[cli][dev] handle no response from edge functions (#8095)

When an edge function has no response during `vc dev`, we were seeing an unhelpful error message:

> The event listener did not respond.

Now, we'll see a much more specific error message:

> Unhandled rejection: Edge Function "api/edge-no-response.ts" did not return a response.
> Error! Failed to complete request to /api/edge-no-response: Error: socket hang up

packages/cli/test/dev/fixtures/edge-function-error/api/edge-no-response.js

@@ -0,0 +1,7 @@
+export const config = {
+  runtime: 'experimental-edge',
+};
+
+export default async function edge(request, event) {
+  // nothing returned
+}

packages/cli/test/dev/fixtures/middleware-no-response/api/hello.js

@@ -0,0 +1,3 @@
+export default function serverless(request, response) {
+  return response.send('hello from a serverless function');
+}

packages/cli/test/dev/fixtures/middleware-no-response/middleware.js

@@ -0,0 +1,7 @@
+export const config = {
+  runtime: 'experimental-edge',
+};
+
+export default async function edge(request, event) {
+  // no response
+}

packages/cli/test/dev/integration-1.test.ts

@@ -56,6 +56,31 @@ test(
   })
 );
 
+test('[vercel dev] throws an error when an edge function has no response', async () => {
+  const dir = fixture('edge-function-error');
+  const { dev, port, readyResolver } = await testFixture(dir);
+
+  try {
+    await readyResolver;
+
+    let res = await fetch(`http://localhost:${port}/api/edge-no-response`);
+    validateResponseHeaders(res);
+
+    const { stdout, stderr } = await dev.kill('SIGTERM');
+
+    expect(await res.status).toBe(500);
+    expect(await res.text()).toMatch('FUNCTION_INVOCATION_FAILED');
+    expect(stdout).toMatch(
+      /Unhandled rejection: Edge Function "api\/edge-no-response.js" did not return a response./g
+    );
+    expect(stderr).toMatch(
+      /Failed to complete request to \/api\/edge-no-response: Error: socket hang up/g
+    );
+  } finally {
+    await dev.kill('SIGTERM');
+  }
+});
+
 test('[vercel dev] should support edge functions returning intentional 500 responses', async () => {
   const dir = fixture('edge-function');
   const { dev, port, readyResolver } = await testFixture(dir);

packages/cli/test/dev/integration-4.test.ts

@@ -442,6 +442,17 @@ test(
   })
 );
 
+test(
+  '[vercel dev] Middleware that has no response',
+  testFixtureStdio('middleware-no-response', async (testPath: any) => {
+    await testPath(
+      500,
+      '/api/hello',
+      'A server error has occurred\n\nEDGE_FUNCTION_INVOCATION_FAILED'
+    );
+  })
+);
+
 test(
   '[vercel dev] Middleware that does basic rewrite',
   testFixtureStdio('middleware-rewrite', async (testPath: any) => {

packages/node/src/dev-server.ts

@@ -148,21 +148,26 @@ async function serializeRequest(message: IncomingMessage) {
   });
 }
 
-async function compileUserCode(entrypoint: string) {
+async function compileUserCode(
+  entrypointPath: string,
+  entrypointLabel: string
+) {
   try {
     const result = await esbuild.build({
       platform: 'node',
       target: 'node14',
       sourcemap: 'inline',
       bundle: true,
-      entryPoints: [entrypoint],
+      entryPoints: [entrypointPath],
       write: false, // operate in memory
       format: 'cjs',
     });
 
     const compiledFile = result.outputFiles?.[0];
     if (!compiledFile) {
-      throw new Error(`Compilation of ${entrypoint} produced no output files.`);
+      throw new Error(
+        `Compilation of ${entrypointLabel} produced no output files.`
+      );
     }
 
     const userCode = new TextDecoder().decode(compiledFile.contents);
@@ -198,6 +203,10 @@ async function compileUserCode(entrypoint: string) {
 
           let response = await edgeHandler(event.request, event);
 
+          if (!response) {
+            throw new Error('Edge Function "${entrypointLabel}" did not return a response.');
+          }
+
           return event.respondWith(response);
         } catch (error) {
           // we can't easily show a meaningful stack trace
@@ -252,9 +261,10 @@ async function createEdgeRuntime(userCode: string | undefined) {
 }
 
 async function createEdgeEventHandler(
-  entrypoint: string
+  entrypointPath: string,
+  entrypointLabel: string
 ): Promise<(request: IncomingMessage) => Promise<VercelProxyResponse>> {
-  const userCode = await compileUserCode(entrypoint);
+  const userCode = await compileUserCode(entrypointPath, entrypointLabel);
   const server = await createEdgeRuntime(userCode);
 
   return async function (request: IncomingMessage) {
@@ -317,17 +327,17 @@ async function createEventHandler(
   config: Config,
   options: { shouldAddHelpers: boolean }
 ): Promise<(request: IncomingMessage) => Promise<VercelProxyResponse>> {
-  const entryPointPath = join(process.cwd(), entrypoint!);
-  const runtime = parseRuntime(entrypoint, entryPointPath);
+  const entrypointPath = join(process.cwd(), entrypoint!);
+  const runtime = parseRuntime(entrypoint, entrypointPath);
 
   // `middleware.js`/`middleware.ts` file is always run as
   // an Edge Function, otherwise needs to be opted-in via
   // `export const config = { runtime: 'experimental-edge' }`
   if (config.middleware === true || runtime === 'experimental-edge') {
-    return createEdgeEventHandler(entryPointPath);
+    return createEdgeEventHandler(entrypointPath, entrypoint);
   }
 
-  return createServerlessEventHandler(entryPointPath, options);
+  return createServerlessEventHandler(entrypointPath, options);
 }
 
 let handleEvent: (request: IncomingMessage) => Promise<VercelProxyResponse>;