[robobun]

Problem

• Bun.serve sends the end of a streamed Response's header section only with the first body chunk. Until then fetch() does not resolve.
• uWS ends the header section only in its first write() or end(). The pending arm of do_render_stream and the no-bytes Source::Bytes arm (src/runtime/server/RequestContext.rs) park before that.

Fix

• AnyResponse::commit_head() sends the rest of an HTTP/1.1 head that has a status and no write, end or try_end. Both arms call it.
• After an awaited handler the commit is a deferred task: a body that ends in that microtask drain keeps Content-Length.
• An S3 body holds its head until the first byte, so a missing key reaches error(). Graceful stop() waits for unsent bytes of a finished response (closeIfIdle(), the same lines as Bun.serve: reset a cut HTTP/1.0 streamed body, and drain a finished response on graceful stop() #46123).
• Verified: test/js/bun/http/serve.test.ts (35 of 41 new cases fail on main), test/js/bun/http/, test/js/bun/s3/. Self-reviewed: 4 findings, all addressed.

Background

• uWS keeps the head open so end() can choose Content-Length.
• The deferred task queue runs after each microtask drain.
• Replaces Bun.serve: flush the response header section before the first stream chunk #33660 (stale-closed), whose unguarded flush cost such bodies Content-Length. Considered a uWS hook per loop turn: every response pays.

Downsides

• A body still pending after the handler's microtasks is chunked. A response that fails after the commit gives a 200 and a cut body.
• An awaited handler's parked stream costs 1 deferred task (507 instructions) and, if pending, 1 send(). Binary: +2,304 bytes.
• HTTP/1.0 requests, HTTP/2, HTTP/3 and FIFO files still wait for the first byte.

Notes

Repro. The script from the report: 8 bodies, the first real chunk after 1500 ms, a raw node:net client that records when the blank line arrives. Release builds, same base.

main:        the head is whole only with the first chunk: start,sse,pull,bytes,direct,flush,gen
             RESULT FAULT: 7 of 7 kinds of body hold the head back
this branch: within the first second: start,sse,pull,bytes,direct,flush,gen,comment
             RESULT right

The same holds for return fetch(upstream), new Response((await fetch(upstream)).body), new Response(req.body) and new Response(proc.stdout). The head is whole at 21 to 242 ms. Main: 1510 to 1878 ms.

On the wire (HTTP/1.1, one response, send() calls / bytes, counted with an LD_PRELOAD interposer, release builds of main and this branch at the same base):

handler, body	main	this branch
sync, body ends in the attach	1 / 76	1 / 76
sync, body ends in its first microtask	1 / 76	1 / 76
awaited, body ends in its first microtask	9 / 76	9 / 76
sync, one chunk and close from a timer	9 / 76, Content-Length	2 / 95, chunked
awaited, one chunk and close from a timer	9 / 76, Content-Length	3 / 95, chunked
sync, a chunk, then a later close	3 / 95	3 / 95
awaited, a chunk, then a later close	3 / 95	4 / 95
sync or awaited, proxied fetch(), upstream body 40 ms after its head	2 / 121	2 / 121
awaited, body writes 1 byte in its first microtask, ends later	3 / 95	3 / 95

Paths that do not park. Calls of the new functions per 1000 requests (gdb breakpoints, release): 0 for a string, a Blob, Bun.file, a static route, HEAD, an awaited string or file, a stream that ends in the attach or in the drain of a sync handler, and each HTTP/1.0 request. A sync handler with a pending stream: 1000 commit_head. An awaited handler with a stream that reaches the pending arm: 1000 defer_head_commit and 1000 tasks. An HTTP/2 or HTTP/3 request context does not contain the commit or the task: MUX is a const parameter.

The deferred task (gdb single-step, release). defer_head_commit is 105 instructions. The task is 402 when the body ended in the drain, plus 233 for the release of the request context, which ran one frame earlier before. The task is 30 when the sink has a flush queued behind it: that flush writes the rest of the head with its bytes, as on main. The task is 1,251 when it sends the head.

Binary (release, same base): text 107,556,528 to 107,558,832 bytes. Data and bss do not change.

S3 bodies. #36733 left S3 bodies with a status line that goes out at once. A whole head at once turns a missing or denied key into 200 OK and a reset (main: an unfinished head and a reset). So an S3 body now follows the HTMLRewriter rule: nothing goes out until the first byte, and a failure before it reaches error(). Main already does this when the failure is known at render time. The alternative is to leave S3 bodies as on main.

