Propagate via rejections

- Propagate errors via promise rejections rather then exceptions to
  ensure consistent behaviour between normal and coroutine functions.

Author: davits

system/include/emscripten/val.h

@@ -714,12 +714,11 @@ class val::awaiter {
   //  - waiting with a given coroutine handle
   //  - completed with a result
   //  - rejected with an error
-  std::variant<val, std::coroutine_handle<val::promise_type>, val, val> state;
+  std::variant<val, std::coroutine_handle<val::promise_type>, val> state;
 
   constexpr static std::size_t STATE_PROMISE = 0;
   constexpr static std::size_t STATE_CORO = 1;
   constexpr static std::size_t STATE_RESULT = 2;
-  constexpr static std::size_t STATE_ERROR = 3;
 
 public:
   awaiter(const val& promise)
@@ -732,7 +731,8 @@ class val::awaiter {
   bool await_ready() { return false; }
 
   // On suspend, store the coroutine handle and invoke a helper that will do
-  // a rough equivalent of `promise.then(value => this.resume_with(value))`.
+  // a rough equivalent of
+  // `promise.then(value => this.resume_with(value)).catch(error => this.reject_with(error))`.
   void await_suspend(std::coroutine_handle<val::promise_type> handle) {
     internal::_emval_coro_suspend(std::get<STATE_PROMISE>(state).as_handle(), this);
     state.emplace<STATE_CORO>(handle);
@@ -746,18 +746,14 @@ class val::awaiter {
     coro.resume();
   }
 
-  void reject_with(val&& error) {
-    auto coro = std::move(std::get<STATE_CORO>(state));
-    state.emplace<STATE_ERROR>(std::move(error));
-    coro.resume();
-  }
+  // When JS invokes `reject_with` with some error value, reject currently suspended
+  // coroutine's promise with that error value and destroy coroutine frame, because
+  // in this case coroutine never reaches final_suspend point to be destroyed automatically.
+  void reject_with(val&& error);
 
   // `await_resume` finalizes the awaiter and should return the result
   // of the `co_await ...` expression - in our case, the stored value.
   val await_resume() {
-    if (state.index() == STATE_ERROR) {
-      throw std::get<STATE_ERROR>(state);
-    }
     return std::move(std::get<STATE_RESULT>(state));
   }
 };
@@ -803,12 +799,25 @@ class val::promise_type {
     }
   }
 
+  // Reject the stored promise due to rejection deeper in the call chain
+  void reject_with(val&& error) {
+    reject(std::move(error));
+  }
+
   // Resolve the stored promise on `co_return value`.
   template<typename T>
   void return_value(T&& value) {
     resolve(std::forward<T>(value));
   }
 };
+
+inline void val::awaiter::reject_with(val&& error) {
+  auto coro = std::move(std::get<STATE_CORO>(state));
+  auto& promise = coro.promise();
+  promise.reject_with(std::move(error));
+  coro.destroy();
+}
+
 #endif
 
 // Declare a custom type that can be used in conjunction with

test/embind/test_val_coro.cpp

@@ -94,24 +94,8 @@ val failingPromise<0>() {
   co_return 65;
 }
 
-val caughtException() {
-  int result = 0;
-  try {
-    co_return co_await throwingCoro<2>();
-  } catch (const val&) { // runtime_error turns into emscripten::val
-    result += 21;
-  }
-  try {
-    co_return co_await failingPromise<2>();
-  } catch (const val&) {
-    result += 21;
-  }
-  co_return result;
-}
-
 EMSCRIPTEN_BINDINGS(test_val_coro) {
   function("asyncCoro", asyncCoro<3>);
   function("throwingCoro", throwingCoro<3>);
   function("failingPromise", failingPromise<3>);
-  function("caughtException", caughtException);
 }

test/test_core.py

@@ -7509,14 +7509,6 @@ def test_embind_val_coro_propogate_js_error(self):
     self.emcc_args += ['-std=c++20', '--bind', '--post-js=post.js', '-fexceptions']
     self.do_runf('embind/test_val_coro.cpp', 'rejected with: bang from JS promise!\n')
 
-  def test_embind_val_coro_caught_exception(self):
-    self.set_setting('EXCEPTION_STACK_TRACES')
-    create_file('post.js', r'''Module.onRuntimeInitialized = () => {
-      Module.caughtException().then(console.log);
-    }''')
-    self.emcc_args += ['-std=c++20', '--bind', '--post-js=post.js', '-fexceptions']
-    self.do_runf('embind/test_val_coro.cpp', '42\n')
-
   def test_embind_dynamic_initialization(self):
     self.emcc_args += ['-lembind']
     self.do_run_in_out_file_test('embind/test_dynamic_initialization.cpp')