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…ot fetch The "native HTTP sink applies backpressure to a stalled client" case counted server pulls while a fetch() client held one chunk unread. e4c2af4 (#39690) replaced fetch's per-chunk stop-and-wait with a 256 KiB read-ahead, so the client now reads about 1.3 MB in fast bursts before it pauses. TCP receive autotuning grows the receive window with each read, up to tcp_rmem[2], which Linux 6.16 raised from 6 MB to 32 MB. On the alpine lanes (kernel 6.18) the kernel then absorbs the whole 12.8 MB body, the sink never sees backpressure, and the pull loop reaches TOTAL while the client is stalled. The client is now a Bun.connect socket that is paused before the request is written and resumed only after the stall is observed. Without reads the receive window stays at its initial size, so what the kernel can absorb is the server's send buffer plus one untouched receive buffer, independent of fetch's read-ahead and of the kernel version. The test also de-chunks the response and compares the gunzipped body with the source.
WalkthroughChangesNative HTTP backpressure
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Actionable comments posted: 1
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Inline comments:
In `@test/js/web/streams/compression.test.ts`:
- Around line 603-605: Update the source stream pull logic around pulls and
onFirstPull to mutate or encode each chunk with its pull index before
enqueueing, while storing every emitted chunk. Replace the decompressed-body
assertion near the existing length/body checks with a comparison against the
stored chunks so reordered complete chunks are detected.
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test/js/web/streams/compression.test.ts
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LGTM — test-only de-flake with a well-traced kernel-level root cause.
What was reviewed:
- The raw
Bun.connect+pause()/resume()client matches the established pattern intest/js/bun/net/socket.test.ts;received.push(data)without cloning matchesserve.test.ts(Bun's socketdatacallback hands out fresh Buffers). numbered()copies viaBuffer.from(Uint8Array)before stamping, so the sharedchunkis never mutated; the final compare rebuildsnumbered(1..TOTAL)and would catch reordering (CodeRabbit's nit, addressed in f8e1635).- The hand-rolled chunked de-framer guards
sizeEnd < 0/NaNand terminates on the0\r\nchunk;Connection: closeguarantees theclosehandler fires soawait closedcan't hang on a clean run.
Extended reasoning...
Overview
This PR touches a single test file, test/js/web/streams/compression.test.ts, rewriting one test case (CompressionStream -> native HTTP sink applies backpressure to a stalled client) that went red on the Alpine 3.23 CI lanes after #39690. The old test used fetch() as the stalled client; #39690 made fetch read ahead in bursts, and on Linux ≥6.16 those reads let TCP receive autotuning grow the window to 32 MB — enough to absorb the whole 12.8 MB body, so the server-side sink never observed backpressure and pullsWhileStalled reached TOTAL. The fix swaps the client for a raw Bun.connect socket that is paused before it writes the request and resumed only after the stall is measured, so the receive window never grows beyond its initial size. The test now also de-chunks and gunzips the full response and byte-compares it against sequence-numbered source chunks.
Security risks
None. This is a test-only change with no production code, no new dependencies, and no external network access (loopback Bun.serve + Bun.connect).
Level of scrutiny
Low-to-medium. It's a targeted test de-flake with no runtime code changes. The main review concerns are (a) whether the new test is itself flaky or platform-sensitive, and (b) whether it still asserts the property the original test protected. On (a): the paused raw socket removes the two hidden inputs the old assertion depended on (fetch read-ahead and kernel autotuning cap); the PR description quantifies worst-case ~66 pulls vs TOTAL = 200, and the pause()-before-write() pattern is already exercised in socket.test.ts. On (b): the assertion pullsWhileStalled < TOTAL is unchanged, and the author verified it still fails when sink backpressure is disabled locally. The new body-integrity check strengthens the test without weakening the original invariant.
Other factors
- CodeRabbit's one finding (identical chunks would let a permutation pass the body compare) was addressed in commit f8e1635 via
numbered(pulls)and the ordered rebuild in the final.equals()— the thread is resolved. Buffer.from(chunk)innumbered()copies (Node semantics forBuffer.from(Uint8Array)), so each enqueued chunk is independent and the shared randomchunkis never mutated in place.- Storing the
datacallback buffer without cloning (received.push(data)) is safe here and matches the identical pattern used repeatedly intest/js/bun/http/serve.test.ts. - The socket has no
errorhandler wired to rejectclosed, but this matches the existingserve.test.tsraw-socket tests and the connection is loopback; a failure would surface as a test timeout with the same signal as before. - The PR description is unusually thorough (kernel commit reference,
ss -tmimeasurements, 10× local runs, negative verification), which raises confidence that the root cause is correctly identified rather than papered over.
