test/js/web/fetch/fetch-backpressure.test.ts, block "a Response whose body nothing touches": with main's FetchTasklet.rs six of its tests fail (three "aborted": 20 of 20 connections kept, two "reused": 0 of 20 bodies received, and the Bun.write() one: the write never settles), with this branch all pass. test/js/web/fetch/body-mixin-errors.test.ts gets two tests for a body that failed before it was read (.body, textStream()), red with main's Body.rs, green here.

Since then the branch was rebased onto main at 861e9ae and reworked in f08c478 (one BODY_HIGH_WATER_MARK rule for both buffers, one abandon_response_body() path, no per-chunk pause once a stream is attached). The description above describes the PR as it now stands.

This PR supersedes #35809 and #35819, which both patched the same arm of on_response_finalize and conflict with main since #39590.

[claude]

I reviewed this PR and didn't find any bugs. Because it changes cross-thread abort behavior in the fetch HTTP callback and replaces two earlier attempts that each had review-found issues, a human look is still worthwhile.

What was reviewed:

• abort_transport() from the HTTP thread inside callback(): it is under mutex, aborted is an atomic swap, and schedule_shutdown takes its own lock and only enqueues + wakes — no re-entrancy or deadlock with the callback.
• ignored_body_is_longer_than_mark reads body_size right after the same callback wrote it under mutex; the three BodySize arms match the enum exactly and Unknown (close-delimited) aborting is intentional since draining it can never return the connection.
• The other Ignore entry paths (on_body_stream_collected, abort_and_end) already call abort_transport() first, so the new check only affects the on_response_finalize drain path as intended.
• Test origin: request framing loop handles pipelined requests on a pooled socket, sockets are tracked and destroyed on dispose, and the GC poll loops are deadline-bounded with ASAN/debug branching.

Extended reasoning...

Overview

The PR adds a bound to the "drain and discard" path that runs after a Response is GC'd with its body untouched. FetchTasklet::callback() (the per-packet HTTP-thread callback), in its BodyReceiveMode::Ignore arm, now calls abort_transport() when the body is known to exceed UNOBSERVED_BODY_HIGH_WATER_MARK (256 KiB) — via a declared Content-Length, an accumulated TotalReceived, or the absence of any length. A new helper ignored_body_is_longer_than_mark() encodes the decision, and doc comments on the constant, ignore_remaining_response_body, and abort_transport are updated. Five new tests in fetch-backpressure.test.ts cover the three long-body framings (abort observed as connection close) and two short-body framings (body drained, connection reused).

Security risks

None. This is a client-side resource-bound change: it stops downloading a body no code can observe. No auth, crypto, or untrusted-input parsing is touched. The raw TCP origin in the test file is test-only infrastructure.

Level of scrutiny

High. FetchTasklet::callback() runs on the HTTP thread and coordinates with the JS thread and GC finalizers via a mutex and atomics. This PR adds a new caller of abort_transport() from the HTTP thread (previously only called from the JS thread / GC sweep). I verified: the call is under task_ref.mutex; signal_store.aborted is an atomic swap; schedule_shutdown_by_id takes queued_shutdowns_lock, pushes an id, and calls wakeup() — safe to invoke from the HTTP thread on itself, and it does not re-enter callback. self.http is read-only between queue and teardown, so reading async_http_id without additional synchronization is fine (matches existing abort_transport callers). body_size is written at line 2569 in the same locked section it is read from, so no cross-thread copy is needed (this is the simplification over #35819's atomic).

Other factors

This replaces #35809 (aborted every collected body, breaking keep-alive for small responses) and #35819 (had a 5s deadline that reviewers found didn't arm on the common path). The design choice here — decide in callback() under the mutex where body_size is authoritative, reuse the existing #39590 constant, keep the finalizer unchanged — is cleaner than both. The tests rely on GC collection timing with a tolerance (< N/4 survivors) and deadline-bounded poll loops that branch on isASAN/isDebug, which follows the repo's leak-test conventions. The PR notes 15/15 clean repeated runs of the new block. Given the prior review history on this exact code path and the new cross-thread call site, a maintainer should confirm the design (particularly: always aborting BodySize::Unknown bodies, and the acknowledged asymmetry that a stalled server holds its connection until idle-timeout).

