Python workers: Better error handling when the handler is not defined (#3495)

Author: hoodmane

src/pyodide/python-entrypoint-helper.ts

@@ -131,9 +131,14 @@ function makeHandler(pyHandlerName: string): Handler {
         'prep_python',
         async () => await preparePython()
       );
-
+      const handler = mainModule[pyHandlerName];
+      if (!handler) {
+        throw new Error(
+          `Python entrypoint "${MAIN_MODULE_NAME}" does not export a handler named "${pyHandlerName}"`
+        );
+      }
       const result = await enterJaegerSpan('python_code', () => {
-        return mainModule[pyHandlerName].callRelaxed(...args);
+        return handler.callRelaxed(...args);
       });
 
       // Support returning a pyodide.ffi.FetchResponse.

src/workerd/server/tests/python/BUILD.bazel

@@ -57,3 +57,12 @@ gen_import_tests(
         ],
     },
 )
+
+py_wd_test(
+    "undefined-handler",
+    # TODO: Requires a new bundle deploy
+    skip_python_flags = [
+        "0.26.0a2",
+        "0.27.1",
+    ],
+)

src/workerd/server/tests/python/undefined-handler/main.py

@@ -0,0 +1,2 @@
+def test():
+    pass

src/workerd/server/tests/python/undefined-handler/server.mjs

@@ -0,0 +1,11 @@
+import { rejects } from 'node:assert';
+
+export default {
+  async test(ctrl, env) {
+    await rejects(env.pythonWorker.fetch(new Request('http://example.com')), {
+      name: 'Error',
+      message:
+        'Python entrypoint "undefined_handler.py" does not export a handler named "on_fetch"',
+    });
+  },
+};

src/workerd/server/tests/python/undefined-handler/undefined-handler.wd-test

@@ -0,0 +1,37 @@
+using Workerd = import "/workerd/workerd.capnp";
+
+const undefinedHandler :Workerd.Worker = (
+  modules = [
+    (name = "undefined_handler.py", pythonModule = embed "undefined_handler.py")
+  ],
+  compatibilityDate = "2024-10-01",
+  compatibilityFlags = [%PYTHON_FEATURE_FLAGS],
+);
+
+const server :Workerd.Worker = (
+  modules = [
+    (name = "server.js", esModule = embed "server.mjs")
+  ],
+  bindings = [
+    ( name = "pythonWorker", service = "undefinedHandler" ),
+  ],
+  compatibilityDate = "2024-10-01",
+  compatibilityFlags = ["nodejs_compat"],
+);
+
+
+const unitTests :Workerd.Config = (
+  services = [
+    ( name = "external",
+      network = (
+        tlsOptions = (trustBrowserCas = true)
+      )
+    ),
+    ( name = "server",
+      worker = .server
+    ),
+    ( name = "undefinedHandler",
+      worker = .undefinedHandler
+    ),
+  ],
+);

src/workerd/server/tests/python/undefined-handler/undefined_handler.py

@@ -0,0 +1,2 @@
+def test():
+    pass