feat: single-tick non-tracking event loop runner

builtins/web/fetch/request-response.cpp

@@ -996,10 +996,6 @@ bool reader_for_outgoing_body_catch_handler(JSContext *cx, JS::HandleObject body
   // `responseDone`. (Note that even though we encountered an error,
   // `responseDone` is the right state: `respondedWithError` is for when sending
   // a response at all failed.)
-  // TODO(TS): investigate why this is disabled.
-  // if (Response::is_instance(body_owner)) {
-  //   FetchEvent::set_state(FetchEvent::instance(), FetchEvent::State::responseDone);
-  // }
   return finish_outgoing_body_streaming(cx, body_owner);
 }
 

host-apis/wasi-0.2.0-rc-2023-10-18/host_api.cpp

@@ -112,6 +112,23 @@ size_t api::AsyncTask::select(std::vector<api::AsyncTask *> &tasks) {
   return ready_index;
 }
 
+std::optional<size_t> api::AsyncTask::ready(std::vector<api::AsyncTask *> &tasks) {
+  auto count = tasks.size();
+  vector<Borrow<Pollable>> handles;
+  for (const auto task : tasks) {
+    handles.emplace_back(task->id());
+  }
+  auto list = list_borrow_pollable_t{
+      reinterpret_cast<HandleOps<Pollable>::borrow *>(handles.data()), count};
+  bindings_list_u32_t result{nullptr, 0};
+  wasi_io_0_2_0_rc_2023_10_18_poll_poll_list(&list, &result);
+  MOZ_ASSERT(result.len > 0);
+  const auto ready_index = result.ptr[0];
+  free(result.ptr);
+
+  return ready_index;
+}
+
 namespace host_api {
 
 HostString::HostString(const char *c_str) {

host-apis/wasi-0.2.0-rc-2023-12-05/host_api.cpp

@@ -116,6 +116,23 @@ size_t api::AsyncTask::select(std::vector<api::AsyncTask *> &tasks) {
   return ready_index;
 }
 
+std::optional<size_t> api::AsyncTask::ready(std::vector<api::AsyncTask *> &tasks) {
+  auto count = tasks.size();
+  vector<Borrow<Pollable>> handles;
+  for (const auto task : tasks) {
+    handles.emplace_back(task->id());
+  }
+  auto list = list_borrow_pollable_t{
+      reinterpret_cast<HandleOps<Pollable>::borrow *>(handles.data()), count};
+  bindings_list_u32_t result{nullptr, 0};
+  wasi_io_0_2_0_rc_2023_11_10_poll_poll(&list, &result);
+  MOZ_ASSERT(result.len > 0);
+  const auto ready_index = result.ptr[0];
+  free(result.ptr);
+
+  return ready_index;
+}
+
 namespace host_api {
 
 HostString::HostString(const char *c_str) {

host-apis/wasi-0.2.0/host_api.cpp

@@ -263,6 +263,18 @@ size_t api::AsyncTask::select(std::vector<AsyncTask *> &tasks) {
   return ready_index;
 }
 
+std::optional<size_t> api::AsyncTask::ready(std::vector<api::AsyncTask *> &tasks) {
+  auto count = tasks.size();
+  for (size_t idx = 0; idx < count; ++idx) {
+    auto task = tasks.at(idx);
+    WASIHandle<host_api::Pollable>::Borrowed poll = { task->id() };
+    if (wasi_io_0_2_0_poll_method_pollable_ready(poll)) {
+      return idx;
+    }
+  }
+  return std::nullopt;
+}
+
 namespace host_api {
 
 HostString::HostString(const char *c_str) {

include/extension-api.h

@@ -148,6 +148,11 @@ class AsyncTask {
    * Select for the next available ready task, providing the oldest ready first.
    */
   static size_t select(std::vector<AsyncTask *> &handles);
+
+  /**
+   * Non-blocking check for a ready task, providing the oldest ready first, if any.
+   */
+  static std::optional<size_t> ready(std::vector<AsyncTask *> &handles);
 };
 
 } // namespace api

runtime/event_loop.cpp

@@ -62,41 +62,54 @@ bool EventLoop::run_event_loop(api::Engine *engine, double total_compute) {
   queue.get().event_loop_running = true;
   JSContext *cx = engine->cx();
 
-  while (true) {
+  do {
     // Run a microtask checkpoint
     js::RunJobs(cx);
 
     if (JS_IsExceptionPending(cx)) {
       exit_event_loop();
       return false;
     }
-    // if there is no interest in the event loop at all, just run one tick
-    if (interest_complete()) {
-      exit_event_loop();
-      return true;
-    }
 
     const auto tasks = &queue.get().tasks;
     size_t tasks_size = tasks->size();
+
     if (tasks_size == 0) {
+      if (interest_complete()) {
+        break;
+      }
       exit_event_loop();
-      MOZ_ASSERT(!interest_complete());
       fprintf(stderr, "event loop error - both task and job queues are empty, but expected "
                       "operations did not resolve");
       return false;
     }
 
+    size_t task_idx;
+
     // Select the next task to run according to event-loop semantics of oldest-first.
-    size_t task_idx = api::AsyncTask::select(*tasks);
+    if (interest_complete()) {
+      // Perform a non-blocking select in the case of there being no event loop interest
+      // (we are thus only performing a "single tick", but must still progress work that is ready)
+      std::optional<size_t> maybe_task_idx = api::AsyncTask::ready(*tasks);
+      if (!maybe_task_idx.has_value()) {
+        break;
+      }
+      task_idx = maybe_task_idx.value();
+    } else {
+      task_idx = api::AsyncTask::select(*tasks);
+    }
 
     auto task = tasks->at(task_idx);
-    tasks->erase(tasks->begin() + task_idx);
     bool success = task->run(engine);
+    tasks->erase(tasks->begin() + task_idx);
     if (!success) {
       exit_event_loop();
       return false;
     }
-  }
+  } while (!interest_complete());
+
+  exit_event_loop();
+  return true;
 }
 
 void EventLoop::init(JSContext *cx) { queue.init(cx); }