Not in this PR

• HTTP/1.0 requests. commit_head() skips them, so they keep the timing of main. The close ends the body of such a response. With a whole head, a cut before the first byte (stop(true), the idle timer) ends with a FIN, and the client reads a whole, empty 200. Main sends an unfinished head and a FIN. Bun.serve: reset a cut HTTP/1.0 streamed body, and drain a finished response on graceful stop() #46123 turns that cut into a reset. After it lands, a follow-up can remove the skip.
• HTTP/3. With HEADERS sent ahead, a body that fails before its first chunk reaches the client as a complete, empty 200 (fetch() resolves, text() is ""). Main rejects with HTTP3StreamReset. The cause is the FIN that Bun.serve(http3): reset the stream when a response body fails mid-body #40598 replaces with a reset. The HTTP/3 arm of commit_head() can follow that PR.
• HTTP/2. With the arm on, HEADERS arrive ahead of DATA, and a proxied body that trails its head takes 1.93 socket writes per response, not 1.00. A node:http2 client also reports end with status 200 when a quiet stream idles out (main: end with no status, rstCode 8 in both). That needs its own decision.
• FIFO and pipe files (new Response(Bun.file(fifo))). A committed chunked head needs a last chunk at EOF. Hardening: fix crashes across the runtime, bundler, dev server, shell and CLI #46021 added it in FileResponseStream::finish(), so the early head for these bodies can follow in its own change. macOS needs io: wait for a named pipe's EOF with select(2) on macOS #40099.
• HTMLRewriter bodies. HTMLRewriter: stream through SinkHandle/SourceHandle; suspend async handlers instead of wait_for_promise #36733 decided that their head waits.
• One send() for an awaited handler. A cork held across the drain gives 9 to 1 sends for a body that ends in the drain, and 4 to 3 for a pending one. A first version of this branch did that. It read a freed socket when pull() ran the event loop (Bun.build() with an async plugin) after the client left. Cork every HttpResponse write so post-await responses leave in one send() #41341 corks every write and is the better place for it.

Other open PRs

• http: consolidate the open fetch, Bun.serve, node:http, node:http2 and HTTP/3 pull requests #46528 (the consolidated HTTP PR) includes this change, in its state before d331334: that branch has no HTTP/1.0 skip.
• Bun.serve: reset a cut HTTP/1.0 streamed body, and drain a finished response on graceful stop() #46123: both PRs make the same four-line edit in closeIfIdle(), so a merge of the two keeps one copy (checked with git merge-tree). Neither PR needs the other.
• Cork every HttpResponse write so post-await responses leave in one send() #41341: two of its Bun.serve tests assume the old order. "async generator body" allows 3 segments, and this PR makes 4. "ReadableStream that produces its only chunk later" expects a Content-Length body, and this PR makes it chunked.
• uws: route every HTTP/1 header-section termination through terminateHeaders() #43866 (terminateHeaders()): no textual conflict. commit_head() calls flushHeaders().

Graceful stop(). closeIfIdle() closed a connection whose finished response still had bytes in the backpressure buffer. Main: a direct stream writes 8 MiB, ends, then the server stops. The client gets 3.4 of 8 MiB, over TLS too. A body that ends through tryEnd parks and was safe. With the head commit, a body that ends after it ends through end(), so the cut would reach it too. Now such a connection is not idle: the stop marks it, and onWritable closes it after the drain. A client that stops reading holds the stop until the idle timeout closes the connection, as it does for a response that is still in flight. With idleTimeout: 0 the stop waits until that client reads or leaves, and stop(true) does not wait. server.closeIdleConnections() leaves such a connection open: it is a one-shot sweep, and this connection still sends a response. The node:http path is not changed.

A request body over maxRequestBodySize while the response is parked. The server cannot send a 413, because the 200 is out. Main ends that 200 with no body, so the client sees a success. With the head sent as chunked, the connection closes with no last chunk, so the client sees a cut body.

A worker that stops inside the drain. The deferred task holds a ref on the request context. A drain that stops early (process.exit() in the worker, worker.terminate() from the parent) does not run the queue, but VirtualMachine::on_exit() does, before the teardown. Probe, debug and release: a later microtask of the drain that queued the task stops the worker, by either call. The client receives the whole head, which only the task sends, and the debug log shows deinit for the context.

Self-review. Four findings survived, all on the first version of this branch:

1. The cork hold read a freed socket (ASAN, uws_res_release_cork). Removed.
2. HTTP/3 and HTTP/2 changed behavior on failure paths. Both are unchanged now.
3. A missing S3 key became a whole 200 OK, then a reset. S3 bodies now hold the head.
4. The FIFO part repeated Bun.serve: terminate FIFO/pipe file responses at EOF #37082 and changed do_sendfile with no test. Left out.

