node:dgram: don't swallow bind errors when a callback is provided by robobun · Pull Request #47218 · oven-sh/bun

robobun · 2026-10-28T17:40:52Z

Problem

socket.bind(port, address, callback) on a taken port reports nothing when the program has no 'error' listener: no event, no callback, no throw. Node.js v26.12.1 raises EADDRINUSE.
The cause is this.on("error", removeListeners) in Socket.prototype.bind (src/js/node/dgram.ts:701), which drops the callback after a failed bind. emit("error") finds that listener and does not throw.

Fix

bind() registers removeListeners under the events.errorMonitor symbol, as dgram: don't swallow bind errors when callback is provided nodejs/node#62602 does. dgram.ts reads the symbol once at load.
Correct because a monitor listener is not a handler: with no 'error' listener of the program, emit("error") throws.
Verified: test/js/bun/udp/dgram.test.ts (10 new tests, 9 fail without the fix) and Node's test-dgram-bind-error-callback.js. All 90 dgram test files pass.

Background

An EventEmitter throws when 'error' is emitted with no listener. Listeners under events.errorMonitor run first and do not change that.
dgram.ts emits a failed bind from six places. All pass this one listener.
Considered: a catch around the bind completion, a bind with no promise hop, a callback slot in socket state. Each changes call orders that equal Node today.

Downsides

Behavior change: a program with a bind callback and no 'error' listener that survived a failed bind now exits with an uncaught 'error', as on Node.js v26.12.1.
In an ES module, socket.close() one await after a failing bind(cb) exits 0 today and on Node. It now raises the error if that bind() makes the process's first process.nextTick call: Bun runs that tick before the continuation (related: process: drain the microtask queue before nextTick callbacks queued from inside it #33088).
Size: +31 bytes of release binary text. Per bind(cb): +2 bytecode instructions, 0 allocations.

Notes

Measurements. Release builds of main (11c3671) and of this PR (3a2f436), and a Node.js v26.12.1 binary. Linux x64.

pending bind(cb) listeners (error/errorMonitor/listening): main 1 0 1, PR 0 1 1, Node v26.12.1 0 1 1
GC cells per bind(cb): main 36, PR 36 (delta 0). Per bind without callback: main 32, PR 32 (delta 0). Counted with BUN_JSC_useGC=0 and heapStats().objectTypeCounts, as the slope between 300 and 600 sockets.
bytecode per bind(cb): +2 instructions executed (bind 189 -> 190, removeListeners 19 -> 20), 0 added calls, allocations, queue entries or syscalls. Read from BUN_JSC_dumpGeneratedBytecodes=1. Each site replaces one constant mov with resolve_scope and get_from_scope. A send() queued behind a bind executes 2 fewer instructions (enqueue 29 -> 28, onListenSuccess 16 -> 15): those two sites lose a get_by_id.
node/dgram.js builtin text: +31 bytes (32,647 -> 32,678). Release binary text: +31 bytes (138,012,530 -> 138,012,561 by size, main 11c3671 vs PR). data and bss do not change. The file size does not change (138,197,144 bytes).
matrix call orders equal to Node v26.12.1: main 6 of 24, PR 21 of 24. Repro exit 0. Upstream test exit 0.
existing dgram tests changing outcome: 0 of 88 files (87 vendored Node.js tests and node-dgram.test.js). In dgram.test.ts the 62 tests from before this PR pass on both builds.
failed bind(cb), no handlers: main exit 124 (hang, killed by timeout 3), PR exit 1, Node v26.12.1 exit 1

The matrix is 24 programs, one process each: bind() with and without a callback on a free port, a taken port and an address that is not local, a lookup that fails (synchronously and asynchronously), a send() queued behind the bind, a retry, a late 'error' listener, close() during the bind, util.promisify(socket.bind), a bad fd, an 'error' the program emits itself, and a buffer size that cannot be applied. Each program prints what it hears (listener, callback, uncaughtException, unhandledRejection) and the listener counts. The output on bun is compared with the output on Node.js v26.12.1.

