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…nd(chunk) Frames that overflow the session's 16 KiB cork went to the JS transport's write() from inside the call that serialized them. When the transport's _write delivers the peer's answer before it returns, the answer was dispatched before the caller of request() could attach listeners, and the request never got 'response'. request() now marks a session that has a JS transport. Until the session's next flush, _write queues into write_buffer instead of calling onWrite, and the session keeps its auto-flush registration so that the deferred tick sends the bytes. flush() runs again after an onWrite dispatch when JS under the dispatch submitted a request or asked for a flush that bailed.
WalkthroughThe HTTP/2 parser now defers JS-backed submissions during re-entrant writes, retries queued output, and preserves wire order. Tests cover buffer detachment, cork handoff, native and TLS transports, and synchronous response timing. ChangesHTTP/2 deferred flush coordination
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🟡 Moderate · up to HTTP/2 clients using upgraded JavaScript transports can still lose response events for large requests, so this path should be corrected before merge. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Updated 3:09 PM PT - Sep 19th, 2026
❌ @robobun, your commit e72a1d6 has 4 failures in
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Status Reproduced with one file and no network. A
The same checks are in the eight new tests in CI: no test under
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Inline comments:
In `@src/runtime/api/bun/h2_frame_parser.rs`:
- Around line 7189-7191: Update the submission guard around transport writes to
set unflushed_submit whenever transport_write_runs_js() is true, not only for
BunSocket::None. Ensure the shared outbound path queues bytes while this flag is
set, including Tls, Tcp, TlsWriteonly, and TcpWriteonly routes through
write_to_js_transport() and generic_write(), while preserving existing
frame-boundary and re-entrant-flush behavior.
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src/runtime/api/bun/h2_frame_parser.rstest/js/node/http2/node-http2.test.js
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…unters Follow-up to review of the first commit. - flush() repeats its onWrite dispatch at most 64 times for JS that submits again under it, then continues from a task queued with enqueue_task_after_yield. A peer that answers inside _write and a handler that submits a large request per answer no longer keep one flush running. - Bytes that unflushed_submit holds back do not count against maxSessionMemory. The transport did not refuse them. - A read that runs under the dispatch of write_buffer treats the bytes in flight as drained, so PINGs a reading peer sends from inside _write do not add up to an outbound ACK flood. - The new tests bound their poll, report session and stream errors in the assertion, and cover the three points above. - Comments are one line each.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline finding, I also checked the new flush_after_yield() ManagedTask ownership: the Box<RefPtr> from heap::into_raw is released exactly once on both paths — heap::take inside run, and release_queued_tasks() → promote_yield_tasks → release_unrun → drop_ctx when the VM tears down with the task still queued (src/runtime/api/bun/h2_frame_parser.rs:2884-2903, src/event_loop/ManagedTask.rs, src/jsc/event_loop.rs:927-939). The MAX_FLUSH_REDISPATCHES exit at h2_frame_parser.rs:2775 hands off to that same task rather than dropping the held bytes, so the bound does not strand write_buffer.
Extended reasoning...
The change is complex native state-machine code in the HTTP/2 frame parser's JS-transport write path, so it is not a candidate for auto-approval, and one confirmed finding is already being posted inline. Since the last review the author pushed two commits that add the redispatch bound, the in-flight-bytes drained check, and the held_bytes exemption, plus a new flush_after_yield ManagedTask. I traced that task's ownership through ManagedTask::new_owned/run/release and the event loop's release_queued_tasks teardown path and found the ref is dropped exactly once on both run and never-run paths; the bound-exhausted branch defers to the same task rather than losing queued bytes. That is the only concrete item I could rule out beyond the inline finding; the remaining flush/backpressure interactions still warrant a human maintainer's look given the subsystem.
