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WalkthroughUndici now implements fetch-backed dispatch, dispatcher lifecycle methods, request and response streaming, abort handling, global dispatcher validation, and dispatcher-aware ChangesUndici dispatcher support
Possibly related issues
Possibly related PRs
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Updated 1:23 PM PT - Aug 21st, 2026
✅ @robobun, your commit 01b6a718a7a2a8ef81f067288d1be7cd8d450030 passed in 🧪 To try this PR locally: bunx bun-pr 39250That installs a local version of the PR into your bun-39250 --bun |
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Found 3 issues this PR may fix:
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Verified all three against this branch: the repro assertions from #8961 pass, |
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Additional findings (outside current diff — PR may have been updated during review):
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🟡
src/js/thirdparty/undici.js:753-760—RetryAgent.close()/destroy()now delegate tothis.#agent(89ccbee), butRetryAgentstill inheritsget closed()/get destroyed()fromDispatcherBase, which read the RetryAgent instance's own#closed/#destroyed— never touched by the overrides. So afterawait retry.close(),retry.closed === falsewhileagent.closed === true(the new test only asserts the latter). Override the two getters to read fromthis.#agent.Extended reasoning...
What the bug is
89ccbee added
close(callback) { return this.#agent.close(callback) }/destroy(err, callback) { return this.#agent.destroy(err, callback) }toRetryAgentso closing a RetryAgent closes the wrapped dispatcher. ButRetryAgent extends DispatcherBase, andDispatcherBasedefines:#closed = false; #destroyed = false; get closed() { return this.#closed; } get destroyed() { return this.#destroyed; }
The overridden
close()/destroy()never callsuper.close()and never write to the RetryAgent instance's ownDispatcherBase.#closed/#destroyedslots — they only forward to the wrapped agent. So the inherited getters keep returning their initialfalsefor the lifetime of the RetryAgent, regardless of the wrapped agent's actual state.Step-by-step proof
const agent = new Agent(); const retry = new RetryAgent(agent); await retry.close();
retry.close()(the override) callsthis.#agent.close(undefined).agent.close()(DispatcherBase.close) setsagent's#closed = true, drains, resolves.retry's ownDispatcherBase.#closedwas never touched — it is stillfalse.retry.closedinvokes the inheritedDispatcherBasegetter, which readsthis.#closedon the RetryAgent instance →false.agent.closed→true.
The same holds for
destroy()/destroyed. The new test attest/js/first_party/undici/undici.test.ts(RetryAgent.close() closes the wrapped dispatcher) only assertsagent.closed === true, so this gap is not caught.Why existing code doesn't prevent it
JS private fields are per-class-declaration;
DispatcherBase.#closedon aRetryAgentinstance is a distinct slot fromDispatcherBase.#closedon the wrappedAgentinstance. Nothing inRetryAgentwrites to its own slot, and there is no getter override, so the inherited accessor reads the never-written slot.Note also that
DispatcherBase.dispatch()guards onthis.#closed/this.#destroyed— which for the RetryAgent are alwaysfalse— soretry.dispatch()afterretry.close()skips the RetryAgent-levelClientClosedErrorguard. It still fails correctly, but only because[kDispatch]callsthis.#agent.dispatch()and the wrapped agent's own#closedcheck firesUND_ERR_CLOSED. So the practical dispatch-after-close behavior is right; only the getters lie about state.Impact
Minor observable inconsistency: code that inspects
retry.closed/retry.destroyed(e.g. to decide whether to reuse a dispatcher) seesfalseafter a successful close/destroy. Nothing crashes and dispatch still errors via the wrapped agent, so this is a fidelity nit rather than a functional break. (Real undici'sRetryAgentextendsDispatcherdirectly rather thanDispatcherBase, so its exact getter surface differs — but this shim chose to exposeclosed/destroyedgetters, and having them contradict the object's actual state is internally inconsistent regardless.)Fix
Override the two getters on
RetryAgentto read from the wrapped dispatcher, mirroring theclose/destroydelegation already added:get closed() { return this.#agent.closed; } get destroyed() { return this.#agent.destroyed; }
and extend the test to also assert
retry.closed === true.