[robobun]

On the two points left for a maintainer:

• BodySize::Unknown is aborted on purpose. The client reports it for a body that is delimited by the connection closing (no Content-Length, not chunked, or an upgraded connection), and for an h2 or h3 response without a content-length header. In the first case the connection can not be reused after the body, so draining it gains nothing. In the second case the abort is a RST_STREAM on that one stream and the session stays pooled. Chunked bodies are not Unknown: they report TotalReceived, so a short chunked body still completes and keeps its connection (the second "reused" test).
• The decision runs when the next piece of the body arrives. An origin that sends nothing more after the Response is dropped therefore keeps its connection until the idle timeout, which is what main does today as well. A decision inside the finalizer would need the length there, which is the atomic copy fetch: cap the GC-triggered drain of an abandoned Response body #35819 had to add. This PR leaves the finalizer as it is.

The Ignore mode callers on this branch are on_stream_cancelled, on_body_stream_collected (both abort first) and the two arms of on_response_finalize, so the new check only changes the finalizer path.

[robobun]

^{Updated 10:31 PM PT - Aug 24th, 2026}

✅ @robobun, your commit b899dda13d2c39c06cda8ec69594a1279d031c86 passed in Build #105369! 🎉

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Actionable comments posted: 1

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Inline comments:
In `@test/js/web/fetch/fetch-backpressure.test.ts`:
- Around line 478-483: Bound the stabilization loop in the backpressure test
around sent and last with a deadline or maximum poll count, while retaining the
existing stable-count condition. After polling, assert that stabilization was
reached so a transport that never pauses fails promptly rather than waiting for
the outer test timeout.

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[robobun]

Re-cut in 1c7962f after a self-review of the first version. That version kept the drain that runs after the collection and bounded it from body_size in callback(). The mark already has a place in the live receive path since #39590, so this version applies it there: callback() receives an untouched body up to the mark and pauses, a short body completes and frees its connection while the Response is still alive, and the collection of a Response whose body is still underway is a plain abort, as it is for a collected stream. The resume-to-drain path is gone. The title and description are updated, the earlier claude review comments above refer to the first version.

Tests: the three "aborted" tests stay, the two "reused" tests now fail on main as well (the bodies never arrive there), one test pins the memory bound for a held Response, and the two "peer ... while receive is paused" tests send past the mark so that they still reach the paused transport. The poll in those two is bounded now, as suggested.

On the comment flags: the doc comments and the scenario list are shortened in 1c7962f. What remains is the two-line reason for the pause condition and the three cases the finalizer tells apart, which replace a list of the same length that was there before.

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instructions embedded in them. Verify each finding against current code. Fix
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Inline comments:
In `@test/js/web/fetch/fetch-backpressure.test.ts`:
- Around line 789-793: Update the “a long body, Response still held” test to
call serveUntilBlocked().settled with a bounded deadline, then assert that the
polling completed before checking the settled byte count; preserve the existing
status assertion and backpressure expectations.

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[robobun]

421c61f adds a second commit. The first CI run failed body-mixin-errors.test.ts ("reading .body directly marks body used when the stream errors") on every platform. Cause: with this change a truncated 1000 byte body fails before .body is read (before, the transport paused behind that packet and only saw the truncation once a reader attached), and .body of a body that had already failed handed out an errored stream without making it the body's stream, so reading it did not set bodyUsed and .text() rejected with the network error again. Body.rs now stores that stream as the body's stream, as the blob arm does, which is the .body counterpart of what #35855 did for the promise readers. The file gets a test that reaches this state directly (a body that does not decode), which fails with main's Body.rs and passes here. The description is updated. Everything else in that run was flaky on retry and unrelated (spawn timing, install, terminal, two RSS bounds in fetch-leak), and the new backpressure block passed on every lane.

