Propagate errors through val call hierarchy.

Fix bug when failure or exception happening in a >1 level of the val
coroutine call hierarchy is not being propagated. As a result only
deep most level coroutine promise is rejected, other val promises
including top most one remain in a perpetual pending state.

Author: davits

src/lib/libemval.js

@@ -467,33 +467,33 @@ var LibraryEmVal = {
     return result.done ? 0 : Emval.toHandle(result.value);
   },
 
-  _emval_coro_suspend__deps: ['$Emval', '_emval_coro_resume'],
-  _emval_coro_suspend: async (promiseHandle, awaiterPtr) => {
-    var result = await Emval.toValue(promiseHandle);
-    __emval_coro_resume(awaiterPtr, Emval.toHandle(result));
+  _emval_coro_suspend__deps: ['$Emval', '_emval_coro_resume',  '_emval_coro_reject'],
+  _emval_coro_suspend: (promiseHandle, awaiterPtr) => {
+    Emval.toValue(promiseHandle)
+      .then(result => __emval_coro_resume(awaiterPtr, Emval.toHandle(result)))
+      .catch(error => __emval_coro_reject(awaiterPtr, Emval.toHandle(error)));
   },
 
-  _emval_coro_make_promise__deps: ['$Emval', '__cxa_rethrow'],
+  _emval_coro_make_promise__deps: ['$Emval'],
   _emval_coro_make_promise: (resolveHandlePtr, rejectHandlePtr) => {
     return Emval.toHandle(new Promise((resolve, reject) => {
-      const rejectWithCurrentException = () => {
-        try {
-          // Use __cxa_rethrow which already has mechanism for generating
-          // user-friendly error message and stacktrace from C++ exception
-          // if EXCEPTION_STACK_TRACES is enabled and numeric exception
-          // with metadata optimised out otherwise.
-          ___cxa_rethrow();
-        } catch (e) {
-          // But catch it so that it rejects the promise instead of throwing
-          // in an unpredictable place during async execution.
-          reject(e);
-        }
-      };
-
       {{{ makeSetValue('resolveHandlePtr', '0', 'Emval.toHandle(resolve)', '*') }}};
-      {{{ makeSetValue('rejectHandlePtr', '0', 'Emval.toHandle(rejectWithCurrentException)', '*') }}};
+      {{{ makeSetValue('rejectHandlePtr', '0', 'Emval.toHandle(reject)', '*') }}};
     }));
   },
+
+  _emval_coro_exception_to_error__deps: ['$Emval', '__cxa_rethrow'],
+  _emval_coro_exception_to_error: () => {
+    try {
+      // Use __cxa_rethrow which already has mechanism for generating
+      // user-friendly error message and stacktrace from C++ exception
+      // if EXCEPTION_STACK_TRACES is enabled and numeric exception
+      // with metadata optimised out otherwise.
+      ___cxa_rethrow();
+    } catch (e) {
+      return Emval.toHandle(e);
+    }
+  },
 };
 
 addToLibrary(LibraryEmVal);

src/lib/libsigs.js

@@ -338,6 +338,7 @@ sigs = {
   _emval_await__sig: 'pp',
   _emval_call__sig: 'dpppp',
   _emval_call_method__sig: 'dppppp',
+  _emval_coro_exception_to_error__sig: 'p',
   _emval_coro_make_promise__sig: 'ppp',
   _emval_coro_suspend__sig: 'vpp',
   _emval_decref__sig: 'vp',

system/include/emscripten/val.h

@@ -24,7 +24,6 @@
 #include <variant>
 #endif
 
-
 namespace emscripten {
 
 class val;
@@ -118,6 +117,7 @@ EM_VAL _emval_iter_next(EM_VAL iterator);
 
 #if __cplusplus >= 202002L
 void _emval_coro_suspend(EM_VAL promise, void* coro_ptr);
+EM_VAL _emval_coro_exception_to_error();
 EM_VAL _emval_coro_make_promise(EM_VAL *resolve, EM_VAL *reject);
 #endif
 
@@ -712,11 +712,13 @@ class val::awaiter {
   //  - initially created with promise
   //  - waiting with a given coroutine handle
   //  - completed with a result
-  std::variant<val, std::coroutine_handle<val::promise_type>, val> state;
+  //  - rejected with an error
+  std::variant<val, std::coroutine_handle<val::promise_type>, val, 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)
@@ -743,9 +745,20 @@ 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();
+  }
+
   // `await_resume` finalizes the awaiter and should return the result
   // of the `co_await ...` expression - in our case, the stored value.
-  val await_resume() { return std::move(std::get<STATE_RESULT>(state)); }
+  val await_resume() {
+    if (state.index() == STATE_ERROR) {
+      throw std::get<STATE_ERROR>(state);
+    }
+    return std::move(std::get<STATE_RESULT>(state));
+  }
 };
 
 inline val::awaiter val::operator co_await() const {
@@ -756,7 +769,7 @@ inline val::awaiter val::operator co_await() const {
 // that compiler uses to drive the coroutine itself
 // (`T::promise_type` is used for any coroutine with declared return type `T`).
 class val::promise_type {
-  val promise, resolve, reject_with_current_exception;
+  val promise, resolve, reject;
 
 public:
   // Create a `new Promise` and store it alongside the `resolve` and `reject`
@@ -766,7 +779,7 @@ class val::promise_type {
     EM_VAL reject_handle;
     promise = val(internal::_emval_coro_make_promise(&resolve_handle, &reject_handle));
     resolve = val(resolve_handle);
-    reject_with_current_exception = val(reject_handle);
+    reject = val(reject_handle);
   }
 
   // Return the stored promise as the actual return value of the coroutine.
@@ -779,7 +792,14 @@ class val::promise_type {
   // On an unhandled exception, reject the stored promise instead of throwing
   // it asynchronously where it can't be handled.
   void unhandled_exception() {
-    reject_with_current_exception();
+    try {
+      std::rethrow_exception(std::current_exception());
+    } catch (const val& error) {
+      reject(error);
+    } catch (...) {
+      val error = val(internal::_emval_coro_exception_to_error());
+      reject(error);
+    }
   }
 
   // Resolve the stored promise on `co_return value`.

system/lib/embind/bind.cpp

@@ -67,6 +67,10 @@ void _emval_coro_resume(val::awaiter* awaiter, EM_VAL result) {
   awaiter->resume_with(val::take_ownership(result));
 }
 
+void _emval_coro_reject(val::awaiter* awaiter, EM_VAL error) {
+  awaiter->reject_with(val::take_ownership(error));
+}
+
 }
 
 namespace {

tools/emscripten.py

@@ -1093,6 +1093,7 @@ def create_pointer_conversion_wrappers(metadata):
     'emscripten_proxy_finish': '_p',
     'emscripten_proxy_execute_queue': '_p',
     '_emval_coro_resume': '_pp',
+    '_emval_coro_reject': '_pp',
     'emscripten_main_runtime_thread_id': 'p',
     '_emscripten_set_offscreencanvas_size_on_thread': '_pp__',
     'fileno': '_p',