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Actionable comments posted: 5
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Inline comments:
In `@src/js/thirdparty/undici.js`:
- Around line 1020-1035: Update the request flow around dispatcher.dispatch so
redirect status responses reject the returned promise when init.redirect is
"error", while preserving the existing manual behavior and follow redirection
handling. Ensure the rejection occurs before resolving the response and uses the
existing promise error path.
- Around line 1069-1075: Update the onData callback to check whether
streamController exists before enqueuing the chunk or reading desiredSize; when
it is null, discard the chunk and return without dereferencing it, while
preserving the current enqueue and backpressure behavior for an available
stream.
- Around line 9-12: Capture FormData, URLSearchParams, Request, Headers, and
Response alongside the existing globals in the module-load destructuring, and
update dispatch code to use those captured bindings instead of live globalThis
lookups. Preserve the tamper-resistant behavior described by the existing
comment.
In `@test/js/first_party/undici/undici.test.ts`:
- Around line 671-677: Update the polling block in the async setup around
abortFn so it explicitly asserts that pulls reached at least 3 before proceeding
to abortFn!(). If the 5-second deadline expires first, fail the test instead of
continuing to the abort assertion.
- Around line 526-541: Strengthen the assertion in the async failure test around
pool.dispatch so it verifies the expected connection-refusal error identity,
such as its documented code or name, rather than only checking that err is an
Error. Keep the existing rejection guards and cleanup behavior unchanged.
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CI status: all test failures in the latest run are known flaky tests that passed on retry or in isolation (bun-lock, napi, bun-install-registry, child_process_ipc_handle, inspect-error-leak, cluster-shared-leak, malformed-integrity-base64). The only hard failure is the Windows x64 build job, which failed on a transient toolchain download timeout (rustup could not reach static.rust-lang.org), unrelated to this JS-only change. The undici test file (45 tests) passes locally, and the miniflare + workerd end-to-end flow described above works under this branch. Ready for review. |
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Inline comments:
In `@src/js/thirdparty/undici.js`:
- Around line 1176-1179: Update the onComplete callback to settle the fetch
promise when completion occurs before onHeaders, rejecting or resolving through
the existing promise settlement mechanism while preserving signal removal and
stream closure. Add a test covering onConnect followed by onComplete without
onHeaders and verify the returned promise settles.
- Line 641: Update the onData callback in the request body-stream path to copy
each dispatcher-owned chunk before passing it to body.push, matching the copy
behavior used by fetchViaDispatcher. Add a request() test that reuses or mutates
the dispatcher buffer across callbacks and verifies the resulting body remains
intact.
In `@test/js/first_party/undici/undici.test.ts`:
- Around line 696-704: Update the test cleanup around getGlobalDispatcher and
setGlobalDispatcher so it restores the pre-test unset global-dispatcher state
without triggering or installing the lazily created Agent. Use the existing
supported reset/clear mechanism, confirming the value accepted by
setGlobalDispatcher, and preserve pool.close cleanup on every path.
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Triage note: #26414 (no-op Checked against upstream undici 5.20 under Node for reference: Still open and overlapping with this PR textually in |
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Inline comments:
In `@src/js/thirdparty/undici.js`:
- Around line 647-655: In the request completion path where body is null, create
the TypeError once, call destroyRequestBody with it before invoking callback,
and pass the same error to callback. Update the CompletesEarly test fixture to
use a Readable body and verify that the body is destroyed after request
rejection.
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Inline comments:
In `@test/js/first_party/undici/undici.test.ts`:
- Around line 826-841: Update the “fetch ignores dispatcher callbacks after
onComplete” test to record whether the late handler.onData and
handler.onComplete calls throw, then assert both late calls complete without
throwing. Keep the existing response-body assertion and exercise the callbacks
through the dispatcher object.
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I reviewed this PR and didn't find any bugs. Because it adds ~850 lines of new user-facing API surface to the undici shim (a fetch-backed dispatch() pipeline, both handler interfaces, a dispatcher-aware fetch(), and full lifecycle semantics) and overlaps with two other open PRs the description calls out, a human look is still worthwhile.