Review follow-up (later commits): the graceful stop() fix, the queued-flush check in the deferred task, a NaN guard in the test helper, a body in the #32111 fixture whose tryEnd() is parked on the drain when the abort arrives, one sink predicate (sends_head_itself()) with one-line comments, no deferred task for HTTP/2 and HTTP/3, and no early head for an HTTP/1.0 request.

Suites. Release builds of main and this branch at the same base: test/js/bun/http/ (74 files), test/js/bun/s3/ (22 files), test/js/workerd/html-rewriter.test.js. Two files differ. serve.test.ts: main fails 37 (the 35 new cases, the changed cancel() test, and the root port test, which fails in my container on both), this branch fails 1 (the root port test). serve-direct-readable-stream.test.ts: main fails 1 (the new row), this branch 0. bun-connect-x509.test.ts fails one case on both. Two dev server tests in bun-serve-html.test.ts fail at random on both builds: 2 of 40 runs on main and 2 of 40 on this branch, interleaved. This PR does not touch the dev server. Debug build with ASAN at this head: serve.test.ts, serve-direct-readable-stream.test.ts, serve-body-leak.test.ts, bun-server.test.ts and serve-close-delimited-framing.test.ts. On an earlier base: test/js/web/fetch/ and test/js/web/streams/ (85 files, release, no difference), and on the debug build serve-http2-protocol.test.ts, serve-http3.test.ts, bun-serve-file.test.ts, html-rewriter.test.js and s3-stream-error-gc.test.ts.

Questions for a maintainer

• The change has no opt-out. Say so if a server option is wanted for the Content-Length of a body that ends in one late step.
• Say so if S3 bodies must keep their first write at once.

---

no test proof · iteration 9 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/http/serve.test.ts, test/js/bun/http/serve-direct-readable-stream.test.ts

[robobun]

^{Updated 12:13 PM PT - Oct 24th, 2026}

✅ @robobun, your commit d3313347bbf79536365237336c31f2c0b877d6fa passed in Build #132113! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 45187

That installs a local version of the PR into your bun-45187 executable, so you can run:

bun-45187 --bun

[robobun]

Status

Reproduced with the script from the report: 8 streamed bodies, the first chunk after 1500 ms, and a raw node:net client that records when the blank line arrives. On main, 7 of 7 body kinds get the end of the header section only with the first chunk. On this branch, 0 of 7.

PR: #45187

[coderabbitai]

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Walkthrough

The server now commits eligible HTTP/1 response heads immediately or through a deferred task. Debug builds check for parked responses with open heads. Tests cover response-head timing, stream completion, graceful shutdown, and abort behavior.

Changes

HTTP response head commitment

Layer / File(s)	Summary
Commit response heads for parked streams src/uws_sys/Response.rs, src/runtime/webcore/streams.rs, src/runtime/server/RequestContext.rs, src/runtime/server/mod.rs	AnyResponse::commit_head commits eligible HTTP/1 headers. RequestContext commits heads immediately or in a deferred task, handles empty byte streams, and checks for parked open heads in debug builds.
Verify response-head timing test/js/bun/http/serve.test.ts, test/js/bun/http/serve-stream-body-error-fixture.ts	Socket-based tests check header timing across HTTP and HTTPS handlers, stream types, upstream bodies, and S3 downloads. They also cover first-microtask completion and failure.
Check stream end and drain paths packages/bun-uws/src/HttpResponse.h, test/js/bun/http/serve-direct-readable-stream.test.ts, test/js/bun/http/serve-async-stream-client-abort-fixture.ts, test/js/bun/http/serve-stream-reject-flush-leak-fixture.ts, test/js/bun/http/serve.test.ts	Tests distinguish first-tick and later-task stream completion, inspect response framing, and exercise client abort, rejection timing, and graceful shutdown while bytes drain.

Suggested reviewers: dylan-conway

Merge Risk: Minimal · up to fc969

The inspected HTTP/1 paths preserve complete response headers and buffered output during graceful shutdown. No merge-blocking issue was established.

Pre-merge checks: 4

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title names the main change: Bun.serve will send the complete HTTP/1.1 response head when a streamed body is not yet ready. The raw summary and PR objectives both describe committing the remaining…
Description check		The description follows the repository template. It has both required sections, "What does this PR do?" (Problem, Fix, Background, Downsides, and a detailed Notes block) and "How did you verify your c…

Full details: Title check

Explanation

The title names the main change: Bun.serve will send the complete HTTP/1.1 response head when a streamed body is not yet ready. The raw summary and PR objectives both describe committing the remaining HTTP/1 head for pending streamed responses, including the Source::Bytes and S3 paths. The title is specific and matches the changeset. It does not mention the deferred commit after microtasks or the closeIfIdle change, but those are secondary details.

