Propagate errors through val coroutine hierarchy. (#23653)

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.

The bug can be reproduced by the following example:
``` cpp
emscripten::val fetch(const std::string& url) {
  co_return co_await emscripten::val::global("fetch")(url);
}
emscripten::val json(const std::string& url) {
  auto response = co_await fetch(url);
  co_return co_await response.call<emscripten::val>("json");
}
EMSCRIPTEN_BINDINGS(module) {
  function("json", &json);
}
```
```js
let p = Module.json('invalid.url'); // JS error is printed, but p remains pending
try {
  let p = await Module.json('invalid.url'); // JS error is printed
} catch(e) {
  console.log('Caught coro error!'); // this line is never printed
}
```

Author: davits

src/lib/libemval.js

@@ -437,33 +437,33 @@ 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)),
+            (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_from_current_cxa_exception__deps: ['$Emval', '__cxa_rethrow'],
+  _emval_from_current_cxa_exception: () => {
+    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

@@ -345,6 +345,7 @@ sigs = {
   _emval_decref__sig: 'vp',
   _emval_delete__sig: 'ipp',
   _emval_equals__sig: 'ipp',
+  _emval_from_current_cxa_exception__sig: 'p',
   _emval_get_global__sig: 'pp',
   _emval_get_method_caller__sig: 'pipi',
   _emval_get_module_property__sig: 'pp',

system/include/emscripten/val.h

@@ -21,10 +21,10 @@
 #include <pthread.h>
 #if __cplusplus >= 202002L
 #include <coroutine>
+#include <exception>
 #include <variant>
 #endif
 
-
 namespace emscripten {
 
 class val;
@@ -118,6 +118,7 @@ EM_VAL _emval_iter_next(EM_VAL iterator);
 
 #if __cplusplus >= 202002L
 void _emval_coro_suspend(EM_VAL promise, void* coro_ptr);
+EM_VAL _emval_from_current_cxa_exception();
 EM_VAL _emval_coro_make_promise(EM_VAL *resolve, EM_VAL *reject);
 #endif
 
@@ -729,7 +730,8 @@ 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);
@@ -743,9 +745,16 @@ class val::awaiter {
     coro.resume();
   }
 
+  // When JS invokes `reject_with` with some error value, reject currently suspended
+  // coroutine's promise with the error value and destroy coroutine frame, because
+  // in this scenario 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() { return std::move(std::get<STATE_RESULT>(state)); }
+  val await_resume() {
+    return std::move(std::get<STATE_RESULT>(state));
+  }
 };
 
 inline val::awaiter val::operator co_await() const {
@@ -756,7 +765,7 @@ 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 +775,7 @@ 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 +788,19 @@ 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_from_current_cxa_exception());
+      reject(error);
+    }
+  }
+
+  // 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`.
@@ -788,6 +809,14 @@ class val::promise_type {
     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

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 {

test/embind/test_val_coro.cpp

@@ -16,10 +16,24 @@ EM_JS(EM_VAL, promise_sleep_impl, (int ms, int result), {
   return handle;
 });
 
+EM_JS(EM_VAL, promise_fail_impl, (), {
+  let promise = new Promise((_, reject) => setTimeout(reject, 1, new Error("bang from JS promise!")));
+  let handle = Emval.toHandle(promise);
+  // FIXME. See https://github.com/emscripten-core/emscripten/issues/16975.
+#if __wasm64__
+  handle = BigInt(handle);
+#endif
+  return handle;
+});
+
 val promise_sleep(int ms, int result = 0) {
   return val::take_ownership(promise_sleep_impl(ms, result));
 }
 
+val promise_fail() {
+  return val::take_ownership(promise_fail_impl());
+}
+
 // Test that we can subclass and make custom awaitable types.
 template <typename T>
 class typed_promise: public val {
@@ -37,7 +51,13 @@ class typed_promise: public val {
   }
 };
 
+template <size_t N>
 val asyncCoro() {
+  co_return co_await asyncCoro<N - 1>();
+}
+
+template <>
+val asyncCoro<0>() {
   // check that just sleeping works
   co_await promise_sleep(1);
   // check that sleeping and receiving value works
@@ -50,12 +70,32 @@ val asyncCoro() {
   co_return 34;
 }
 
+template <size_t N>
 val throwingCoro() {
+  co_await throwingCoro<N - 1>();
+  co_return 56;
+}
+
+template <>
+val throwingCoro<0>() {
   throw std::runtime_error("bang from throwingCoro!");
   co_return 56;
 }
 
+template <size_t N>
+val failingPromise() {
+  co_await failingPromise<N - 1>();
+  co_return 65;
+}
+
+template <>
+val failingPromise<0>() {
+  co_await promise_fail();
+  co_return 65;
+}
+
 EMSCRIPTEN_BINDINGS(test_val_coro) {
-  function("asyncCoro", asyncCoro);
-  function("throwingCoro", throwingCoro);
+  function("asyncCoro", asyncCoro<3>);
+  function("throwingCoro", throwingCoro<3>);
+  function("failingPromise", failingPromise<3>);
 }

test/test_core.py

@@ -7585,7 +7585,7 @@ def test_embind_val_coro(self):
     self.emcc_args += ['-std=c++20', '--bind', '--pre-js=pre.js', '-sINCOMING_MODULE_JS_API=[onRuntimeInitialized]', '--no-entry']
     self.do_runf('embind/test_val_coro.cpp', '34\n')
 
-  def test_embind_val_coro_caught(self):
+  def test_embind_val_coro_propogate_cpp_exception(self):
     self.set_setting('EXCEPTION_STACK_TRACES')
     create_file('pre.js', r'''Module.onRuntimeInitialized = () => {
       Module.throwingCoro().then(
@@ -7596,6 +7596,17 @@ def test_embind_val_coro_caught(self):
     self.emcc_args += ['-std=c++20', '--bind', '--pre-js=pre.js', '-fexceptions', '-sINCOMING_MODULE_JS_API=[onRuntimeInitialized]', '--no-entry']
     self.do_runf('embind/test_val_coro.cpp', 'rejected with: std::runtime_error: bang from throwingCoro!\n')
 
+  def test_embind_val_coro_propogate_js_error(self):
+    self.set_setting('EXCEPTION_STACK_TRACES')
+    create_file('post.js', r'''Module.onRuntimeInitialized = () => {
+      Module.failingPromise().then(
+        console.log,
+        err => console.error(`rejected with: ${err.message}`)
+      );
+    }''')
+    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_dynamic_initialization(self):
     self.emcc_args += ['-lembind']
     self.do_run_in_out_file_test('embind/test_dynamic_initialization.cpp')

tools/emscripten.py

@@ -1164,6 +1164,7 @@ 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',

tools/system_libs.py

@@ -1932,7 +1932,7 @@ class libwebgpu_cpp(MTLibrary):
   src_files = ['webgpu_cpp.cpp']
 
 
-class libembind(Library):
+class libembind(MTLibrary):
   name = 'libembind'
   never_force = True