What was reviewed:
- The
fetchDispatchtransport loop for abort/pause/resume ordering, and that every terminal exit inrequest()andfetchViaDispatcherflips its guard flag and nullsstreamControllerso late callbacks are no-ops. - Path handling —
pathmust start with/and is concatenated onto the origin so//other.host/xcannot change the request authority;parseOriginrejects non-http(s) and origins with a path/query/hash. headersFromDispatchOptsemits an entries array (so array-valued/repeated headers append separately) and iteratesHeaders/MapviaSymbol.iteratorbefore theObject.keysfallback.close()/destroy()drain and abort thekPendingset correctly, andgetGlobalDispatcher()does not reroute barefetch().
Extended reasoning...
Overview
This PR replaces the empty Dispatcher/Pool/Client/Agent/BalancedPool/RetryAgent stubs in src/js/thirdparty/undici.js with a working implementation: a fetch()-backed fetchDispatch() that drives both the legacy (onConnect/onHeaders/onData/onComplete/onError) and v7 controller (onRequestStart/onResponseStart/…) handler shapes, a DispatcherBase with close()/destroy() lifecycle and in-flight tracking, a Dispatcher.request() built on dispatch() returning a DispatchBodyReadable with the undici body mixin, and a dispatcher-aware exported fetch(). It also gives the error classes their undici name/code, validates setGlobalDispatcher(), and adds ~1200 lines of tests covering both handler interfaces, abort/pause/resume, close/drain/destroy, signal support, path validation, header shapes, and hardening against contract-violating custom dispatchers. Net: +2055 / −23 across two files.
Security risks
Low. The one authority-relevant path — building the request URL from a Pool's origin plus opts.path — validates that path starts with / and concatenates onto base.origin rather than URL-resolving, so //evil.example/x cannot redirect the request off-origin (tested). parseOrigin rejects non-http(s) protocols and origins carrying a path/query/hash. Header records handed to callers and per-request context/trailers objects are null-prototype. Globals (AbortController, ReadableStream, Blob, ArrayBuffer, and the Undici.cpp bindings) are captured at module load. No auth, crypto, or filesystem code is touched.
Level of scrutiny
High. This is not a mechanical change: it implements a substantial third-party API surface from scratch, with a non-trivial design choice (transporting over Bun.fetch rather than a lower-level client, which the PR itself acknowledges leaves a GET/HEAD-body transport gap and means the dispatcher-routed Response has no .url/.redirected). The PR description also flags textual overlap with #27338 and #26414 and a complementary specifier-resolution direction (#36102/#30561) that "needs a maintainer call" — a maintainer should decide how these efforts relate. Per the repo's guidance on API design and Node/Web compat, new user-facing surface of this size warrants a human sign-off.
Other factors
The PR has been through many bot-review iterations; every prior inline finding (double-final onHeaders, late-callback guards, signal-listener balance, pre-aborted-signal short-circuit, header array/iterable handling, protocol validation, dead-code removal, outer-catch completed flip, orphaned streamController nulling) is marked resolved with a follow-up commit and a test, and this run's bug-hunting pass found nothing new. Test coverage is broad and each guard has a test that exercises it. What tips this to defer rather than approve is purely the scale and the acknowledged overlap with other open PRs — the code itself looks solid.
…tchers Implements the undici Dispatcher API over Bun.fetch: dispatch() with both the legacy and the v7 controller handler interfaces, request() with the body mixin, close() and destroy() lifecycle, and a dispatcher-aware fetch() so miniflare can route requests through a custom dispatcher. Co-authored-by: Matthew Bell <33056264+matthew2564@users.noreply.github.com>
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robobun ur doing great keep going much love we gotta get some buns in the oven going |
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Thanks. The PR is rebased and green, and it now waits on a maintainer review. |
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I reviewed this PR and the automated bug-hunting pass found no issues; the last inline finding (late 1xx firing opts.onInfo after a terminal callback) was addressed in 01b6a71. Given the scope — ~850 lines of new dispatcher/fetch machinery in the built-in undici shim plus a dispatcher-aware fetch() — and the noted overlap with #27338/#26414/#36102, a maintainer look at the API-surface and resolution-semantics choices would still be worthwhile.
