[EH] Print message for uncaught exceptions

[aheejin]

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:

emscripten/system/lib/libcxxabi/src/cxa_exception_emscripten.cpp

Lines 75 to 111 in f6c4657

	// Given a thrown_object, puts the information about its type and message into
	// 'type' and 'message' output parameters. 'type' will contain the string
	// representation of the type of the exception, e.g., 'int'. 'message' will
	// contain the result of 'std::exception::what()' method if the type of the
	// exception is a subclass of std::exception; otherwise it will be NULL. The
	// caller is responsible for freeing 'type' buffer and also 'message' buffer, if
	// it is not NULL.
	void __get_exception_message(void* thrown_object, char** type, char** message) {
	__cxa_exception* exception_header =
	cxa_exception_from_thrown_object(thrown_object);
	const __shim_type_info* thrown_type =
	static_cast<const __shim_type_info*>(exception_header->exceptionType);
	const char* type_name = thrown_type->name();
	int status = 0;
	char* demangled_buf = __cxa_demangle(type_name, 0, 0, &status);
	if (status == 0 && demangled_buf) {
	*type = demangled_buf;
	} else {
	if (demangled_buf) {
	free(demangled_buf);
	}
	*type = (char*)malloc(strlen(type_name) + 1);
	strcpy(*type, type_name);
	}
	*message = NULL;
	const __shim_type_info* catch_type =
	static_cast<const __shim_type_info*>(&typeid(std::exception));
	int can_catch = catch_type->can_catch(thrown_type, thrown_object);
	if (can_catch) {
	const char* what =
	static_cast<const std::exception*>(thrown_object)->what();
	*message = (char*)malloc(strlen(what) + 1);
	strcpy(*message, what);
	}
	}

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.

[aheejin]

This second argument is what I should've deleted in #17979 and forgot. (I first added an option in the JS f unction and then deleted it after feedback, but forgot to delete the caller side, which didn't error out)

[aheejin]

Will convert this to a draft while fixing CI errors.

[RReverser]

Unrelated, but I wonder if we could drop the [object WebAssembly.Exception], here. It seems to be just unnecessary visual noise.

[aheejin]

I don't think It's easy to consistently take out that part, because different engines can have different error formats. For example, d8 prints:

exiting due to exception: [object WebAssembly.Exception],Error: std::runtime_error,my message
    at __cxa_throw (wasm://wasm/00462286:wasm-function[44]:0x3b0f)
    at bar() (wasm://wasm/00462286:wasm-function[9]:0x98c)
    at foo() (wasm://wasm/00462286:wasm-function[10]:0x992)
    at __original_main (wasm://wasm/00462286:wasm-function[11]:0x9c4)
    at main (wasm://wasm/00462286:wasm-function[12]:0x9e1)
    at test.js:934:22
    at callMain (test.js:1874:15)
    at doRun (test.js:1928:23)
    at run (test.js:1943:5)

Whereas Node prints:

/usr/local/google/home/aheejin/test/stack/test.js:147
      throw ex;
      ^

WebAssembly.Exception: std::runtime_error,my message
    at __cxa_throw (wasm://wasm/00462286:wasm-function[44]:0x3b0f)
    at bar() (wasm://wasm/00462286:wasm-function[9]:0x98c)
    at foo() (wasm://wasm/00462286:wasm-function[10]:0x992)
    at __original_main (wasm://wasm/00462286:wasm-function[11]:0x9c4)
    at main (wasm://wasm/00462286:wasm-function[12]:0x9e1)
    at /usr/local/google/home/aheejin/test/stack/test.js:934:22
    at callMain (/usr/local/google/home/aheejin/test/stack/test.js:1874:15)
    at doRun (/usr/local/google/home/aheejin/test/stack/test.js:1928:23)
    at run (/usr/local/google/home/aheejin/test/stack/test.js:1943:5)

Node.js v17.5.0

So while both of them have WebAssembly.Exception, their printing formats differ, making taking it out in a consistent way difficult. We can just simply replace WebAssembly.Exception with an empty string, but not sure if that's better..?

[sbc100]

Leaving it as is sounds fine for to me then. We can attempt to refine in a followup perhaps.

[RReverser]

This is great, thanks!

[kripken]

Nice!