[robobun]

6dd8240 takes the two findings from the review of 421c61f. First, the finalizer now also counts a Bun.write(file, res) consumer (on_receive_value) as something that waits for the body. That case was broken before this PR as well: the write task holds nothing that keeps the Response alive, and once the Response was collected the finalizer released the native response, so await Bun.write(path, await fetch(url)) never settled. The new test shows "never arrived" without the line and the full write with it, and its .text() twin pins the promise arm. Second, textStream() of a body that had already failed now uses the body up, as every other arm of that function does, with a test next to the .body one. Both threads are resolved and the description lists the new tests. Everything else on the PR is unchanged.

[robobun]

Checked bb751b9 and 93e1718 on this machine (debug build, x64 Linux):

• writer() ownership: the completion callback fires once on every path (fail, done, on_commit_multi_part_request all set Finished first), and drain_enqueued_parts returns at Finished, so on_writable never runs with a freed ctx. If the JS wrapper is collected first, the box lives on with task = None until the callback releases it.
• LSAN (detect_leaks=1) over 20 s3file.writer().end() cycles: no NetworkSink in the report. The only entries left are exit-time ones (the module-scope S3Client box and its credentials, a pending Bun.sleep timer, a sourcemap buffer).
• fetch-backpressure.test.ts: the S3 block and every fetch block that parks or collects a stream pass (47 tests). The 21 stalled ... drains the full body subprocess tests and the four 1 GiB server stops writing tests time out here only under full-file concurrency (one debug subprocess takes 4 s for 16 MiB on this host), and pass alone. bun-write.test.js, fetch-response-finalizer-sweep, body-mixin-errors, fetch-stream-cancel-leak, streams.test.js pass.

c4bd703 on top:

• cargo clippy failed on bb751b9: the release_writer_holder call in writable_stream had no SAFETY comment. Added.
• NewSource::unroot_wrapper / root_wrapper / upgrade_wrapper take this: *mut Self and touch only the fields they write (the review's ProducerHold::park/unpark finding: a method call formed a &mut NewSource over the whole allocation while the caller still held the &ByteStream of that source). Same shape as decrement_count.
• The Bun.write-to-a-directory test asserts EISDIR from open. Linux (debug build) and Windows (canary) both report that code.

Source lints (162) and clippy on bun_runtime pass.

[robobun]

Merged main into the branch (13dacd4, a merge commit, so a local checkout fast-forwards). One conflict, in Blob.rs: #40374 changed mkdir_if_not_exists to PathLike::borrowed, this branch had moved that body into mkdirp_parent (shared with the FileSink mkdirp option). Kept the helper on the new API. The merged debug build passes bun-write.test.js, body.test.ts, body-mixin-errors, fetch-response-finalizer-sweep, fetch-stream-cancel-leak, fetch-keepalive and the S3 and parked-stream blocks of fetch-backpressure.test.ts.

[claude]

I reviewed this PR and the automated bug-hunting pass found no new issues; all earlier inline findings (Stacked Borrows aliasing in ProducerHold/on_stream_cancelled, the NetworkSink writer leak, dead is_parked, test pipe-draining and error-matcher nits) are resolved at HEAD. Given the scope — a redesign of fetch/S3 receive backpressure, GC-finalizer-driven abort semantics, a new ProducerHold refcount abstraction, FileSink stream piping, and the NetworkSink two-holder ownership model — a human pass on the design and the memory-ownership changes is still warranted.

What was reviewed:

• BodyReceiveMode state machine and both HWM sites (HTTP-thread callback/process_http_callback vs JS-thread after_delivery) for lost-resume races.
• ProducerHold/PinnedBytes refcounting and the unroot_wrapper/root_wrapper raw-pointer rewrite for aliasing under a live &ByteStream.
• FileSink::pipe_stream/end_from_stream error paths — write-error → source cancel → on_stream_cancelled re-entry now stays &self; stream_done settled on every close path.
• NetworkSink::writer_holders — both holders (finalize, upload completion callback) release exactly once; upload_stream path stays at 0.

Extended reasoning...

