Bun.serve: send the completed responses of a pipelined read before JavaScript runs again

[robobun]

Problem

• Since Bun.serve: keep the socket corked across the requests of one read #42762, Bun.serve holds the responses to HTTP/1.1 requests pipelined in one read until every handler of that read has run. A fast request ahead of a handler that blocks for 500 ms prints 502 ms: R:0 R:1. Bun 1.4.2 prints 2 ms: R:0.
• A later handler that exits, calls server.stop(true), or calls ws.terminate() in open() drops the responses completed before it.
• Cause: HttpContext::onData (packages/bun-uws/src/HttpContext.h:385) corks once per read and uncorks after the last request.

Fix

• AsyncSocket::sendCorked() sends the cork buffer and keeps the socket corked. prepare_js_request_context (src/runtime/server/mod.rs) and the WebSocket upgrade fallback (server_body.rs) call it before they call a user function.
• Only user JavaScript can block without a bound, so only a dispatch to JavaScript sends early. Static routes, the early 413, and node:http are unchanged. Not covered: a response that completes from a body callback or a microtask (notes).
• Cost: one send() per response for JavaScript handlers. Depth 16: 1.4.2 17.5k req/s, main 340k to 515k, this PR 130k to 175k. Static routes stay at 1.2M req/s. Depth 1 is unchanged.
• Verified: test/js/bun/http/serve.test.ts, "requests pipelined in one read". Four tests fail on main and pass here. Self-reviewed (notes).

Background

• Cork: uWS collects the writes of a socket in a 16 KB buffer and sends them with one send() at uncork.
• Pipelining: a client sends several HTTP/1.1 requests without waiting for responses. onData parses every request of one read and calls each handler.
• prepare_js_request_context is the common entry of the Bun.serve paths that call a user function (fetch, route functions, bake).

Notes

Self-review. Addressed: the send_corked() call moved below the early 413 (see below), the process.exit() and blocked handler tests got a static route directly before the handler, and the comments and this description now claim the dispatch of a later request only. The items under "Not covered" come from the same review.

Relation to #42762. #42762 is in no release tag. This PR replaces its "What changes for a client" paragraph: the response to request N no longer waits for the JavaScript handlers of the requests behind it. It also closes the ws.terminate() in open() item that #42762 listed as not addressed.

Decision. Three options were measured first. Jarred chose the third.

option	fetch handler, depth 16	static route, depth 16	first byte of [fast, 500 ms]
keep main	340k to 515k req/s	0.8M to 1.26M req/s	502 ms
send before every dispatch	174k req/s	190k req/s	1 ms
send before JavaScript (this PR)	130k to 175k req/s	0.8M to 1.26M req/s	1 ms

Not covered.

• A response that completes from a request body callback or from a microtask still waits for the JavaScript that runs after it in the same onData call. Example: one fetch() POST, no pipelining. The handler does await req.json(), starts work that it does not await, and returns a 202. The client gets the response after 11 ms on 1.4.2 and after 503 ms on main and on this branch. process.exit() or server.stop(true) in that trailing work loses the response. This also comes from Bun.serve: keep the socket corked across the requests of one read #42762. It needs a send at the completion of a response that JavaScript produced. Bun.serve: send a response that JavaScript produced when it completes #42815 does that on top of this PR. It is separate because it changes the mixed sequence of the segment test from 3 segments to 5.
• HTTP/2 and HTTP/3 do not use the cork buffer. This PR does not change how they batch the frames of one read.
• The messages that a WebSocket handler sends while it handles several frames of one read still leave together.
• The plugin load of an HTML bundle route ([serve.static] plugins) can run JavaScript from a request. It does not go through the two entry points.

Measurements. Release builds, loopback, one server process, 8 client connections that write batches of depth requests, 2,304,000 requests per run (230,400 for 1.4.2). send() counted with an LD_PRELOAD shim. The container is shared, so req/s varies between runs. The send() counts and the sys CPU do not.

server	depth	1.4.2	main 7d792a5	this PR
fetch: () => new Response("hi")	16	17.5k req/s, 14.1 send() per request, 51.6 us sys per request	340k to 515k req/s, 1 send() per 16 requests, 0.5 us sys	130k to 175k req/s, 1 send() per request, 3.9 to 5.0 us sys
same	64	17.5k req/s	596k req/s	125k to 166k req/s
routes: { "/": () => new Response("hi") }	16		397k req/s	127k to 135k req/s
routes: { "/": new Response("hi") }	16	232k req/s, 1 send() per request	0.8M to 1.26M req/s, 1 send() per 16	0.8M to 1.26M req/s, 1 send() per 16
same	64		1.82M req/s	1.93M req/s
fetch handler	1	102k req/s	98k req/s	104k req/s

One loopback send() costs about 3.8 us of sys CPU. The handler itself costs about 1.9 us.

Time to the first response byte, [fast, 500 ms busy loop] written in one segment: 1.4.2 1 ms, main 502 ms, this PR 1 ms. Over TLS: main 504 ms, this PR 9 ms.