What was reviewed:
fetchDispatchhandler-callback ordering, abort/pause/resume, and pending-set drain for close()/destroy().Dispatcher.request()andfetchViaDispatcherre-entry guards against contract-violating dispatchers (all sibling callbacks now gate oncompleted/resolved, including the 1xx onInfo path).- Path handling:
//other.host/xcannot change authority;parseOriginrejects non-http(s) — no origin-escape found. - Ruled out: close() on an already-destroyed dispatcher rejecting instead of resolving — matches undici.
Extended reasoning...
Overview
This PR replaces the empty Dispatcher/Pool/Client/Agent/BalancedPool/RetryAgent stubs in src/js/thirdparty/undici.js with a working fetch-backed implementation: dispatch() driving both the legacy (onConnect/onHeaders/onData/onComplete/onError) and v7 controller handler interfaces, DispatcherBase lifecycle (close() drains, destroy() aborts), Dispatcher.request() built on dispatch() with a body-mixin Readable, and a dispatcher-aware exported fetch() (fetchViaDispatcher). ~850 net lines in the shim, ~1200 lines of new tests. It targets miniflare/wrangler and closes four long-standing issues.
Security risks
The one security-adjacent surface is request-authority handling: fetchDispatch concatenates base.origin + path (rejecting paths not starting with /) rather than URL-resolving, so //other.host/x cannot swap the authority, and parseOrigin rejects non-http(s) protocols and origins carrying a path/query/hash. Header records handed to handlers are null-prototype. No credential, TLS, or auth logic is touched. I did not identify an origin-escape or header-injection vector.
Level of scrutiny
High. This is a large addition of new user-facing API surface to a shim that bare require('undici') resolves to for every Bun user, and it changes the exported fetch() to route through dispatcher.dispatch() when one is set (including via setGlobalDispatcher). The PR description itself flags textual overlap with two other open PRs and a complementary resolution-semantics direction that "needs a maintainer call". That's a design-level decision a human should weigh in on, not something to auto-approve.
Other factors
The PR has been through eight review iterations; every prior inline finding (double-callback guards, signal-listener balance, header normalization for arrays/Headers/Map, pre-aborted signals, redirect-error ordering, dead-code removal, and the late-1xx onInfo guard) was addressed with a targeted fix and a test. This run's finder pass raised only one candidate (close() on an already-destroyed dispatcher rejecting), which the verifiers ruled out as matching undici. Test coverage is thorough (60+ new tests exercising both handler shapes, abort paths, drain/destroy, path validation, and contract-violating dispatchers). CI is green on debug+ASAN and release. The remaining reason to defer is scope and API-surface ownership, not correctness concerns.
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good now we need to release it. all pray for robobun our saviour |
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Thanks. Merge and release timing is a maintainer decision. The branch stays rebased and green in the meantime. |
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be the maintainer most will never be |
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A second report with the same root cause came in: On bun 1.4.3-canary.1+6a92015fc every proxied request answers 500 with I ran That report also lists |
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Problem
undici.Pool(andClient,Agent, every dispatcher) in the builtin shim has nodispatch(),close()ordestroy(): theDispatcherbase class insrc/js/thirdparty/undici.jswas an emptyclass Dispatcher extends EventEmitter {}.pool.dispatch(options, handler)and tears down withpool.close(), sowrangler dev/@cloudflare/vite-pluginunder Bun dies withTypeError: this.#runtimeDispatcher?.close is not a function.Pool.prototype.requestwas an empty method returningundefined, a silent no-op.Dispatcherhas nodispatch),undici.Agentis missingasync close()method #14498 (Agenthas noclose()), @elastic/elasticsearch@8.11.0 not working due to missing undici.Pool.request #7920 (Pool.prototype.requestno-op breaks @elastic/elasticsearch).Poollacksdispatch()/close()/destroy(), breaking miniflare + workerd (wrangler dev, @cloudflare/vite-plugin) #39247. Fixes bug: EventEmitter or something different? #8961. Fixesundici.Agentis missingasync close()method #14498. Fixes @elastic/elasticsearch@8.11.0 not working due to missing undici.Pool.request #7920. Fixes wrangler's createTestHarness() never answers over its listening socket when the parent process is Bun #42231.Fix
