reproduce error in test

.changeset/quick-shoes-jump.md

@@ -0,0 +1,5 @@
+---
+"@cloudflare/vitest-pool-workers": patch
+---
+
+fix mock modules not re-evaluated in watch mode

packages/vitest-pool-workers/src/worker/lib/cloudflare/test-runner.ts

@@ -230,28 +230,10 @@ export default class WorkersTestRunner extends VitestTestRunner {
 			await scheduler.wait(100);
 		}
 
-		// Reset the module graphs so module mock will be re-evalated on watch mode
-		// See https://github.com/cloudflare/workers-sdk/issues/6844
-		// This is basically `vi.resetModules()`
-		// From https://github.com/vitest-dev/vitest/blob/9584be337f00e22475895360cb5cfa5dc0775e79/packages/vitest/src/runtime/utils.ts#L50-L68
-		// const skipPaths = [
-		// 	// Vitest
-		// 	/\/vitest\/dist\//,
-		// 	/\/vite-node\/dist\//,
-		// 	// yarn's .store folder
-		// 	/vitest-virtual-\w+\/dist/,
-		// 	// cnpm
-		// 	/@vitest\/dist/,
-		// 	// don't clear mocks
-		// 	/^mock:/,
-		// ];
-		// this.state.moduleCache.forEach((mod, path) => {
-		// 	if (skipPaths.some((re) => re.test(path))) {
-		// 		return;
-		// 	}
-		// 	this.state.moduleCache.invalidateModule(mod);
-		// });
-		vi.resetModules();
+		// Unlike the official threads and forks pool, we do not recycle the miniflare instances to maintains the module cache.
+		// However, this creates a side effect where the module mock will not be re-evaluated on watch mode.
+		// This fixes https://github.com/cloudflare/workers-sdk/issues/6844 by resetting the module graph.
+		// vi.resetModules();
 
 		// Ensure all `ctx.waitUntil()` calls complete before disposing the runtime
 		// (if using `vitest run`) and aborting all objects. `ctx.waitUntil()`s may

packages/vitest-pool-workers/test/helpers.ts

@@ -45,6 +45,7 @@ export interface Process {
 	readonly stdout: string;
 	readonly stderr: string;
 	readonly exitCode: Promise<number>;
+	readonly write: (chunk: unknown) => void;
 }
 
 function wrap(proc: childProcess.ChildProcess): Process {
@@ -75,6 +76,9 @@ function wrap(proc: childProcess.ChildProcess): Process {
 		get exitCode() {
 			return closePromise.then(([exitCode]) => exitCode ?? -1);
 		},
+		write(chunk) {
+			proc.stdin?.write(chunk);
+		},
 	};
 }
 

packages/vitest-pool-workers/test/watch.test.ts

@@ -106,3 +106,71 @@ test("automatically re-runs integration tests", async ({
 		expect(result.stdout).toMatch("Tests  1 passed");
 	});
 });
+
+test("automatically reset module graph", async ({
+	expect,
+	seed,
+	vitestDev,
+}) => {
+	await seed({
+		"vitest.config.mts": dedent`
+			import { defineWorkersConfig } from "@cloudflare/vitest-pool-workers/config";
+			export default defineWorkersConfig({
+				test: {
+					poolOptions: {
+						workers: {
+							main: "./index.ts",
+							singleWorker: true,
+						},
+					},
+				}
+			});
+		`,
+		"index.ts": dedent`
+			import { getAnswer } from "./answer";
+
+			export default {
+				async fetch(request, env, ctx) {
+					const answer = getAnswer();
+					return new Response(answer);
+				}
+			}
+		`,
+		"answer.ts": dedent`
+			export function getAnswer() {
+				return "wrong";
+			}
+		`,
+		"index.test.ts": dedent`
+			import { env, createExecutionContext, waitOnExecutionContext } from "cloudflare:test";
+			import { it, expect, vi } from "vitest";
+			import worker from "./index";
+			import { getAnswer } from './answer';
+
+			vi.mock('./answer.ts');
+
+			it("mocks module properly in watch mode", async () => {
+				vi.mocked(getAnswer).mockReturnValue("Correct");
+
+				const request = new Request("https://example.com");
+				const ctx = createExecutionContext();
+				const response = await worker.fetch(request, env, ctx);
+				await waitOnExecutionContext(ctx);
+				expect(await response.text()).toBe("correct");
+			});
+		`,
+	});
+	const result = vitestDev();
+
+	await waitFor(() => {
+		expect(result.stdout).toMatch("Tests  1 passed");
+	});
+
+	const enterKey = "\x0d";
+	result.write(enterKey);
+
+	// result.stdout
+	await waitFor(() => {
+		expect(result.stdout).toMatch("Tests  1 passed");
+	});
+});