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CI on f8e1635 (build 105766) and 838e100 (build 105867): The red tests in those builds are not touched by this diff:
The remaining failures passed on retry. |
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LGTM — well-reasoned test de-flake, test-only change.
What was reviewed:
- The paused-socket client removes fetch's read-ahead and TCP autotuning as hidden inputs to the stall assertion;
pause()before write +resume()afterwaitUntilStablematches existingBun.connectusage in the suite. numbered()copies viaBuffer.from(chunk)so the sharedchunkis never mutated; the final ordered compare is deterministic.received.push(data)without cloning matches the established pattern inserve.test.ts; the chunked-transfer parser handles Bun.serve's output shape and throws on malformed framing.- CI on f8e1635 confirmed the previously-red alpine 3.23 lanes now pass; CodeRabbit's reorder-detection concern was addressed in f8e1635.
Extended reasoning...
Overview
This PR touches one file, test/js/web/streams/compression.test.ts, rewriting a single test case ("CompressionStream -> native HTTP sink applies backpressure to a stalled client") that went red on the alpine 3.23 CI lanes after #39690 changed fetch's read-ahead behavior. The fix replaces the fetch client with a raw Bun.connect socket that is paused before writing the request and resumed after the stall is observed, so TCP receive autotuning never grows the window and the kernel cannot absorb the whole 12.8 MB body. The test also now de-chunks and gunzips the full response and compares it byte-for-byte against the numbered source chunks, which is a strictly stronger assertion than the old test had.
Security risks
None. This is a test-only change with no runtime code touched. The test uses port: 0, connects only to localhost, and involves no auth, crypto, or untrusted input handling.
Level of scrutiny
Low-to-medium. Test-only, single-file, fixing a documented CI flake with a thorough root-cause analysis (kernel version → tcp_rmem[2] change → autotuning interaction with fetch read-ahead). The PR description includes empirical verification (ss -tmi measurements, 10× runs on debug, 5× on pre-#39690 release, and confirmation that disabling sink backpressure makes the test fail as expected). CI on build 105766 confirmed the previously-red alpine lanes pass.
Other factors
- The change follows harness conventions closely:
await using server,using socket,Promise.withResolversfor observable conditions (firstPull,closed) instead of sleeps,port: 0. Buffer.from(typedArray)copies, sonumbered(n)produces a fresh buffer each call and the sharedchunkrandom bytes are never mutated — the finalArray.from({length: TOTAL}, (_, i) => numbered(i + 1))reconstruction is deterministic.received.push(data)in the socketdatahandler without cloning matches the established pattern used repeatedly intest/js/bun/http/serve.test.ts.- The chunked-encoding parser is minimal but sufficient for Bun.serve's output (no chunk extensions or trailers), and throws a clear error on malformed framing rather than silently mis-parsing.
- CodeRabbit raised one minor point (identical chunks would let a permutation pass); this was addressed in f8e1635 by stamping the pull index into each chunk's first 4 bytes, and the thread is resolved.
- No socket
errorhandler is wired to rejectclosed, but on a localhost loopback connection withConnection: closethis is acceptable and matches the serve.test.ts precedent; the test would time out rather than hang forever if the socket errored.