Dispatchermirrors undici's abstract base (throwingdispatch/close/destroystubs) and gains a workingrequest()built onthis.dispatch(); its result body carries undici's body mixin (text/json/arrayBuffer/bytes/blob/bodyUsed), and destroying it early cancels the request.DispatcherBaseimplements the lifecycle:close()/destroy()in promise and callback form,closed/destroyedgetters, and in-flight request tracking soclose()resolves after pending requests settle anddestroy(err)aborts them witherrorClientDestroyedError.dispatch()validates opts, handler shape (undici's synchronousinvalid onError method), and closed/destroyed state, routing failures through the handler's error callback (UND_ERR_CLOSED/UND_ERR_DESTROYED).Agent/Pool/Client/BalancedPool/RetryAgentdispatch real requests overfetch(), driving both handler interfaces: the legacy callbacks (onConnect/onHeaders/onData/onComplete/onError) and the undici v7 controller callbacks (onRequestStart/onResponseStart/onResponseData/onResponseEnd/onResponseError), with abort (including from a paused body loop or on the final chunk), pause/resume backpressure, andopts.signal(EventTarget or EventEmitter style).fetch()honors the{ dispatcher }init option by routing the request throughdispatcher.dispatch()and assembling theResponsefrom the handler callbacks (falling back toBun.fetchwhen no dispatcher is given). This is how miniflare rewrites every request into workerd./and are concatenated onto the origin rather than URL-resolved, so//other.host/xcannot change the request authority.Readable/iterable request bodies stream through fetch instead of being buffered.getGlobalDispatcher()returns anAgent(undici's behavior);setGlobalDispatcher()validates its argument. Error classes carry undici'snameandcode. Header records and handler-facing context/trailers objects are null-prototype and per-request.test/js/first_party/undici/undici.test.tscovering both handler interfaces,request()body streaming and mixin, close/drain/destroy semantics, callback forms, abort paths, signal support, path validation, and the global dispatcher. All fail on current Bun, pass with this change; existing shim tests andundici-primordials.test.tsstill pass.typeofassertions pass,await new Agent().close()resolvesnull(undici.Agentis missingasync close()method #14498), and@elastic/elasticsearch@8.11.0indices.createround-trips against a local stub server (@elastic/elasticsearch@8.11.0 not working due to missing undici.Pool.request #7920, fails on current Bun inside@elastic/transport).Pool.prototypeown props are["constructor"], andawait pool.close()resolves.miniflare@5.20260811.1-alpha+ real workerd: on current Bun,setOptions()(the reconfigure@cloudflare/vite-pluginperforms at startup) crashes with the issue's exactthis.#runtimeDispatcher?.close is not a function; with this change, boot,dispatchFetch,setOptions, a seconddispatchFetch, anddispose()all succeed.Background
Dispatcheris the transport abstraction:dispatch(options, handler)performs one request and reports progress through handler callbacks instead of returning a response object;request()/stream()/etc. are sugar built on it.close()finishes outstanding work and rejects new dispatches;destroy()does so immediately.onHeaders(status, rawHeadersAsBuffers, resume, statusText)andonData(chunk)(returningfalsepauses untilresume()); the controller one receives a controller object (abort/pause/resume) plusonResponseStart/Data/End/Error. Consumers like miniflare pass either, so the shim detects which callbacks the handler defines.fetch(), which decompresses bodies,content-encoding/content-lengthare dropped from the headers handed to the handler so they describe the bytes actually delivered.request()/stream()on Pool/Client but leavesDispatcherempty and adds nodispatch(); fix(undici): add Dispatcher close() and destroy() methods #26414 adds no-opclose()/destroy()stubs only. Neither unblocks miniflare, which needs a functionaldispatch()and a dispatcher-awarefetch(). Changing specifier resolution so installed undici wins (Resolve bare undici to the installed package, keep the shim as fallback #36102 / undici: remove polyfill, resolve to real npm package #30561) is a complementary direction that needs a maintainer call on resolution semantics; this PR fixes the shim itself, which is what bareundiciresolves to today and would remain the fallback for projects without undici installed.[review] gate passed · iteration 8 · 2 files touched
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