Full details: Description check

Explanation

The description follows the repository template. It has both required sections, "What does this PR do?" (Problem, Fix, Background, Downsides, and a detailed Notes block) and "How did you verify your code works?" (suite results, repro table, and debug/ASAN runs). It explains the change, the S3 behavior change, the known limitations, and the open PR interactions. It also describes what the author tested and where results differ between main and this branch. Two items are not fully resolved: platform-specific tests (serve.test.ts and serve-direct-readable-stream.test.ts) were not run locally and are deferred to CI, and the author lists open questions for a maintainer about an opt-out option and S3 first-write timing. These gaps do not make the description incomplete, so the check passes.

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Files selected for processing: 5

• src/runtime/server/RequestContext.rs
• src/runtime/server/mod.rs
• src/runtime/webcore/streams.rs
• test/js/bun/http/serve-direct-readable-stream.test.ts
• test/js/bun/http/serve.test.ts

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Actionable comments posted: 2

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @src/runtime/server/RequestContext.rs:
- Around line 2150-2168: In on_deferred_head_commit, make the sink requirement
explicit by adding a debug assertion that this.sink is present before the
conditional sink handling; preserve the existing response-ending checks and
commit_head behavior.

Review comments at @test/js/bun/http/serve.test.ts:
- Around line 5061-5071: Add a `Number.isNaN` check immediately after parsing
the chunk size in `dechunk`; throw an error containing the malformed size line
so invalid framing fails clearly instead of looping.

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Reviewing files that changed from the base of the PR and between c69a636 and 9a1fc03.

📒 Files selected for processing: 8

• src/runtime/server/RequestContext.rs
• src/runtime/server/mod.rs
• src/uws_sys/Response.rs
• test/js/bun/http/serve-async-stream-client-abort-fixture.ts
• test/js/bun/http/serve-direct-readable-stream.test.ts
• test/js/bun/http/serve-stream-body-error-fixture.ts
• test/js/bun/http/serve-stream-reject-flush-leak-fixture.ts
• test/js/bun/http/serve.test.ts

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[claude]

Findings marked 🟡 are optional suggestions and need no follow-up push.

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🟡 src/runtime/server/RequestContext.rs — Clients that stream an oversized chunked request body to a handler whose streamed response is parked now get a chunked 200 cut off mid-body, where the base branch delivered a complete, empty 200 and a clean close. After commit_head at src/runtime/server/RequestContext.rs:2367 the response has HTTP_WRITE_CALLED, so end_without_body at src/runtime/server/RequestContext.rs:4347 (and the twin at 4241) no longer terminates the head or the chunked body: libuwsockets.cpp:1161 sees headers_open false and writes nothing before the close. Fix: when the head is already committed, end the parked stream through a path that writes the terminating chunk at both 413 sites.

  Why this was flagged

  A sync handler returns new Response(stream) whose pull() awaits req.text() or req.body; the request body has no Content-Length and exceeds maxRequestBodySize. The pending arm commits the head at src/runtime/server/RequestContext.rs:2367 (flushHeaders writes Transfer-Encoding: chunked and the blank line, HttpResponse.h:729-733, state gains HTTP_WRITE_CALLED). The over-limit chunk then reaches src/runtime/server/RequestContext.rs:4344-4347: write_status is a no-op and end_without_body runs. In src/uws_sys/libuwsockets.cpp:1161-1175 headers_open is false because HTTP_WRITE_CALLED is set, so no "\r\n" and no "0\r\n\r\n" is written; markDone and HTTP_CONNECTION_CLOSE close the socket. The client parsed Transfer-Encoding: chunked and sees EOF without a last chunk: a truncation error. On the base branch the head was still open at that point, so libuwsockets.cpp:1170-1173 wrote the blank line and the client got a complete empty 200. The same sequence exists at src/runtime/server/RequestContext.rs:4238-4241.

  Verification: The pending arm at src/runtime/server/RequestContext.rs:2358-2368 commits the head, setting HTTP_WRITE_CALLED. The over-limit chunk reaches the 413 arm at :4347 (twin at :4241), where write_status is a no-op and end_without_body runs. In src/uws_sys/libuwsockets.cpp:1161-1175 headers_open is false, so no 0\r\n\r\n is written before the close. No test pins this.

[robobun]

On the finding about maxRequestBodySize (the 413 sites, outside the diff): no code change.