The symbol is read once at module load. events.errorMonitor is a writable export. The first commit of this PR read EventEmitter.errorMonitor at each call, as Node does and as the send queue of dgram.ts did. After require("node:events").errorMonitor = "error", the cleanup listeners were then ordinary 'error' listeners and a failed bind was not raised. Node.js v26.12.1 does the same: the program of the new test row prints nothing there. dgram.ts now reads the symbol into kErrorMonitor when the module loads and uses it at all four sites (two in bind(), two in the send queue). An overwrite before the first load of node:dgram is not covered. The first commit measured +36 bytes of text and +4 instructions per bind(cb).

The 3 call orders that still differ are the buffer size rows. startBunSocket completes the bind inside a promise reaction (dgram.ts:912-938). An error that leaves that reaction is raised as unhandledRejection. Node raises uncaughtException. The same is true of a bind callback that throws and of a 'listening' listener that throws. This PR does not change that. With a callback, the buffer size error was silent before and is raised now.

The ES module case in Downsides. Probes, each a .mjs entry point:

import dgram from "node:dgram";
const s = dgram.createSocket("udp4");
s.bind(0, "192.0.2.1", () => console.log("callback")); // EADDRNOTAVAIL
await null;
s.close(); // or: s.on("error", e => console.log("heard", e.code));

after `await null`	Node v26.12.1	main	this PR
`s.close()`	exit 0	exit 0	uncaught `EADDRNOTAVAIL`, exit 1
`s.on("error", ...)`	listener hears it	nothing is heard, exit 0	uncaught `EADDRNOTAVAIL`, exit 1

The address lookup of an IP literal answers through process.nextTick. When that is the first process.nextTick call of the process, bun runs the tick before the continuation of the await. After any earlier process.nextTick call all three columns are equal. In CommonJS the PR equals Node (both raise the error).

Other differences from Node that this PR does not change.

bind() with a port out of range (70000, -1): bun emits ERR_INVALID_ARG_TYPE, Node binds the port modulo 65536. With a callback this was silent and is now raised.
The bind error has no .port and its message has no port (bind EADDRINUSE 127.0.0.1). node:dgram: a bind error carries a negative errno #46679 is open for the error shape.
Socket.prototype.bindSync() and connectSync() do not exist. docs: list the Node.js v26.4 to v26.12 APIs that Bun lacks on the compatibility page #47138 lists them.
node:events: make errorMonitor an unregistered Symbol #47134 makes events.errorMonitor an unregistered symbol. This change reads the export of node:events, so it holds the right symbol with and without that PR.

The one internal caller with a callback is parseHandle for a received dgram.Socket (src/js/builtins/Ipc.ts:98). If the native adoption of the received descriptor fails, the error now leaves bind(). src/runtime/ipc.rs:2200 then closes the descriptor and returns the error. I read this path in the source. I did not trigger it.

Tests.

test/js/node/test/parallel/test-dgram-bind-error-callback.js is byte-identical to the file at the Node.js tag v26.12.1. On main it does not exit (timeout 8 exit 124).
New tests in test/js/bun/udp/dgram.test.ts: the listener counts while a bind is pending (0/1/1) and inside the callback (0/0/0), a synchronous lookup failure that throws from bind(), and one child process for each of: bind(port, address, cb), bind({ port, address }, cb), a queued send(), an overwritten events.errorMonitor, an asynchronous lookup failure, a cluster worker whose primary answers with an error code, and a buffer size that cannot be applied. One more child is a control: a listener of the program hears the error and nothing is raised. The expected output of each child is what Node.js v26.12.1 prints for the same program, with two exceptions. The buffer size row accepts either process event. The overwrite row expects the error, and Node prints nothing.
Ran with the debug build: test/js/bun/udp/dgram.test.ts (72 pass), test/js/node/dgram/node-dgram.test.js, and the 88 vendored test-dgram-*, test-cluster-dgram-* and test-child-process-dgram-* files through scripts/runner.node.ts: 0 fail.
Not run locally on macOS or Windows.

