Fix Miniflare#dispose() immediately after new Miniflare() (#705)

We previously waited for Miniflare to be ready before `dispose()`ing.
Unfortunately, we weren't waiting for the `workerd` config to finish
being written to stdin. Calling `dispose()` immediately after
`new Miniflare()` would stop waiting for socket ports to be reported,
and kill the `workerd` process while data was still being written.
This threw an unhandled `EPIPE` error.

This changes makes sure we don't report that Miniflare is ready until
after the config is fully-written.

Closes #680

packages/miniflare/src/index.ts

@@ -1180,7 +1180,7 @@ export class Miniflare {
 		}
 	}
 
-	async #waitForReady() {
+	async #waitForReady(disposing = false) {
 		// If `#init()` threw, we'd like to propagate the error here, so `await` it.
 		// Note we can't use `async`/`await` with getters. We'd also like to wait
 		// for `setOptions` calls to complete before resolving.
@@ -1191,6 +1191,10 @@ export class Miniflare {
 		// waiters on the mutex to avoid logging ready/updated messages to the
 		// console if there are future updates)
 		await this.#runtimeMutex.drained();
+		// If we called `dispose()`, we may not have a `#runtimeEntryURL` if we
+		// `dispose()`d synchronously, immediately after constructing a `Miniflare`
+		// instance. In this case, return a discard URL which we'll ignore.
+		if (disposing) return new URL("http://[100::]/");
 		// `#runtimeEntryURL` is assigned in `#assembleAndUpdateConfig()`, which is
 		// called by `#init()`, and `#initPromise` doesn't resolve until `#init()`
 		// returns.
@@ -1465,7 +1469,7 @@ export class Miniflare {
 	async dispose(): Promise<void> {
 		this.#disposeController.abort();
 		try {
-			await this.ready;
+			await this.#waitForReady(/* disposing */ true);
 		} finally {
 			// Remove exit hooks, we're cleaning up what they would've cleaned up now
 			this.#removeTmpPathExitHook();

packages/miniflare/src/runtime/index.ts

@@ -1,6 +1,6 @@
 import assert from "assert";
 import childProcess from "child_process";
-import type { Abortable } from "events";
+import { Abortable, once } from "events";
 import rl from "readline";
 import { Readable } from "stream";
 import { red } from "kleur/colors";
@@ -146,9 +146,10 @@ export class Runtime {
 		const controlPipe = runtimeProcess.stdio[3];
 		assert(controlPipe instanceof Readable);
 
-		// 3. Write config
+		// 3. Write config, and wait for writing to finish
 		runtimeProcess.stdin.write(configBuffer);
 		runtimeProcess.stdin.end();
+		await once(runtimeProcess.stdin, "finish");
 
 		// 4. Wait for sockets to start listening
 		return waitForPorts(controlPipe, options);

packages/miniflare/test/index.spec.ts

@@ -709,6 +709,12 @@ test("Miniflare: listens on ipv6", async (t) => {
 	t.true(response.ok);
 });
 
+test("Miniflare: dispose() immediately after construction", async (t) => {
+	const mf = new Miniflare({ script: "", modules: true });
+	await mf.dispose();
+	t.pass();
+});
+
 test("Miniflare: getBindings() returns all bindings", async (t) => {
 	const tmp = await useTmp(t);
 	const blobPath = path.join(tmp, "blob.txt");