Problem
createConnection), anode:http2client loses'response'when the request's frames overflow the 16 KiB cork and the transport's_writedelivers the answer before it returns. The frames reachsocket.write()insiderequest()orend(chunk), before the caller attaches listeners. Node v26.3.0 delivers it.req.end(largeBody)beforereq.on("response"), 230 requests in one synchronous block.H2FrameParser::_write(src/runtime/api/bun/h2_frame_parser.rs) callsonWriteat once when the cork overflows. node:http2: submit a request's frames after request() returns #42353 fixes another path to this symptom.Fix
request()setsunflushed_submiton a session with a JS transport. Until that session's nextflush(),_writeappends towrite_bufferinstead of callingonWrite. The deferred flush sends the bytes. Node defers every session write too.'drain'follows an unseen write. Held bytes do not count againstmaxSessionMemoryor the ACK flood counter.onWritedispatch,flush()repeats if JS under it submitted a request or had a flush bail. After 64 repeats it yields to the event loop.test/js/node/http2/node-http2.test.js(main fails all eight), the rest oftest/js/node/http2/, node'stest-http2-*. A structured self-review did not finish, see Notes.Background
DeferredTaskQueueat the end of each microtask checkpoint.onWritecallssocket.write(), which runs the user's_writesynchronously.write_bufferholds bytes the transport did not take. A nestedflush()bails whileflush()hands it over.Notes
Relation to #42353. #42353 removes the
native.flush()calls inHttp2Stream._final, the path a request that ends with its HEADERS takes (a plain GET). This PR covers the paths where the cork overflows. The two do not touch the same lines, and #42353 applies cleanly on top of this branch. Measured against a peer that answers from inside_write(node v26.3.0 delivers'response'in every row):endStream: falsereq.end(60 KB)beforeon("response")Two interplays with open PRs, both checked with local builds:
on_auto_flushdrop the auto-flush registration when the session does not hold the cork. If both land, itsstill_owedcondition should also countunflushed_submit. No test reaches the difference today.h2-conformance.test.tstests for "a large header block reaches the transport inside request()". With this PR and node:http2: apply SETTINGS_HEADER_TABLE_SIZE to the HPACK encoder and decoder #43345 they all pass (a GET still flushes in_final). With this PR, node:http2: submit a request's frames after request() returns #42353 and node:http2: apply SETTINGS_HEADER_TABLE_SIZE to the HPACK encoder and decoder #43345 together, one of them fails on its own reach check (insideRequest: true), because the block no longer reaches the transport insiderequest(). The case it is for still happens (the SETTINGS frame arrives during that write), so the check can go when the third of the three lands.I will rebase this PR if #42353 or #43345 lands first.
Why the flag lives until the next flush, not until
request()returns. The caller attaches listeners afterrequest()returns, in the same synchronous block. A body written in that block (req.end(chunk)) drains the corked HEADERS into its batch and pushes them out with it. A secondrequest()that overflows the cork pushes out the first one's HEADERS. So everything the session writes to a JS transport waits until a flush. The flush is the deferred tick, the transport's'drain'or'connect', or the flush at the end of an inbound read.Why the registration is kept.
uncork()releases the auto-flush registration together with the cork slot. When another session takes the slot, this session is uncorked without a flush, and its corked bytes now go towrite_buffer. Nothing on the JS side flushes them, sounregister_auto_flush()returns early while the flag is set.settle_unflushed_submit()releases the registration when a flush takes over.Why
flush()loops, and why the loop is bounded. Bytes held inwrite_buffergo out through the dispatch that setsjs_socket_in_flight. A flush requested under that dispatch bails (for example the flush at the end of an inbound read, after the peer answered from inside_write). Before this change that path was rare (real transport backpressure only). Now it is the normal path for held bytes, so a bailed flush is repeated once the dispatch returns, and frames the stream queue serialized into the cork are uncorked. Without this, a WINDOW_UPDATE or an END_STREAM corked under the dispatch waits for a later deferred tick, and an in-process pair can have no later tick. A peer that answers inside_writeand a handler that submits a large request per answer would keep that loop running, so after 64 repeatsflush()queues a task withenqueue_task_after_yieldand returns. Timers and I/O run before the task continues (a 0 ms timer fires after 65 of 200 chained requests). On main the same chain recursed until the stack overflowed.Held bytes and the two counters.