What still batches. The writes of one response (status line, headers, body) always share one send(). Responses that need no JavaScript (static routes) collect until a handler runs JavaScript or the read is consumed. The segment test pins this: 8 fetch responses are 8 segments, 8 static responses are 1, and [static static static js static static js static] is 3.

The 413 for an oversized body. uWS writes the 100 Continue of an Expect: 100-continue request into the cork buffer before the route handler runs. prepare_js_request_context answers a Content-Length over maxRequestBodySize with a 413 and closes. The send_corked() call sits below that check, so the 100 Continue and the 413 still share one send(), as on 1.4.2 and main. A client that reads the 100 Continue alone starts its upload, and the close resets it. The segment test covers this (tooLarge). The first push of this PR had the call above the check and sent two segments.

Entry points. Every Bun.serve HTTP/1 path that dispatches a request to a user function goes through prepare_js_request_context, except the fetch fallback of a WebSocket upgrade (on_web_socket_upgrade), which creates its request context inline. Both call send_corked().

Tests. On a release build of main: the segment test reports 1 segment for the four pipelined servers, the process.exit() test and the ws.terminate() test receive nothing, and the blocked handler test times out its Atomics.wait and answers timed-out. The process.exit() test and the blocked handler test put a static route directly before the handler that exits or blocks. That response needs no JavaScript, so only the send at dispatch can deliver it. The process.exit() test is skipped on Windows: the exit resets the connection there, and the client reads ECONNRESET. I checked that on a Windows debug build. The other tests of the block that run on Windows pass there (the segment test is Linux and Android only).

Suites run with the debug build. serve.test.ts, bun-server.test.ts, request-smuggling.test.ts, bun-serve-body-json-async.test.ts, http-server-chunking.test.ts, bun-serve-static.test.ts, bun-serve-routes.test.ts, bun-serve-file.test.ts, bun-serve-headers.test.ts, serve-close-delimited-framing.test.ts, serve-if-none-match.test.ts, hspec.test.ts, and websocket-server*.test.ts. Three tests fail in this container for reasons of the container (a root user can bind port 1, no IPv6, an egress proxy answers for the external address). They fail the same way with a release build of main. websocket-server.test.ts hits test timeouts under the ASAN debug build. It passes 142 of 142 with release builds of main and of this branch.

[robobun]

Status

How I reproduced the problem:

1. Start Bun.serve({ port: 0, fetch(req) { ... } }) with a release build of main (7d792a5). The handler runs a 500 ms busy loop for ?n=1 and returns new Response("R:" + n).
2. Open one TCP connection and write GET /?n=0 and GET /?n=1 in one write().
3. Print the time of each data event on the client.

Result on main: 502 ms: R:0 R:1. Result on Bun 1.4.2: 1 ms: R:0, then 502 ms: R:1. With this PR: 1 ms: R:0, then 501 ms: R:1. Over TLS the first response arrives after 504 ms on main and after 9 ms with this PR.

The throughput numbers in the description come from a client that keeps 8 connections open and writes batches of 16 or 64 requests. The send() calls of the server are counted with an LD_PRELOAD shim.

PR: #42796

[coderabbitai]

Walkthrough

The change adds cork-preserving response flushing through the uSockets FFI, invokes it before later request processing, and expands pipelined-response tests for route handling, oversized requests, process termination, blocked handlers, and WebSocket upgrades.

Changes

Corked response flushing

Layer / File(s)	Summary
Add the send_corked API packages/bun-uws/src/AsyncSocket.h, src/uws_sys/Response.rs, src/uws_sys/libuwsockets.cpp	Adds cork-preserving response flushing for TLS and non-TLS sockets through the C++ and Rust FFI layers.
Flush responses during request processing src/runtime/server/mod.rs, src/runtime/server/server_body.rs, packages/bun-uws/src/HttpResponse.h	Flushes buffered responses before later JavaScript or WebSocket processing and documents the corking behavior.
Validate pipelined response delivery test/js/bun/http/serve.test.ts	Tests batching, route sequences, oversized requests, process-exit delivery, blocked handlers, and WebSocket upgrades.

Suggested reviewers: alii

Priority: ➖ Normal

Merge Risk: 🟡 Moderate · up to 3f484

The oversized-request coverage does not exercise valid HTTP framing, leaving the response-batching regression unprotected. Correct the test before merging.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: flushing completed responses from a pipelined read before JavaScript runs again.
Description check	✅ Passed	The description explains the problem, implementation, behavior changes, performance impact, limitations, and verification results. It does not use the exact template headings, but it provides the requ…

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@test/js/bun/http/serve.test.ts`:
- Line 4884: Update the worker setup around the CommonJS worker entry to use a
temporary-directory module file instead of evaluated source. Define the worker
module with module-scope imports for parentPort and workerData, then start it as
a module worker while preserving the existing HTTP response ordering and
blocking assertions.

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[robobun]

Two review points are handled, and the PR has no open review threads.

• CodeRabbit: the blocked handler test started its worker from evaluated CommonJS source. The worker now runs from a module file in a tempDir with module scope imports (e237052). The test still fails on a release build of main (the handler answers timed-out) and passes with this branch.
• The comment check flagged the comment at the send_corked() call in prepare_js_request_context two times. The comment is one line now (b14e5b9).