The status is already 200 when the body passes the limit, on main and on this branch, so a 413 cannot be sent. Main then ends that 200 with no body (HTTP/1.1 200 OK, Connection: close, blank line, FIN). The client reads a success for a request that the server refused. On this branch the head is out as chunked, the connection closes with no last chunk, and the body read fails. I think the cut body is the better signal, so I did not add a terminating chunk. The case is in the Notes of the description.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.

Still open from earlier reviews (1):

• Unresolved: 1 minor or pre-existing.

[claude]

Beyond the inline finding, I also checked the new defer_head_commit/on_deferred_head_commit ref pair in src/runtime/server/RequestContext.rs for the abort-in-drain case: the task still runs with the context pinned, and the resp.get() / sink_mut() early returns both drop the adopted ref, so the count stays balanced. commit_head in src/uws_sys/Response.rs masks the state bits rather than comparing for equality, and the !MUX guard keeps the H2/H3 arms from posting a task.

Extended reasoning...

The latest push (five commits) changes HTTP/1 head commit for parked streamed bodies in RequestContext.rs, adds commit_head to Response.rs and sends_head_itself to streams.rs, plus debug-only invariant checks and new tests; no auth, crypto or injection surface. One confirmed regression (graceful stop truncating a slow client's late-ended body) is posted inline, so a human still needs to look. This note only records the refcount and state-mask checks that were examined and found sound.

[claude]

This automated review ran on the latest push and found no new bugs; commit b714e9c restores the closeIfIdle() drain guard and the graceful stop() test rows, which covers the body-truncation concern raised on the previous push. Because the change rewires HTTP/1 head framing for every parked streamed body in Bun.serve and adds a deferred task with its own refcount, a human look is still worthwhile before merge.

What was reviewed:

• defer_head_commit / on_deferred_head_commit: the ref taken at post time is adopted into a RequestContextRef and dropped on every exit of the task (detached resp, no sink, commit), and HAS_DEFERRED_HEAD_COMMIT is set and cleared only by that pair.
• commit_head(): the state test is bitwise against STATUS_CALLED|RESPONSE_PENDING with WRITE/END_CALLED excluded, H2/H3 return early, and the Pending arm is gated on !MUX so HTTP/2 and HTTP/3 contexts post no task.
• closeIfIdle(): the restored guard relies on App::closeIdle setting HTTP_CLOSE_WHEN_IDLE and onWritable's shouldCloseConnection() gate to close after drain; the node:http branch is unchanged.

Extended reasoning...

The PR (six commits, ~640 lines) adds AnyResponse::commit_head() in src/uws_sys/Response.rs, a deferred head-commit task and HAS_DEFERRED_HEAD_COMMIT flag in src/runtime/server/RequestContext.rs, a sends_head_itself() predicate in src/runtime/webcore/streams.rs, a closeIfIdle() drain guard in packages/bun-uws/src/HttpResponse.h, debug-only assertions in mod.rs, and a large new test block plus fixture updates. It touches no auth, crypto or input-parsing surface; the sensitive area is request-context lifetime and HTTP/1 response framing on the hot server path. The latest commit b714e9c addresses the blocking finding from the prior run (graceful stop() truncating a late-ended body), and the remaining open thread of mine is a nit. Deferring rather than approving because the change alters wire-level behavior (chunked vs Content-Length, head-before-body) for all HTTP/1 streamed responses and adds refcounted deferred work in core server code that a maintainer should weigh.

[claude]

Findings marked 🟡 are optional suggestions and need no follow-up push.

[claude]

The HTTP/1.0 skip in commit d331334 addresses the earlier commit_head() concern: an ancient request now keeps the base's open-head timing. Beyond the inline finding, I also checked that the new HTTP_ANCIENT_REQUEST bit in src/uws_sys/Response.rs (1 << 8) matches packages/bun-uws/src/HttpResponseData.h:105, and that the test under commit_head reads the flag with bitwise-AND against the mirrored value.

Extended reasoning...

The latest push adds HTTP_ANCIENT_REQUEST to the skip set in AnyResponse::commit_head and gates the pending arm on it, which removes the HTTP/1.0 cut-reads-as-empty-200 path flagged on the prior version; the bit value was cross-checked against the C++ enum. The remaining inline finding (deferred head-commit ref not released on Worker termination) is a refcount-balance nit that still needs an author look.

Still open from earlier reviews (1):

• Unresolved: 1 minor or pre-existing.

[robobun]

#46528 (the consolidated HTTP PR) includes this change. Its Bun.serve list cites this PR.

• That branch has the port from before d331334, so it also sends the early head to HTTP/1.0 requests. I left a note there.
• This branch now conflicts with main, in test/js/bun/http/serve.test.ts only. The source files merge cleanly. I will rebase it if a maintainer wants this PR merged on its own.