Fix potential EBADF error when restarting workerd process in Miniflare

.changeset/fix-runtime-ebadf-on-restart.md

@@ -0,0 +1,9 @@
+---
+"miniflare": patch
+---
+
+Fix potential EBADF error when restarting workerd process
+
+Previously, when the workerd process was restarted (e.g., via `setOptions()` or Vite server restart), the stdio pipes from the previous process were not explicitly destroyed. This could lead to `EBADF` (Bad File Descriptor) errors during spawn on some systems.
+
+The `Runtime#dispose()` method now explicitly destroys all stdio streams (stdin, stdout, stderr, and the control pipe) before killing the process to ensure file descriptors are properly released.

packages/miniflare/src/runtime/index.ts

@@ -315,13 +315,35 @@ export class Runtime {
 	}
 
 	dispose(): Awaitable<void> {
+		const runtimeProcess = this.#process;
+		if (runtimeProcess === undefined) {
+			return;
+		}
+
+		// Clear reference to prevent potential race conditions
+		this.#process = undefined;
+
+		// Explicitly destroy all stdio streams to ensure file descriptors are
+		// properly released. This prevents EBADF errors when spawning a new
+		// process after restart.
+		// See https://github.com/cloudflare/workers-sdk/issues/11675
+		runtimeProcess.stdin?.destroy();
+		runtimeProcess.stdout?.destroy();
+		runtimeProcess.stderr?.destroy();
+		// The control pipe at stdio[3] is a Readable stream
+		const controlPipe = runtimeProcess.stdio[3];
+		if (controlPipe instanceof Readable) {
+			controlPipe.destroy();
+		}
+
 		// `kill()` uses `SIGTERM` by default. In `workerd`, this waits for HTTP
 		// connections to close before exiting. Notably, Chrome sometimes keeps
 		// connections open for about 10s, blocking exit. We'd like `dispose()`/
 		// `setOptions()` to immediately terminate the existing process.
 		// Therefore, use `SIGKILL` which force closes all connections.
 		// See https://github.com/cloudflare/workerd/pull/244.
-		this.#process?.kill("SIGKILL");
+		runtimeProcess.kill("SIGKILL");
+
 		return this.#processExitPromise;
 	}
 }

packages/miniflare/test/index.spec.ts

@@ -3401,6 +3401,46 @@ test("Miniflare: custom Node outbound service", async () => {
 	);
 });
 
+test("Miniflare: setOptions: can restart workerd multiple times in succession", async () => {
+	// Regression test for https://github.com/cloudflare/workers-sdk/issues/11675
+	// EBADF errors can occur when spawning a new workerd process if the stdio
+	// pipes from the previous process are not properly cleaned up. This is
+	// especially relevant when other parts of the system (like worker_threads
+	// used by Miniflare or other Vite plugins) are also managing file descriptors.
+	// While this test doesn't reliably reproduce the race condition, it validates
+	// that the restart mechanism works correctly after the fix.
+	const mf = new Miniflare({
+		port: 0,
+		modules: true,
+		script: `export default {
+			fetch() {
+				return new Response("version 1");
+			}
+		}`,
+	});
+	useDispose(mf);
+
+	// First request to ensure initial startup is complete
+	let res = await mf.dispatchFetch("http://localhost");
+	expect(await res.text()).toBe("version 1");
+
+	// Perform multiple rapid setOptions calls to trigger workerd restarts.
+	// This tests that the stdio pipe cleanup in dispose() works correctly.
+	for (let i = 2; i <= 5; i++) {
+		await mf.setOptions({
+			port: 0,
+			modules: true,
+			script: `export default {
+				fetch() {
+					return new Response("version ${i}");
+				}
+			}`,
+		});
+		res = await mf.dispatchFetch("http://localhost");
+		expect(await res.text()).toBe(`version ${i}`);
+	}
+});
+
 test("Miniflare: MINIFLARE_WORKERD_CONFIG_DEBUG controls workerd config file creation", async () => {
 	const originalEnv = process.env.MINIFLARE_WORKERD_CONFIG_DEBUG;
 	const configFilePath = "workerd-config.json";