no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/udp/dgram.test.ts

socket.bind(..., callback) registered its cleanup function as an 'error' listener. A failed bind then found a listener, so the error was not raised. A program with no 'error' listener of its own heard nothing. bind() now registers the cleanup function under events.errorMonitor, as Node.js does since nodejs/node#62602 (commit 788976c626). A failed bind with a callback and no 'error' listener raises the error. test-dgram-bind-error-callback.js is the test of that commit. Co-authored-by: armanmikoyan <arman.mikoyan1@gmail.com>

robobun · 2026-10-28T17:41:17Z

Status

Reproduced on bun 1.4.3-canary and on main (11c3671) with this program. It prints nothing on main. It prints uncaughtException EADDRINUSE on Node.js v26.12.1 and with this PR.

const dgram = require("node:dgram");
process.on("uncaughtException", err => console.log("uncaughtException", err.code));
const holder = dgram.createSocket("udp4");
holder.bind(0, "127.0.0.1", () => {
  holder.unref();
  dgram.createSocket("udp4").bind(holder.address().port, "127.0.0.1", () => console.log("callback"));
});

CI on 3a2f436 passed (build 134616). All test-bun lanes are green: Linux (Debian, Ubuntu, Alpine, ASAN), Windows and macOS, on x64 and aarch64. The only failed jobs are the 7 soft-fail test-aot-drawn lanes. They fail in test/aot/ tests of bun build --compile --aot, which this PR does not touch.

PR: #47218

robobun · 2026-10-28T17:41:19Z

^{Updated 6:38 PM PT - Oct 28th, 2026}

❌ @robobun, your commit 3a2f436 has 7 failures in Build #134616 (All Failures):

test/aot/perf/generated-code-of-idioms-9.test.ts - code 1 on 🐧 13 x64 - test-aot-drawn
test/aot/perf/generated-code-of-idioms-7.test.ts - code 1 on 🐧 13 x64 - test-aot-drawn
test/aot/perf/generated-code-of-operations-1.test.ts - code 1 on 🐧 13 aarch64 - test-aot-drawn
test/aot/generated-births-and-uses-of-typed-two-kinds-of-list-or-collection-at-one-use-4.test.ts - code 1 on 🐧 13 x64 - test-aot-drawn
test/aot/perf/generated-code-of-idioms-12.test.ts - code 1 on 🐧 13 aarch64 - test-aot-drawn
test/aot/perf/generated-code-of-idioms-15.test.ts - code 1 on 🐧 13 aarch64 - test-aot-drawn
test/aot/perf/generated-code-of-idioms-16.test.ts - code 1 on 🐧 13 aarch64 - test-aot-drawn

🧪 To try this PR locally:

bunx bun-pr 47218

That installs a local version of the PR into your bun-47218 executable, so you can run:

bun-47218 --bun

coderabbitai · 2026-10-28T17:48:40Z

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src/js/node/dgram.ts
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Walkthrough

Socket.prototype.bind now uses a captured errorMonitor symbol for temporary listener setup and cleanup. New tests cover listener cleanup and callback-based bind failures, including cluster, lookup, and address-in-use errors.

Changes

UDP bind error callback handling

Layer / File(s)	Summary
Bind listener cleanup `src/js/node/dgram.ts`, `test/js/bun/udp/dgram.test.ts`	The bind path uses the captured `kErrorMonitor` symbol for temporary cleanup listeners and queued-send cleanup. Tests check listener counts and synchronous lookup-error behavior.
Bind failure outcomes `test/js/bun/udp/dgram-cluster-bind-callback-error-fixture.ts`, `test/js/bun/udp/dgram.test.ts`, `test/js/node/test/parallel/test-dgram-bind-error-callback.js`	Tests cover cluster bind errors, address-in-use failures, queued sends, asynchronous lookup errors, oversized receive buffers, and duplicate binds.

