fix(coding-agent): handle undici mid-stream client errors

closes #6133

packages/coding-agent/CHANGELOG.md

@@ -8,6 +8,7 @@
 
 ### Fixed
 
+- Fixed a crash when undici emits an internal client error while terminating a mid-stream HTTP response ([#6133](https://github.com/earendil-works/pi/issues/6133)).
 - Fixed the compaction event regression test to cover status indicator cleanup and keep CI passing.
 - Fixed interactive status indicators so ending work, retry, compaction, or branch-summary indicators no longer shrink the TUI when clear-on-shrink is enabled ([#6026](https://github.com/earendil-works/pi/pull/6026)).
 - Fixed `--session` and `SessionManager.open()` to reject non-empty invalid session files without overwriting them ([#6002](https://github.com/earendil-works/pi/issues/6002)).

packages/coding-agent/src/core/http-dispatcher.ts

@@ -1,3 +1,4 @@
+import { EventEmitter } from "node:events";
 import * as undici from "undici";
 
 export const DEFAULT_HTTP_IDLE_TIMEOUT_MS = 300_000;
@@ -46,18 +47,50 @@ export function applyHttpProxySettings(httpProxy: string | undefined): void {
 	process.env.HTTPS_PROXY ??= proxy;
 }
 
+const ignoreUndiciDispatcherError = (_error: unknown): void => {};
+
+// Undici can emit an internal Client "error" while terminating a mid-stream
+// fetch body. The body stream still rejects through reader.read(); this listener
+// only prevents EventEmitter's unhandled "error" special case from crashing pi.
+function withUndiciErrorListener<T extends undici.Dispatcher>(dispatcher: T): T {
+	if (dispatcher instanceof EventEmitter) {
+		EventEmitter.prototype.on.call(dispatcher, "error", ignoreUndiciDispatcherError);
+	}
+	return dispatcher;
+}
+
+function createUndiciClient(origin: string | URL, options: object): undici.Dispatcher {
+	return withUndiciErrorListener(new undici.Client(origin, options as undici.Client.Options));
+}
+
+function createUndiciOriginDispatcher(origin: string | URL, options: object): undici.Dispatcher {
+	const dispatcherOptions = options as undici.Pool.Options;
+	if (dispatcherOptions.connections === 1) {
+		return createUndiciClient(origin, dispatcherOptions);
+	}
+	return withUndiciErrorListener(
+		new undici.Pool(origin, {
+			...dispatcherOptions,
+			factory: createUndiciClient,
+		}),
+	);
+}
+
 export function configureHttpDispatcher(timeoutMs: number = DEFAULT_HTTP_IDLE_TIMEOUT_MS): void {
 	const normalizedTimeoutMs = parseHttpIdleTimeoutMs(timeoutMs);
 	if (normalizedTimeoutMs === undefined) {
 		throw new Error(`Invalid HTTP idle timeout: ${String(timeoutMs)}`);
 	}
-	undici.setGlobalDispatcher(
+	const dispatcher = withUndiciErrorListener(
 		new undici.EnvHttpProxyAgent({
 			allowH2: false,
 			bodyTimeout: normalizedTimeoutMs,
 			headersTimeout: normalizedTimeoutMs,
+			clientFactory: createUndiciClient,
+			factory: createUndiciOriginDispatcher,
 		}),
 	);
+	undici.setGlobalDispatcher(dispatcher);
 	// Keep fetch and the dispatcher on the same undici implementation. Node 26.0's
 	// bundled fetch can otherwise consume compressed responses through npm undici's
 	// dispatcher without decompressing them, causing response.json() failures.