write_buffercounts towardmaxSessionMemory, and a non-emptywrite_bufferat the start of an inbound read means the peer does not read, which feeds the outbound ACK flood counter (1000). Held bytes are neither refused nor unread, so they are excluded from the memory count until a flush takes them, and a read that runs under the dispatch ofwrite_buffertreats the bytes in flight as drained. Both came from review of the first push.Request bodies that queue behind held bytes still count, as in node. With
maxSessionMemory: 1and 30 POSTs that eachend(64000 bytes)in one synchronous block, node v26.3.0 answers 17 and rejects 13 withERR_HTTP2_STREAM_ERROR(Http2Stream::DoWritecounts queued writes as session memory). This branch answers 17 or 18. Main answered all 30, because it wrote each body insideend(chunk)and the in-process peer opened the window again at once. With 40 KB header blocks under the same limit node accepts 39 of 40 and 199 of 200, which is what the exemption for held header blocks mirrors.Server sessions.
respond()uses the same native call, so a server session over a JS transport also holds its bytes until the flush at the end of the inbound read that ran the'stream'handler, or until the deferred tick.Changed tests. Six cases in
http2 DATA payload survives its ArrayBuffer being detached/resized by transport JS mid-sendused a request's own HEADERS as the corked bytes, so that transport JS ran in the middle of a later write. Those HEADERS now wait for the session's flush, so the tests would still pass but no longer run the Duplex mid-send. They now cork something else and assert the precondition (firedDuringWrite: true), on main and on this branch:straddling the cork flushcases fill the cork with another stream's DATA frame, one tick after both HEADERS frames left.Not covered (same result as main, node delivers
'response').tls.connect({ socket: duplex }). Its socket is native (BunSocket::Tlson anUpgradedDuplex), but its writes also run the user's_write, so it is the same class (transport_write_runs_js()names both). It is left out on purpose:generic_write()andgeneric_flush()borrowwrite_bufferacross the socket write and have no guard for a flush that re-enters under them, which the JS arm has (js_socket_in_flight, the frame tracker). Held bytes on that arm need that guard first, and the case needs a TLS peer that answers synchronously in the same process.request(), one event-loop turn, thenreq.end(largeBody)beforereq.on("response"), against a peer that answers when it sees END_STREAM. The flag is gone after the first flush, so the body is written from insideend(chunk)._write, whose'response'listener is attached after anawait. The flush at the end of the inbound read sends it before microtasks run. Node schedules that write. Bun can do the same only when a deferred task posted during a deferred pass is sure to run, which node:http2: submit a request's frames after request() returns #42353 adds.Teardown in the tick of the request.
request(40 KB header block)thensession.destroy()in one synchronous block now puts nothing on the wire, like a small request on main and like node v26.3.0 in both cases. Main wrote HEADERS and CONTINUATION first, because the block left insiderequest().Differential fuzz. Random synchronous blocks of
request(),write(),end()andping(), some issued from inside the transport's_write, over transports with sync or async write callbacks and a 1 byte or 16 KiBhighWaterMark. 96 runs each: main loses at least one'response'in 42 runs, this branch in none. Frame order and leftover bytes are clean in every run of both. 6 runs of each show DATA that stops at the flow-control window. Main has that stall too, this change does not cause it.Self-review. A structured adversarial review of this diff was started four times and was stopped by infrastructure each time before it returned a result, so no completed review of that kind stands behind this PR. In its place I ran, on main and on this branch: the differential fuzz above, and probe scripts for wire order, cork handover between two sessions, teardown in the tick of the request, requests submitted from inside the transport's
_write, and transport backpressure. Those runs led to the six re-triggered test cases above, the two cork-handover tests (checked by disabling the registration guard: the large-header case then times out), the two entries under "Not covered" that match main, and the teardown note. The second push answers the review comments on the first: the loop bound, themaxSessionMemorycount, the flood counter, bounded polling and error capture in the new tests, one-line comments, and the stated exclusion of the TLS arm. The three tests added with it fail on the first push and pass on the second.Suites run (debug build).
test/js/node/http2/(all files.h2-conformance.test.tsshows thestream release after a queued END_STREAMflake that #42357 tracks, over real sockets). All 261 nodetest-http2-*files, 13 of which use a JS transport: 260 pass, andtest-http2-forget-closed-streams.js(10000 requests over TCP, 1 s on a release build) exceeds my 90 s cap on the debug build. In-process client and server overduplexPair()with 30 KB header blocks, 50 KB bodies and 200 KB responses, parallel and sequential: same results as main or better.[human-review] gate passed · iteration 0 · 2 files touched
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