Suggested reviewers: jarred-sumner

Priority: Normal

Merge Risk: Low · up to 3a2f4

A pre-import mutation can still hide a failed UDP bind, so the internal monitor symbol should be used before merging.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the primary change: preventing bind errors from being swallowed when a callback is provided.
Description check		The description explains the problem, fix, behavior changes, verification results, known differences, and test coverage. It does not use the exact template headings, but it provides the required infor…
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.

Full details: Description check

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Actionable comments posted: 1

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Inline comments:
Review comments at @src/js/node/dgram.ts:
- Line 701: In the bind cleanup path, replace reliance on the mutable
EventEmitter.errorMonitor property with the module-local symbol matching the
internal error-monitor symbol; use that same symbol for listener registration
and removal so overriding the public property cannot turn cleanup into an
ordinary 'error' listener.

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Files selected for processing: 4

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claude

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 three callers whose behavior shifts with this change and ruled them out: the cluster-worker bind path (dgram.ts:795-799) and the IPC-received socket path (bindServerHandle at dgram.ts:717-720) now raise an uncaught 'error' instead of silently dropping a failed bind, which matches Node, and the synchronous bind({ fd }) throw at dgram.ts:725/730 leaves the same orphan it did before, now under errorMonitor rather than 'error'. The switch to EventEmitter.errorMonitor mirrors the existing use at dgram.ts:1102, errorMonitor is exported by src/js/node/events.ts, and there is no ahead-of-time copy of dgram to keep in sync.

Extended reasoning...

Two-line production change in src/js/node/dgram.ts Socket.prototype.bind swapping the internal cleanup listener from 'error' to EventEmitter.errorMonitor, plus new in-process, subprocess, cluster-fixture and ported Node tests. No security-sensitive surface. One inline finding stands (a pre-existing unhandledRejection-vs-uncaughtException divergence that this change widens), and the fix is a user-visible behavior change for programs that previously survived a failed bind(cb), so a human maintainer should still weigh the compat trade-off.

bind() and the send queue read EventEmitter.errorMonitor at each call. The export is writable. A program that overwrote it turned the cleanup listeners of node:dgram into 'error' listeners, and a failed bind was not raised. The module now reads the symbol once into kErrorMonitor and uses it at all four sites.

claude

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

LGTM — the fix is Node's own two-line change (register the bind-callback cleanup under errorMonitor instead of 'error'), the second commit's load-time kErrorMonitor capture is consistent across all three listener sites, and the remaining inline note is a nit about a self-inflicted edge case.

Extended reasoning...

The diff changes four lines of logic in src/js/node/dgram.ts: Socket.prototype.bind now adds/removes its removeListeners cleanup under EventEmitter.errorMonitor (captured once at module load as kErrorMonitor) rather than under 'error', and the existing enqueue/onListenSuccess sites switch to the same cached constant. No other internal 'error' listeners remain in dgram.ts (grep shows only emit sites), so the whole class of swallowed bind failures is covered by this one listener. It touches no security-sensitive surface. The tests add listener-count assertions, a sync-throw case, a concurrent subprocess matrix covering both bind overloads, queued send, overwritten errorMonitor export, async lookup failure, the user-listener control, a cluster fixture, and a verbatim port of Node's test-dgram-bind-error-callback.js. The one confirmed finding (a program overwriting events.errorMonitor before first requiring node:dgram) is an edge case that stems from Bun's export being writable where Node's is not, and does not affect the correctness of this fix for ordinary programs.

robobun assigned Jarred-Sumner Oct 28, 2026

github-actions Bot added the claude label Oct 28, 2026

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Comment thread src/js/node/dgram.ts Resolved

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Comment thread test/js/bun/udp/dgram.test.ts Resolved

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Comment thread src/js/node/dgram.ts Resolved

robobun mentioned this pull request Nov 1, 2026

node:dgram: run 'message' in the async context of createSocket() #47609

Open

robobun mentioned this pull request Nov 8, 2026

node:dgram: read the port of bind() as Node's handle does #48220

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node:dgram: don't swallow bind errors when a callback is provided - #47218

node:dgram: don't swallow bind errors when a callback is provided#47218
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