[embind] Update exceptionLast in _emval_throw (#26276)

aheejin · web-flow · commit 8551146567cc · 2026-02-17T18:32:52.000-08:00
When `_emval_throw` throws an instance of C++ exception in Emscripten EH mode, it needs to update `exceptionLast` accordingly, as we do in `__cxa_throw` and `__cxa_rethrow` in `src/lib/libexceptions.js`: https://github.com/emscripten-core/emscripten/blob/2d257177915c49c807a3e03d6f404173bb4e5e72/src/lib/libexceptions.js#L100 Here we can't use `storeException` because we are not creating a C++ exception from scratch; we are throwing an already created C++ exception. Fixes #26213.
diff --git a/src/lib/libemval.js b/src/lib/libemval.js
@@ -392,9 +392,37 @@ ${functionBody}
     return delete object[property];
   },
 
-  _emval_throw__deps: ['$Emval'],
+  _emval_throw__deps: ['$Emval',
+#if !WASM_EXCEPTIONS
+    '$exceptionLast',
+#endif
+  ],
   _emval_throw: (object) => {
     object = Emval.toValue(object);
+#if !WASM_EXCEPTIONS
+    // If we are throwing Emcripten C++ exception, set exceptionLast, as we do
+    // in __cxa_throw. When EXCEPTION_STACK_TRACES is set, a C++ exception will
+    // be an instance of EmscriptenEH, and when EXCEPTION_STACK_TRACES is not
+    // set, it will be a pointer (number).
+    //
+    // This is different from __cxa_throw() in libexception.js because
+    // __cxa_throw() is called from the user C++ code when the 'throw' keyword
+    // is used, and the value thrown is a C++ pointer. When
+    // EXCEPTION_STACK_TRACES is true, we wrap it with CppException. But this
+    // _emval_throw is called when we throw whatever is contained in 'object',
+    // which can be anything including a CppException object, or a number, or
+    // other JS object. So we don't use storeException() wrapper here and we
+    // throw it as is.
+#if EXCEPTION_STACK_TRACES
+    if (object instanceof CppException) {
+      exceptionLast = object;
+    }
+#else
+    if (object === object+0) { // Check if it is a number
+      exceptionLast = object;
+    }
+#endif
+#endif
     throw object;
   },
 
diff --git a/test/embind/test_embind_throw_cpp_exception.cpp b/test/embind/test_embind_throw_cpp_exception.cpp
@@ -0,0 +1,34 @@
+#include <emscripten/val.h>
+#include <iostream>
+using namespace emscripten;
+
+int main() {
+  val error;
+  try {
+    throw std::runtime_error("oopsie");
+  } catch (const std::runtime_error&) {
+    error = val::take_ownership(
+      emscripten::internal::_emval_from_current_cxa_exception());
+  }
+  try {
+    error.throw_();
+  } catch (const std::runtime_error& ex) {
+    std::clog << "Caught1: " << ex.what() << '\n';
+  }
+
+  try {
+    throw std::runtime_error("oopsie");
+  } catch (const std::runtime_error&) {
+    error = val::take_ownership(
+      emscripten::internal::_emval_from_current_cxa_exception());
+  }
+  try {
+    throw std::runtime_error("this is bad");
+  } catch (const std::runtime_error&) {
+    try {
+      error.throw_(); // Rethrow oopsie
+    } catch (const std::runtime_error& ex) {
+      std::clog << "Caught2: " << ex.what() << '\n';
+    }
+  }
+}
diff --git a/test/embind/test_embind_throw_cpp_exception.out b/test/embind/test_embind_throw_cpp_exception.out
@@ -0,0 +1,2 @@
+Caught1: oopsie
+Caught2: oopsie
diff --git a/test/test_core.py b/test/test_core.py
@@ -7741,6 +7741,10 @@ def test_embind_no_rtti_followed_by_rtti(self):
   def test_embind_wasm_workers(self):
     self.do_run_in_out_file_test('embind/test_embind_wasm_workers.cpp', cflags=['-lembind', '-sWASM_WORKERS'])
 
+  @with_all_eh_sjlj
+  def test_embind_throw_cpp_exception(self):
+    self.do_run_in_out_file_test('embind/test_embind_throw_cpp_exception.cpp', cflags=['-lembind', '-std=c++20'])
+
   @parameterized({
     '': ('DEFAULT', False),
     'all': ('ALL', False),
