[EH] Print message for uncaught exceptions (#18003)

After #17979, when `ASSERTIONS` is set, uncaught exceptions carry stack
traces that are printed to the screen for debugging help. This adds
an exception message to the exception objects in addition to that.

The message it adds is produced by `__get_exception_message` function:
https://github.com/emscripten-core/emscripten/blob/f6c46570e3780e52050bf822a07b342ec4bdddbe/system/lib/libcxxabi/src/cxa_exception_emscripten.cpp#L75-L111

If an exception is a subclass of `std::exception`, it returns
`std::exception::what()`. If not, it just prints its type.

If an exception is uncaught and its type is a subclass of
`std::exception`, now we print the `what()` message along with the stack
trace when `ASSERTIONS` is set. In case our exception is
`std::runtime_error` and the message is "my exception", when uncaught,
this prints:
```
exiting due to exception: [object WebAssembly.Exception],Error:
std::runtime_error,my exception
    at __cxa_throw (wasm://wasm/009a7c9a:wasm-function[1551]:0x24367)
    ...
```

Fixes #6330.

ChangeLog.md

@@ -21,6 +21,8 @@ See docs/process.md for more on how version tagging works.
 3.1.24 (in development)
 -----------------------
 - In Wasm exception mode (`-fwasm-exceptions`), when `ASSERTIONS` is enabled,
+  uncaught exceptions will display stack traces and what() message. (#17979 and
+  #18003)
   uncaught exceptions will display stack traces. (#17979)
 - It is now possible to specify indirect dependencies on JS library functions
   directly in C/C++ source code.  For example, in the case of a EM_JS or EM_ASM

emcc.py

@@ -2633,6 +2633,7 @@ def get_full_import_name(name):
   # using the JS API, which needs this C++ tag exported.
   if settings.ASSERTIONS and settings.WASM_EXCEPTIONS:
     settings.EXPORTED_FUNCTIONS += ['___cpp_exception']
+    settings.EXPORT_EXCEPTION_HANDLING_HELPERS = True
 
   # Make `getExceptionMessage` and other necessary functions available for use.
   if settings.EXPORT_EXCEPTION_HANDLING_HELPERS:
@@ -2642,7 +2643,7 @@ def get_full_import_name(name):
     # What you need to do is different depending on the kind of EH you use
     # (https://github.com/emscripten-core/emscripten/issues/17115).
     settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE += ['$getExceptionMessage', '$incrementExceptionRefcount', '$decrementExceptionRefcount']
-    settings.EXPORTED_FUNCTIONS += ['getExceptionMessage', '___get_exception_message']
+    settings.EXPORTED_FUNCTIONS += ['getExceptionMessage', '___get_exception_message', '_free']
     if settings.WASM_EXCEPTIONS:
       settings.EXPORTED_FUNCTIONS += ['___cpp_exception', '___cxa_increment_exception_refcount', '___cxa_decrement_exception_refcount', '___thrown_object_from_unwind_exception']
 

src/library_exceptions.js

@@ -396,11 +396,11 @@ var LibraryExceptions = {
   $getExceptionMessageCommon__deps: ['__get_exception_message', 'free', '$withStackSave'],
   $getExceptionMessageCommon: function(ptr) {
     return withStackSave(function() {
-      var type_addr_addr = stackAlloc(4);
-      var message_addr_addr = stackAlloc(4);
-      ___get_exception_message(ptr, type_addr_addr, message_addr_addr);
-      var type_addr = HEAP32[type_addr_addr >> 2];
-      var message_addr = HEAP32[message_addr_addr >> 2];
+      var type_addr_addr = stackAlloc({{{ POINTER_SIZE }}});
+      var message_addr_addr = stackAlloc({{{ POINTER_SIZE }}});
+      ___get_exception_message({{{ to64('ptr') }}}, {{{ to64('type_addr_addr') }}}, {{{ to64('message_addr_addr') }}});
+      var type_addr = {{{ makeGetValue('type_addr_addr', 0, '*') }}};
+      var message_addr = {{{ makeGetValue('message_addr_addr', 0, '*') }}};
       var type = UTF8ToString(type_addr);
       _free(type_addr);
       var message;
@@ -431,9 +431,10 @@ var LibraryExceptions = {
   // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Exception
   // In release builds, this function is not needed and the native
   // _Unwind_RaiseException in libunwind is used instead.
-  __throw_exception_with_stack_trace__deps: ['$getCppExceptionTag'],
+  __throw_exception_with_stack_trace__deps: ['$getCppExceptionTag', '$getExceptionMessage'],
   __throw_exception_with_stack_trace: function(ex) {
     var e = new WebAssembly.Exception(getCppExceptionTag(), [ex], {traceStack: true});
+    e.message = getExceptionMessage(e);
     // The generated stack trace will be in the form of:
     //
     // Error

system/lib/libcxxabi/src/cxa_exception.cpp

@@ -256,7 +256,7 @@ exception.
 
 #if defined(__USING_WASM_EXCEPTIONS__) && !defined(NDEBUG)
 extern "C" {
-void __throw_exception_with_stack_trace(_Unwind_Exception*, bool);
+void __throw_exception_with_stack_trace(_Unwind_Exception*);
 } // extern "C"
 #endif
 
@@ -293,7 +293,7 @@ __cxa_throw(void *thrown_object, std::type_info *tinfo, void (*dest)(void *)) {
 #else
     // In debug mode, call a JS library function to use WebAssembly.Exception JS
     // API, which enables us to include stack traces
-    __throw_exception_with_stack_trace(&exception_header->unwindHeader, true);
+    __throw_exception_with_stack_trace(&exception_header->unwindHeader);
 #endif
 #else
     _Unwind_RaiseException(&exception_header->unwindHeader);

test/test_other.py

@@ -7905,10 +7905,12 @@ def test_wasm_nope(self):
       self.assertContained('no native wasm support detected', out)
 
   @requires_v8
-  def test_wasm_exceptions_stack_trace(self):
+  def test_wasm_exceptions_stack_trace_and_message(self):
     src = r'''
+      #include <stdexcept>
+
       void bar() {
-        throw 3;
+        throw std::runtime_error("my message");
       }
       void foo() {
         bar();
@@ -7921,8 +7923,8 @@ def test_wasm_exceptions_stack_trace(self):
     emcc_args = ['-g', '-fwasm-exceptions']
     self.v8_args.append('--experimental-wasm-eh')
 
-    # Stack trace example for this example code:
-    # exiting due to exception: [object WebAssembly.Exception],Error
+    # Stack trace and message example for this example code:
+    # exiting due to exception: [object WebAssembly.Exception],Error: std::runtime_error, my message
     #     at __cxa_throw (wasm://wasm/009a7c9a:wasm-function[1551]:0x24367)
     #     at bar() (wasm://wasm/009a7c9a:wasm-function[12]:0xf53)
     #     at foo() (wasm://wasm/009a7c9a:wasm-function[19]:0x154e)
@@ -7932,7 +7934,12 @@ def test_wasm_exceptions_stack_trace(self):
     #     at callMain (test.js:4567:15)
     #     at doRun (test.js:4621:23)
     #     at run (test.js:4636:5)
-    stack_trace_checks = ['at __cxa_throw', 'at bar', 'at foo', 'at main']
+    stack_trace_checks = [
+      'std::runtime_error,my message',
+      'at __cxa_throw',
+      'at bar',
+      'at foo',
+      'at main']
 
     # We attach stack traces to exception objects only when ASSERTIONS is set
     self.set_setting('ASSERTIONS')