[embind] Manage exceptions' states correctly

src/lib/libemval.js

@@ -11,6 +11,9 @@
 var LibraryEmVal = {
   // Stack of handles available for reuse.
   $emval_freelist: [],
+#if !DISABLE_EXCEPTION_CATCHING || WASM_EXCEPTIONS
+  $emval_exception_decrefs: [],
+#endif
   // Array of alternating pairs (value, refcount).
   // reserve 0 and some special values. These never get de-allocated.
   $emval_handles: [
@@ -80,13 +83,27 @@ var LibraryEmVal = {
     }
   },
 
-  _emval_decref__deps: ['$emval_freelist', '$emval_handles'],
+  _emval_decref__deps: ['$emval_freelist', '$emval_handles',
+#if !DISABLE_EXCEPTION_CATCHING || WASM_EXCEPTIONS
+    '$emval_exception_decrefs',
+#endif
+  ],
   _emval_decref: (handle) => {
     if (handle > {{{ EMVAL_LAST_RESERVED_HANDLE }}} && 0 === --emval_handles[handle + 1]) {
   #if ASSERTIONS
       assert(emval_handles[handle] !== undefined, `Decref for unallocated handle.`);
   #endif
+      var value = emval_handles[handle];
       emval_handles[handle] = undefined;
+#if !DISABLE_EXCEPTION_CATCHING || WASM_EXCEPTIONS
+      // In case the value is a C++ exception, decrement the refcount, so the
+      // memory can be freed correctly
+      var destructor = emval_exception_decrefs[handle];
+      if (destructor) {
+        emval_exception_decrefs[handle] = undefined;
+        destructor(value);
+      }
+#endif
       emval_freelist.push(handle);
     }
   },
@@ -392,18 +409,40 @@ ${functionBody}
     return delete object[property];
   },
 
+#if !DISABLE_EXCEPTION_CATCHING || WASM_EXCEPTIONS
+  $isCppExceptionObject__deps: ['$Emval'],
+  $isCppExceptionObject: (object) => {
+#if !DISABLE_EXCEPTION_CATCHING
+    return object instanceof CppException;
+#else // WASM_EXCEPTIONS
+    return object instanceof WebAssembly.Exception;
+#endif
+  },
+#endif
+
   _emval_throw__deps: ['$Emval',
-#if !WASM_EXCEPTIONS && !DISABLE_EXCEPTION_CATCHING
+#if !DISABLE_EXCEPTION_CATCHING || WASM_EXCEPTIONS
+#if !DISABLE_EXCEPTION_CATCHING
     '$exceptionLast',
+    '$ExceptionInfo',
+#endif
+    '$incrementExceptionRefcount',
+    '$incrementUncaughtExceptionCount',
+    '$isCppExceptionObject',
 #endif
   ],
   _emval_throw: (object) => {
     object = Emval.toValue(object);
-#if !WASM_EXCEPTIONS && !DISABLE_EXCEPTION_CATCHING
-    // If we are throwing Emcripten C++ exception, set exceptionLast, as we do
-    // in __cxa_throw. C++ exception will be an instance of CppEmscripten.
-    if (object instanceof CppException) {
+#if !DISABLE_EXCEPTION_CATCHING || WASM_EXCEPTIONS
+    if (isCppExceptionObject(object)) {
+#if !DISABLE_EXCEPTION_CATCHING
+      var info = new ExceptionInfo(object.excPtr);
+      info.set_caught(false);
+      info.set_rethrown(false);
       exceptionLast = object;
+#endif
+      incrementUncaughtExceptionCount();
+      incrementExceptionRefcount(object);
     }
 #endif
     throw object;
@@ -446,7 +485,15 @@ ${functionBody}
     }));
   },
 
-  _emval_from_current_cxa_exception__deps: ['$Emval', '__cxa_rethrow'],
+  _emval_from_current_cxa_exception__deps: ['$Emval', '__cxa_rethrow',
+#if !DISABLE_EXCEPTION_THROWING || WASM_EXCEPTIONS
+    '$decrementUncaughtExceptionCount',
+#endif
+#if !DISABLE_EXCEPTION_CATCHING || WASM_EXCEPTIONS
+    '$decrementExceptionRefcount',
+    '$emval_exception_decrefs',
+#endif
+  ],
   _emval_from_current_cxa_exception: () => {
     try {
       // Use __cxa_rethrow which already has mechanism for generating
@@ -455,7 +502,16 @@ ${functionBody}
       // with metadata optimised out otherwise.
       ___cxa_rethrow();
     } catch (e) {
-      return Emval.toHandle(e);
+#if !DISABLE_EXCEPTION_THROWING || WASM_EXCEPTIONS
+      // ___cxa_rethrow incremented uncaughtExceptionCount.
+      // Since we caught it in JS, we need to manually decrement it to balance.
+      decrementUncaughtExceptionCount();
+#endif
+      var handle = Emval.toHandle(e);
+#if !DISABLE_EXCEPTION_CATCHING || WASM_EXCEPTIONS
+      emval_exception_decrefs[handle] = decrementExceptionRefcount;
+#endif
+      return handle;
     }
   },
 };

src/lib/libexceptions.js

@@ -366,6 +366,12 @@ var LibraryExceptions = {
     return ___thrown_object_from_unwind_exception(unwind_header);
   },
 
