[vitest-pool-workers] Fix dynamic import() in entrypoint and DO handlers (cloudflare#13056)

Author: penalosa

.changeset/fix-dynamic-import-cross-do.md

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+---
+"@cloudflare/vitest-pool-workers": patch
+---
+
+fix: Support dynamic `import()` inside entrypoint and Durable Object handlers
+
+Previously, calling `exports.default.fetch()` or `SELF.fetch()` on a worker whose handler used a dynamic `import()` would hang and fail with "Cannot perform I/O on behalf of a different Durable Object". This happened because the module runner's transport — which communicates over a WebSocket owned by the runner Durable Object — was invoked from a different DO context.
+
+The fix patches the module runner's transport via the `onModuleRunner` hook so that all `invoke()` calls are routed through the runner DO's I/O context, regardless of where the `import()` originates.

fixtures/vitest-pool-workers-examples/dynamic-import/src/greeting.ts

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+export function greet(name: string): string {
+	return `Hello, ${name}!`;
+}

fixtures/vitest-pool-workers-examples/dynamic-import/src/index.ts

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+import { DurableObject } from "cloudflare:workers";
+
+// Durable Object that uses dynamic import() in fetch handler.
+// Regression test for https://github.com/cloudflare/workers-sdk/issues/5387
+export class GreeterDO extends DurableObject {
+	async fetch(request: Request): Promise<Response> {
+		const { greet } = await import("./greeting");
+		return new Response(greet("DO"));
+	}
+}
+
+export default {
+	async fetch(
+		request: Request,
+		_env: unknown,
+		_ctx: ExecutionContext
+	): Promise<Response> {
+		// Dynamic import inside a fetch handler — this is the pattern that
+		// triggers the cross-DO I/O violation in vitest-pool-workers 0.13.x
+		// when called via `exports.default.fetch()` in tests.
+		// See: https://github.com/cloudflare/workers-sdk/issues/12924
+		const { greet } = await import("./greeting");
+		return new Response(greet("World"));
+	},
+} satisfies ExportedHandler;

fixtures/vitest-pool-workers-examples/dynamic-import/src/tsconfig.json

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+{
+	"extends": "../../tsconfig.workerd.json",
+	"include": ["./**/*.ts"]
+}

fixtures/vitest-pool-workers-examples/dynamic-import/src/worker-configuration.d.ts

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+declare namespace Cloudflare {
+	interface GlobalProps {
+		mainModule: typeof import("./index");
+	}
+	interface Env {
+		GREETER: DurableObjectNamespace;
+	}
+}
+interface Env extends Cloudflare.Env {}

fixtures/vitest-pool-workers-examples/dynamic-import/test/dynamic-import.test.ts

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+import { env } from "cloudflare:workers";
+import { exports } from "cloudflare:workers";
+import { it } from "vitest";
+
+// Regression test for https://github.com/cloudflare/workers-sdk/issues/12924
+//
+// Calling exports.default.fetch() on a worker whose fetch handler uses a
+// dynamic import() would hang with "Cannot perform I/O on behalf of a
+// different Durable Object". Pre-loading the module (e.g. via a static
+// import of the worker) masks the bug by caching the module.
+it("exports.default.fetch() with dynamic import()", async ({ expect }) => {
+	const response = await exports.default.fetch(
+		new Request("https://example.com/")
+	);
+	expect(response.status).toBe(200);
+	expect(await response.text()).toBe("Hello, World!");
+});
+
+// Regression test for https://github.com/cloudflare/workers-sdk/issues/5387
+//
+// Dynamic import() inside a Durable Object fetch handler has the same
+// cross-context I/O violation when the module isn't already cached.
+it("Durable Object fetch with dynamic import()", async ({ expect }) => {
+	const id = env.GREETER.idFromName("test");
+	const stub = env.GREETER.get(id);
+	const response = await stub.fetch(new Request("https://example.com/"));
+	expect(response.status).toBe(200);
+	expect(await response.text()).toBe("Hello, DO!");
+});

fixtures/vitest-pool-workers-examples/dynamic-import/test/tsconfig.json

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+{
+	"extends": "../../tsconfig.workerd-test.json",
+	"include": ["./**/*.ts", "../src/**/*.ts"]
+}

fixtures/vitest-pool-workers-examples/dynamic-import/tsconfig.json

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+{
+	"extends": "../tsconfig.node.json",
+	"include": ["./*.ts"]
+}

fixtures/vitest-pool-workers-examples/dynamic-import/vitest.config.ts

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+import { cloudflareTest } from "@cloudflare/vitest-pool-workers";
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+	plugins: [
+		cloudflareTest({
+			wrangler: {
+				configPath: "./wrangler.jsonc",
+			},
+		}),
+	],
+
+	test: {},
+});

fixtures/vitest-pool-workers-examples/dynamic-import/wrangler.jsonc

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+{
+	"name": "dynamic-import",
+	"main": "src/index.ts",
+	// don't provide compatibility_date so that vitest will infer the latest one
+	"durable_objects": {
+		"bindings": [{ "name": "GREETER", "class_name": "GreeterDO" }],
+	},
+	"migrations": [{ "tag": "v1", "new_classes": ["GreeterDO"] }],
+}