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Closing in favor of #40966, which deflakes this test together with the h2 stream-release cases in h2-conformance.test.ts. The analysis here (receive autotuning up to the 32 MB |
…cket, not fetch (#41134) ### Problem - `compression.test.ts > CompressionStream -> native HTTP sink applies backpressure to a stalled client` fails on the alpine 3.23 lanes (x64 and aarch64): `expect(pullsWhileStalled).toBeLessThan(RUNAWAY)`, `Expected: < 512`, `Received: 512`. In the last 400 Buildkite builds it was the flaky test in 96 of them, every hit on alpine, and 36 of those hits were after #40966 raised the bound to 512. - The stalled client is a `fetch()` whose reader holds one chunk. Fetch reads ahead in bursts, and each read lets TCP receive autotuning grow the window up to `tcp_rmem[2]`. That is 32 MiB on Linux 6.16 and later, which the alpine lanes run. With the server's 4 MiB send buffer the kernel alone holds about 580 chunks, so the source reaches 512 without the sink ever pushing back. ### Fix - The client is a `Bun.connect` socket, paused in `open()` before it sends the request. Nothing reads until the stall is observed, so the receive window stays at its initial size and only the two socket buffers absorb data. Here it parks at 43 pulls on every run, release and debug, on a 6.17 kernel with the same 32 MiB cap. The old client parked anywhere from 93 to 105. - Correct because the test is about the server: `HTTPServerWritable` returns a pending promise when uWS did not take all the bytes, and the native `CompressionStream` parks on it. Fetch's read-ahead and the kernel's autotuning cap were hidden inputs to the assertion. - Each source block carries its pull number in its first 4 bytes. The test de-chunks the response and compares the gunzipped body with the numbered blocks in order, so a lost, duplicated or reordered block across the stall and the resume fails it. - Verified: `bun bd test test/js/web/streams/compression.test.ts` (65 pass). The new case passes 6 of 6 with the released bun and 4 of 4 with the debug build. ### Background - `Bun.serve` writes a streaming response through `HTTPServerWritable` (`src/runtime/webcore/streams.rs`). When the socket buffer is full its `write()` returns a pending promise, and a native `CompressionStream` piped into it waits on `m_nativeSinkReadyPromise`. - TCP receive autotuning sizes a socket's receive buffer from the application's read rate. A socket that never reads keeps its initial buffer (128 KiB on Linux). - `socket.pause()` on a `Bun.connect` socket stops reading from the fd. A pause in `open()` applies before the server's first byte arrives. <details><summary>Notes</summary> - #40487 proposed this client before #40966 merged and was closed in favor of it. The comment on #40966 (#40966 (comment)) noted that the 512 bound is below what the kernel can hold on the alpine lanes. The failures since confirm it. - Parked pull counts measured here with the released bun 1.4.1, 6 runs each: new client 43, 43, 43, 43, 43, 43. Old client 99, 105, 104, 105, 93, 99. - `Connection: close` makes the server close the socket after the response, which is how the client learns the body is complete. - The request-body case (`DecompressionStream propagates backpressure to the client`) is unchanged. It stalls the server, not the client, and has not appeared in the Buildkite annotations. </details> <!-- robobun:evidence:begin --> --- **[auto-merge]** gate passed · iteration 0 · 1 files touched <details><summary>passes on PR (with fix)</summary> ```console Test-only change. Debug/ASAN (expected pass): $ bun bd test 'test/js/web/streams/compression.test.ts' $ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test test/js/web/streams/compression.test.ts bun test v1.4.1 (a6c4cc2) test/js/web/streams/compression.test.ts: (pass) TransformStream.prototype getters reject native transform subclasses (0) [9.56ms] (pass) TransformStream.prototype getters reject native transform subclasses (1) [2.81ms] (pass) TransformStream.prototype getters reject native transform subclasses (2) [2.10ms] (pass) TransformStream.prototype getters reject native transform subclasses (3) [2.34ms] (pass) CompressionStream and DecompressionStream > brotli > compresses data with brotli [15.07ms] (pass) CompressionStream and DecompressionStream > brotli > decompresses brotli data [20.70ms] (pass) CompressionStream and DecompressionStream > brotli > round-trip compression with brotli [48.48ms] (pass) CompressionStream and DecompressionStream > zstd > compresses data with zstd [9.32ms] (pass) CompressionStream and DecompressionStream > zstd > decompresses zstd data [19.09ms] (pass) CompressionStream and DecompressionStream > zstd > round-trip compression with zstd [47.25ms] (pass) CompressionStream and DecompressionStream > zstd > decompresses a multi-frame zstd stream [15.50ms] (pass) CompressionStream and DecompressionStream > zstd > decompresses a multi-frame zstd stream split across writes (next = zstd frame) [18.47ms] (pass) CompressionStream and DecompressionStream > zstd > decompresses a multi-frame zstd stream split across writes (next = skippable frame) [7.70ms] (pass) CompressionStream and DecompressionStream > zstd > decompresses many concatenated zstd frames larger than one output chunk [13.80ms] (pass) CompressionStream and DecompressionStream > zstd > decompresses a zstd stream with a leading skippable frame [9.25ms] (pass) CompressionStream and DecompressionStream > zstd > rejects trailing garbage after a zstd frame [9.77ms] (pass) CompressionStream and DecompressionStream > all formats > works with all ... (truncated) Exit: 0 ``` </details> <details><summary>diff hotspot</summary> ``` test/js/web/streams/compression.test.ts | 78 +++++++++++++++++++++++++++++---- 1 file changed, 69 insertions(+), 9 deletions(-) ``` </details> **gate history** · 1 passed · 0 rejected · iteration 0 <details><summary>evidence per changed file</summary> ``` file reads edits tests test/js/web/streams/compression.test.ts 2 6 19 ``` </details> <!-- robobun:evidence:end -->
1 participant
Problem
compression.test.ts > CompressionStream -> native HTTP sink applies backpressure to a stalled clientis red on main on the alpine 3.23 lanes since e4c2af4 (fetch/S3: one high-water-mark rule for response-body backpressure; Bun.write(dest, response) streams to disk #39690):Expected: < 200,Received: 200atexpect(pullsWhileStalled).toBeLessThan(TOTAL).fetch()client holds one chunk unread. fetch/S3: one high-water-mark rule for response-body backpressure; Bun.write(dest, response) streams to disk #39690 made fetch read ahead 256 KiB in fast bursts. Each read lets TCP receive autotuning grow the window, up totcp_rmem[2]: 32 MB since Linux 6.16, which only the alpine lanes run (6.18). The kernel absorbs the whole 12.8 MB body, so the sink never pushes back.Fix
Bun.connectsocket, paused before it writes the request and resumed once the stall is observed. Nothing reads, so the window keeps its initial size and the kernel absorbs only the server's send buffer (2.7 MB here, 43 pulls on the old and the new bun).bun bd test test/js/web/streams/compression.test.ts(65 pass). With sink backpressure disabled inconsumerFull(local experiment) the case fails withReceived: 200.Background
HTTPServerWritable(src/runtime/webcore/streams.rs) returns a pending promise fromwrite()when uWS did not take all the bytes. A nativeCompressionStreamwrites into that sink directly and parks onm_nativeSinkReadyPromiseuntil it drains.tcp_rcv_space_adjust) sizes a socket's receive buffer from the application's read rate. A socket nobody reads keeps its initial 128 KiB.Notes
net/ipv4/tcp.ctcp_init:max_rshare = min(6UL*1024*1024, limit)through v6.15,min(32UL*1024*1024, limit)from v6.16.max_wsharestays 4 MB.ss -tmiduring the stall on this 6.17 kernel. Fetch client, debug build: clientrb1262364, serverSend-Q 2.6 MB, 80 pulls. Raw paused client: clientrb131072, serverSend-Q 2.57 MB, 43 pulls, same on the pre-fetch/S3: one high-water-mark rule for response-body backpressure; Bun.write(dest, response) streams to disk #39690 release and on main.BUN_DEBUG_FetchTasklet=1 BUN_DEBUG_fetch=1): the client takes 303 KB, 467 KB and 524 KB off the socket in three bursts, thenparkBodyStreamwithbuffered=728997and stays paused. The client-side high-water mark works as designed. The bursts are what grow the kernel window.waitUntilStablethe repro held the stall for 300 ms. With the raw clientpullsdid not move (43 -> 43). With the old fetch client on the pre-fetch/S3: one high-water-mark rule for response-body backpressure; Bun.write(dest, response) streams to disk #39690 build the count still moved during the hold (44 -> 82), so the old assertion had less margin than its comment suggested.sk_sndbufexpands from the initial cwnd to about 2.6 MB and can reachtcp_wmem[2](4 MB). Worst case is about 66 pulls againstTOTAL = 200.Bun.connectpause()/resume()is public API and used the same way intest/js/bun/net/socket.test.ts. Unix sockets would give a smaller, fixed capacity, butBun.connect({ unix })is skipped on Windows in the existing tests.[stamp-90s] gate passed · iteration 0 · 1 files touched
passes on PR (with fix)
diff hotspot
gate history · 1 passed · 0 rejected · iteration 0
evidence per changed file