feat(dx): add default message for missing handler (#264)

This commit adds a default message for the HTTP handler to the 500
that is returned when the event loop has run but the request is still
not being responded to via the fetch-event integration.

In these cases, the user would normally receive a 500 with no
information on what went wrong (e.g. if run with `wasmtime serve`), so
with this PR we can do a little more to point towards the issue.

Author: vados-cosmonic

builtins/web/fetch/fetch_event.cpp

@@ -15,6 +15,7 @@
 
 #include <iostream>
 #include <memory>
+#include <optional>
 
 using builtins::web::event::Event;
 using builtins::web::event::EventTarget;
@@ -32,6 +33,8 @@ JSString *fetch_type_atom;
 JS::PersistentRootedObject INSTANCE;
 host_api::HttpOutgoingBody *STREAMING_BODY;
 
+constexpr const std::string_view DEFAULT_NO_HANDLER_ERROR_MSG = "ERROR: no fetch-event handler triggered, was one registered?";
+
 void inc_pending_promise_count(JSObject *self) {
   MOZ_ASSERT(FetchEvent::is_instance(self));
   auto count = JS::GetReservedSlot(self, FetchEvent::Slots::PendingPromiseCount).toInt32();
@@ -322,10 +325,18 @@ bool FetchEvent::respondWith(JSContext *cx, unsigned argc, JS::Value *vp) {
   return true;
 }
 
-bool FetchEvent::respondWithError(JSContext *cx, JS::HandleObject self) {
+  bool FetchEvent::respondWithError(JSContext *cx, JS::HandleObject self, std::optional<std::string_view> body_text) {
   MOZ_RELEASE_ASSERT(state(self) == State::unhandled || state(self) == State::waitToRespond);
 
   auto headers = std::make_unique<host_api::HttpHeaders>();
+  if (body_text) {
+    auto header_set_res = headers->set("content-type", "text/plain");
+    if (auto *err = header_set_res.to_err()) {
+      HANDLE_ERROR(cx, *err);
+      return false;
+    }
+  }
+
   auto *response = host_api::HttpOutgoingResponse::make(500, std::move(headers));
 
   auto body_res = response->body();
@@ -334,6 +345,11 @@ bool FetchEvent::respondWithError(JSContext *cx, JS::HandleObject self) {
     return false;
   }
 
+  if (body_text) {
+    auto body = std::move(body_res.unwrap());
+    body->write(reinterpret_cast<const uint8_t*>(body_text->data()), body_text->length());
+  }
+
   return send_response(response, self, FetchEvent::State::respondedWithError);
 }
 
@@ -504,7 +520,9 @@ bool handle_incoming_request(host_api::HttpIncomingRequest *request) {
   }
 
   if (!FetchEvent::response_started(fetch_event)) {
-    FetchEvent::respondWithError(ENGINE->cx(), fetch_event);
+    // If at this point no fetch event handler has run, we can
+    // send a specific error indicating that there is likely no handler registered
+    FetchEvent::respondWithError(ENGINE->cx(), fetch_event, DEFAULT_NO_HANDLER_ERROR_MSG);
     return true;
   }
 

builtins/web/fetch/fetch_event.h

@@ -7,6 +7,8 @@
 
 #include "../event/event.h"
 
+#include <optional>
+
 namespace builtins::web::fetch::fetch_event {
 
 class FetchEvent final : public BuiltinNoConstructor<FetchEvent> {
@@ -57,7 +59,19 @@ class FetchEvent final : public BuiltinNoConstructor<FetchEvent> {
   static bool init_incoming_request(JSContext *cx, JS::HandleObject self,
                                     host_api::HttpIncomingRequest *req);
 
-  static bool respondWithError(JSContext *cx, JS::HandleObject self);
+  /**
+   * @brief Responds with an error that contains some text for the HTTP response body
+   *
+   * @param cx The Javascript context
+   * @param self A handle to the `FetchEvent` object
+   * @param body_text optional text to send as the body
+   *
+   * @return True if the response was sent successfully
+   * @throws None directly, but surfaces errors to JS via `HANDLE_ERROR`
+   */
+  static bool respondWithError(JSContext *cx, 
+                               JS::HandleObject self, 
+                               std::optional<std::string_view> body_text = std::nullopt);
   static bool is_active(JSObject *self);
 
   static State state(JSObject *self);