The behavior of the change is the same as in the first push.

[claude]

I reviewed this PR and didn't find any bugs. Because it edits vendored uWS corking internals and the Bun.serve hot dispatch path, and deliberately trades pipelined throughput for latency (a maintainer decision the description says was already made), a human sign-off on that trade-off is still worthwhile.

What was reviewed:

• AsyncSocket::sendCorked() — no-op when not corked or offset == 0, so first request of a read and non-corked callers are safe; uncork()+cork() re-acquires a slot immediately.
• Both JS entry points covered (prepare_js_request_context and the WS-upgrade fetch fallback in server_body.rs); H2/H3 and static routes intentionally untouched.
• FFI shim uws_res_send_corked matches the SSL/non-SSL pattern of neighboring uws_res_* functions.
• Test edits: segment-count expectations changed intentionally (8/8/1/3); new process.exit, Atomics.wait, and ws.terminate() tests await real conditions with failure events wired, and the Windows skip has a stated reason.

Extended reasoning...

Overview

The PR adds AsyncSocket::sendCorked() to bun-uws (flush the cork buffer via uncork()+cork() while remaining corked), plumbs it through uws_res_send_corked in libuwsockets.cpp and Response::send_corked() in src/uws_sys/Response.rs, and calls it at the two Bun.serve entry points that are about to run user JavaScript: prepare_js_request_context in src/runtime/server/mod.rs and the WebSocket-upgrade fetch fallback in src/runtime/server/server_body.rs. Tests in test/js/bun/http/serve.test.ts are updated to pin the new per-JS-handler send behavior (TCP_INFO segment counts) and to prove earlier responses survive a later handler that exits, blocks synchronously, or terminates an upgraded socket.

Security risks

None identified. The change only alters when already-buffered response bytes are flushed to a socket the client already owns; it introduces no new input parsing, no allocation sized by untrusted data, and no new trust boundary. sendCorked() early-returns when the socket has no cork slot or an empty buffer, so it cannot flush another socket's data or write uninitialized bytes.

Level of scrutiny

High. This sits directly in the Bun.serve request-dispatch hot path and edits vendored packages/bun-uws internals. The PR itself documents a deliberate ~2-3× throughput regression for pipelined fetch handlers at depth 16 in exchange for correct latency and not dropping completed responses when a later handler exits/blocks/terminates. That is a product/performance trade-off a maintainer should confirm on the record, and vendored-code edits warrant a human look per the repo's landing rules. The existing TCP_INFO segment test's expectations are relaxed (1 → 8 for fetch/route), which is intentional here but is exactly the kind of test-weakening REVIEW.md asks a human to acknowledge explicitly.

Other factors

The implementation is small and follows established patterns: the C shim mirrors neighboring uws_res_* SSL/non-SSL branching, and the Rust wrapper mirrors uncork(). Both sibling JS-entry paths are covered, matching the "fix the whole class" rule; static routes and node:http are unchanged as stated. New tests use port: 0, bunEnv/bunExe, tempDir, wire error/close to reject via the existing exchange helper, poll observable conditions instead of sleeping, and skip on the platforms where the assertion cannot hold (isLinux/isAndroid for TCP_INFO, isWindows for the process-exit connection-reset case) with reasons stated. The bug-hunt ran to a dry streak with no findings and no ruled-out candidates.

[robobun]

The automated review found no bugs, so this push stays as it is. On the trade-off it names: the throughput cost for pipelined JavaScript handlers is the choice recorded under Notes, Decision. Three options were measured first, and Jarred chose the flush before JavaScript. The segment test expectations changed for that reason (8 fetch responses are 8 segments now, static routes are still 1).

[robobun]

^{Updated 8:41 AM PT - Sep 15th, 2026}

✅ @robobun, your commit 3f4841ff35b67740e18ec0b1e46b9ab5f143ed97 passed in Build #115983! 🎉

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🧪   To try this PR locally:

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That installs a local version of the PR into your bun-42796 executable, so you can run:

bun-42796 --bun

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Inline comments:
In `@src/runtime/server/mod.rs`:
- Around line 712-714: Add a focused HTTP/1.1 regression test covering two
pipelined requests on one connection. Verify the first response arrives before
the later request’s JavaScript handler completes, exercising the ordering
enforced by prepare_js_request_context and resp_ref.send_corked().

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Actionable comments posted: 1

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Inline comments:
In `@test/js/bun/http/serve.test.ts`:
- Line 4831: Update the oversized-request test’s raw HTTP request string and
response parsing near the socket write and parser to use actual CRLF escape
sequences rather than literal backslash sequences, so HTTP headers are correctly
delimited and the assertions detect both 100 Continue and 413 responses.

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

A note on the "Merge Risk: Moderate" line in the CodeRabbit summary above: it repeats the finding about \\r\\n in the oversized request case of the segment test. CodeRabbit withdrew that finding in its review thread (the string sits inside the template literal of a bun -e script, so the double escape is correct). The case reports 1 segment with this branch and 2 segments with the send_corked() call above the 413 check. The PR has no open review threads.