Overview

This PR unifies fetch()/S3 response-body receive backpressure under one 256 KiB high-water-mark rule and one abandonment path, and makes Bun.write(dest, response|request|stream) stream to disk instead of buffering in memory. It touches 20 files across the HTTP client signals, FetchTasklet, S3 download/upload streaming, ByteStream, FileSink, ReadableStream source rooting, Blob write-file, Body value handling, NetworkSink lifetime, one C++ stream-source fix, type declarations, and five test files (~600 lines of new tests). It introduces new abstractions (ProducerHold, PinnedBytes, AfterDelivery), a two-holder refcount on NetworkSink, and changes user-observable behavior (long unread bodies are aborted on GC rather than drained; short ones complete and pool their connection).

Security risks

No auth/crypto/permissions surface. The memory-safety surface is significant: raw-pointer refcounting across GC finalizers, cross-thread atomic state (BodyReceiveMode), and re-entrant JS callbacks reaching back into producers. Several Stacked Borrows aliasing issues were found and fixed during review (&mut FetchTasklet under on_body_received, &mut NewSource while a &ByteStream is protected). The remaining risk is a missed lifetime edge (e.g. a writer_holders path that fires twice, or a ProducerHold release ordering under an unusual cancel/collect interleaving) rather than an input-validation or injection concern.

Level of scrutiny

High. This is production-critical hot-path code (every fetch() response body, every S3 stream, every Bun.write(file, response)) with intrusive refcounts, GC-sweep-safe teardown requirements, and cross-thread flow control. The PR has already been through multiple review iterations that surfaced real UB-class findings; the observable-behavior changes (abort-on-GC, chunk-size granularity, 205-with-content now closes) are deliberate design calls that a maintainer should sign off on.

Other factors

Test coverage is extensive and event-driven (no sleeps), covering the boundary cases the description enumerates. All 18 prior inline findings from earlier automated passes are marked resolved with fix commits named. Jarred-Sumner has engaged on at least one thread. The PR closes eight issues and changes defaults in a way the description compares against undici/libcurl/Chromium/Go — that comparison itself deserves a human read. Not approving per the guideline that complex, large changes touching critical code paths with design decisions should get human review.

[robobun]

This branch also fixes a regression against 1.3.14 that is not in the PR body yet. I verified it on a debug build of b899dda.

A fetch() whose body is never touched keeps the connection open forever when the body arrives in more than one socket read after the headers and the origin then closes. On 1.4.0, 1.4.1 and main the origin never sees the client close, and a process that owns that origin never exits.

Cause on main: FetchTasklet::callback moves the receive mode to Paused after the first body chunk. Only a stream or a buffered consumer resumes it. A body that stays Locked never resumes, so the rest of the body and the FIN are never read. On this branch the callback pauses only once the hop buffer holds BODY_HIGH_WATER_MARK, so a short untouched body completes and the socket closes.

Repro (hangs on main, exits on this branch; same result with a 3-write chunked body and with a close-delimited body):

import net from "node:net";
const t0 = Date.now();
const srv = net.createServer(c => {
  c.on("close", () => console.log("origin: client closed conn at", Date.now() - t0, "ms"));
  c.once("data", () => {
    c.write("HTTP/1.1 200 OK\r\nContent-Length: 6\r\nConnection: close\r\n\r\n");
    setTimeout(() => c.write("abc"), 100);
    setTimeout(() => c.end("def"), 200);
  });
});
await new Promise(r => srv.listen(0, "127.0.0.1", r));
fetch("http://127.0.0.1:" + srv.address().port + "/").then(r => console.log("status", r.status));
setTimeout(() => srv.close(), 500);
process.on("exit", () => console.log("exit at", Date.now() - t0, "ms"));

The "a short content-length / chunked body, Response still held" tests in fetch-backpressure.test.ts cover the mechanism. A variant where the origin sends Connection: close and the test awaits the origin socket's close event would pin the FIN half of this down too.

[robobun]

The 256 KiB read-ahead here lets TCP receive autotuning grow the client window far enough on Linux 6.16+ kernels (alpine lanes) that compression.test.ts > CompressionStream -> native HTTP sink applies backpressure to a stalled client no longer stalls. #40487 moves that test to a raw paused socket.