+  $incrementUncaughtExceptionCount__deps: ['__increment_uncaught_exception'],
+  $incrementUncaughtExceptionCount: '__increment_uncaught_exception',
+
+  $decrementUncaughtExceptionCount__deps: ['__decrement_uncaught_exception'],
+  $decrementUncaughtExceptionCount: '__decrement_uncaught_exception',
+
   $incrementExceptionRefcount__deps: ['__cxa_increment_exception_refcount', '$getCppExceptionThrownObjectFromWebAssemblyException'],
   $incrementExceptionRefcount: (ex) => {
     var ptr = getCppExceptionThrownObjectFromWebAssemblyException(ex);
@@ -384,7 +390,20 @@ var LibraryExceptions = {
     return getExceptionMessageCommon(ptr);
   },
 
-#elif !DISABLE_EXCEPTION_CATCHING
+#else
+#if !DISABLE_EXCEPTION_THROWING
+  $incrementUncaughtExceptionCount__deps: ['$uncaughtExceptionCount'],
+  $incrementUncaughtExceptionCount: () => {
+    uncaughtExceptionCount++;
+  },
+
+  $decrementUncaughtExceptionCount__deps: ['$uncaughtExceptionCount'],
+  $decrementUncaughtExceptionCount: () => {
+    uncaughtExceptionCount--;
+  },
+#endif
+
+#if !DISABLE_EXCEPTION_CATCHING
   $incrementExceptionRefcount__deps: ['__cxa_increment_exception_refcount'],
   $incrementExceptionRefcount: (exn) => ___cxa_increment_exception_refcount(exn.excPtr),
 
@@ -394,6 +413,7 @@ var LibraryExceptions = {
   $getExceptionMessage__deps: ['$getExceptionMessageCommon'],
   $getExceptionMessage: (exn) => getExceptionMessageCommon(exn.excPtr),
 
+#endif
 #endif
 };
 

system/lib/libcxxabi/src/cxa_exception_js_utils.cpp

@@ -123,6 +123,17 @@ char* __get_exception_terminate_message(void* thrown_object) {
   free(type);
   return result;
 }
+
+#ifdef __WASM_EXCEPTIONS__
+void __increment_uncaught_exception() throw() {
+  __cxa_get_globals()->uncaughtExceptions += 1;
+}
+
+void __decrement_uncaught_exception() throw() {
+  __cxa_get_globals()->uncaughtExceptions -= 1;
+}
+#endif
+
 } // extern "C"
 
 } // namespace __cxxabiv1

test/embind/test_embind_throw_val_uncaught_and_refcount.cpp

@@ -0,0 +1,47 @@
+#include <emscripten/val.h>
+#include <exception>
+#include <iostream>
+
+using namespace emscripten;
+
+int main() {
+  val error = val::null();
+  try {
+    throw std::runtime_error("test exception");
+  } catch (const std::exception& e) {
+    // Capture the exception
+    error = val::take_ownership(
+      emscripten::internal::_emval_from_current_cxa_exception());
+  }
+
+  std::cout << "Captured exception." << std::endl;
+
+  int uncaught_before = std::uncaught_exceptions();
+  std::cout << "Uncaught before throw 1: " << uncaught_before << std::endl;
+
+  // First throw
+  try {
+    std::cout << "Throwing 1..." << std::endl;
+    error.throw_();
+  } catch (const std::exception& e) {
+    std::cout << "Caught 1: " << e.what() << std::endl;
+    int uncaught_during = std::uncaught_exceptions();
+    std::cout << "Uncaught during catch 1: " << uncaught_during << std::endl;
+  }
+
+  int uncaught_between = std::uncaught_exceptions();
+  std::cout << "Uncaught between throws: " << uncaught_between << std::endl;
+
+  // Second throw - if refcount was messed up, this might fail/crash
+  try {
+    std::cout << "Throwing 2..." << std::endl;
+    error.throw_();
+  } catch (const std::exception& e) {
+    std::cout << "Caught 2: " << e.what() << std::endl;
+    int uncaught_during = std::uncaught_exceptions();
+    std::cout << "Uncaught during catch 2: " << uncaught_during << std::endl;
+  }
+
+  std::cout << "Done." << std::endl;
+  return 0;
+}

test/embind/test_embind_throw_val_uncaught_and_refcount.out

@@ -0,0 +1,10 @@
+Captured exception.
+Uncaught before throw 1: 0
+Throwing 1...
+Caught 1: test exception
+Uncaught during catch 1: 0
+Uncaught between throws: 0
+Throwing 2...
+Caught 2: test exception
+Uncaught during catch 2: 0
+Done.

test/test_core.py

@@ -7782,6 +7782,10 @@ def test_embind_wasm_workers(self):
   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'])
 
+  @with_all_eh_sjlj
+  def test_embind_throw_val_uncaught_and_refcount(self):
+    self.do_run_in_out_file_test('embind/test_embind_throw_val_uncaught_and_refcount.cpp', cflags=['-lembind', '-std=c++20'])
+
   @parameterized({
     '': ('DEFAULT', False),
     'all': ('ALL', False),