Export C++ exception tag

[hoodmane]

I am looking into how to do a similar task to #16343 with -fwasm-exceptions. If there is an uncaught C++ exception with -fwasm-exceptions, a WasmException object is thrown which contains a single i32 which is the same pointer that is thrown as a number when using -fexceptions. The WasmException API has a getArg API which can be used to get the pointer like exception.getArg(tag, 0). The problem is that the tag isn't exported and without the tag the WasmException is completely opaque.

Once I have this tag, we can get access to the pointer from the WasmException and then handle it similarly to as in the -fexceptions case. I have checked that this works by using wasm2wat then adding (export "cpp_exception_tag" (tag 0)) to the wat, then using wat2wasm. (Though there is still the remaining issue that __cxa_can_catch doesn't exist in this case.)

[hoodmane]

Actually I am not sure what to do with the pointer we get out either. But I think the first step is definitely getting access to it.

[aheejin]

Closing this, given that #17064 landed. As I described in that PR, using this to extract thrown values still requires multiple options, which is inconvenient at the moment, but it is at least possible. Refer to the example test to see how it is used and which options should be turned on:

emscripten/tests/test_core.py

Lines 1636 to 1713 in 2a3bb65

	@no_wasm64('MEMORY64 does not yet support exceptions')
	@with_both_eh_sjlj
	def test_exception_message(self):
	self.set_setting('DEFAULT_LIBRARY_FUNCS_TO_INCLUDE', ['$getExceptionMessage', '$incrementExceptionRefcount', '$decrementExceptionRefcount'])
	self.set_setting('EXPORTED_FUNCTIONS', ['_main', 'getExceptionMessage', '___get_exception_message'])
	if '-fwasm-exceptions' in self.emcc_args:
	exports = self.get_setting('EXPORTED_FUNCTIONS')
	self.set_setting('EXPORTED_FUNCTIONS', exports + ['___cpp_exception', '___increment_wasm_exception_refcount', '___decrement_wasm_exception_refcount'])
	# FIXME Temporary workaround. See 'FIXME' in the test source code below for
	# details.
	if self.get_setting('DISABLE_EXCEPTION_CATCHING') == 0:
	self.emcc_args.append('-D__USING_EMSCRIPTEN_EXCEPTION__')
	self.maybe_closure()
	src = '''
	#include <emscripten.h>
	#include <exception>
	#include <stdexcept>
	using namespace std;
	class myexception : public exception {
	virtual const char* what() const throw() { return "My exception happened"; }
	} myex;
	EMSCRIPTEN_KEEPALIVE extern "C" void throw_exc(int x) {
	if (x == 1) {
	throw 1000;
	}
	if (x == 2) {
	throw 'c';
	}
	if (x == 3) {
	throw runtime_error("abc");
	}
	if (x == 4) {
	throw myex;
	}
	if (x == 5) {
	throw "abc";
	}
	}
	int main() {
	EM_ASM({
	for (let i = 1; i < 6; i++){
	try {
	_throw_exc(i);
	} catch(p) {
	// Because we are catching and handling the exception in JS, the normal
	// exception catching C++ code doesn't kick in, so we need to make sure we free
	// the exception, if necessary. By incrementing and decrementing the refcount
	// we trigger the free'ing of the exception if its refcount was zero.
	#ifdef __USING_EMSCRIPTEN_EXCEPTION__
	// FIXME Currently Wasm EH and Emscripten EH increases
	// refcounts in different places. Wasm EH sets the refcount to
	// 1 when throwing, and decrease it in __cxa_end_catch.
	// Emscripten EH sets the refcount to 0 when throwing, and
	// increase it in __cxa_begin_catch, and decrease it in
	// __cxa_end_catch. Fix this inconsistency later.
	incrementExceptionRefcount(p);
	#endif
	console.log(getExceptionMessage(p));
	decrementExceptionRefcount(p);
	}
	}
	});
	}
	'''
	expected = '''\
	exception of type int
	exception of type char
	exception of type std::runtime_error: abc
	exception of type myexception: My exception happened
	exception of type char const*
	'''
	self.do_run(src, expected)

(The test uses EM_ASM, but you can do the same thing with normal JS code as well)

I'm planning to make this easier to use, possibly creating a single option that adds all necessary imports and exports the tag, but I'd like to let you know that using this is at least possible now. I'll CC this issue again when the one-stop option is added later too.

[aheejin]

You can use these methods to access the tag and thrown values, and print what() message (if any) for an exception:

• getCppExceptionTag
• getCppExceptionThrownValue
• getExceptionMessage
  

  emscripten/src/library_exceptions.js

  Lines 396 to 447 in 2a3bb65

  	#if WASM_EXCEPTIONS
  	$getCppExceptionTag: function() {
  	return Module['asm']['__cpp_exception'];
  	},
  	$getCppExceptionThrownValue__deps: ['$getCppExceptionTag'],
  	$getCppExceptionThrownValue: function(ex) {
  	return ex.getArg(getCppExceptionTag(), 0);
  	},
  	$incrementExceptionRefcount__deps: ['__increment_wasm_exception_refcount', '$getCppExceptionThrownValue'],
  	$incrementExceptionRefcount: function(obj) {
  	var ptr = getCppExceptionThrownValue(obj);
  	___increment_wasm_exception_refcount(ptr);
  	},
  	$decrementExceptionRefcount__deps: ['__decrement_wasm_exception_refcount', '$getCppExceptionThrownValue'],
  	$decrementExceptionRefcount: function(obj) {
  	var ptr = getCppExceptionThrownValue(obj);
  	___decrement_wasm_exception_refcount(ptr);
  	},
  	$getExceptionMessage__deps: ['__get_exception_message', 'free', '$getCppExceptionThrownValue'],
  	$getExceptionMessage: function(ptr) {
  	// In Wasm EH, the thrown object is a WebAssembly.Exception. Extract the
  	// thrown value from it.
  	var obj = getCppExceptionThrownValue(ptr);
  	var utf8_addr = ___get_exception_message(obj);
  	var result = UTF8ToString(utf8_addr);
  	_free(utf8_addr);
  	return result;
  	},
  	#elif !DISABLE_EXCEPTION_CATCHING
  	$incrementExceptionRefcount__deps: ['__cxa_increment_exception_refcount'],
  	$incrementExceptionRefcount: function(ptr) {
  	___cxa_increment_exception_refcount(ptr);
  	},
  	$decrementExceptionRefcount__deps: ['__cxa_decrement_exception_refcount'],
  	$decrementExceptionRefcount: function(ptr) {
  	___cxa_decrement_exception_refcount(ptr);
  	},
  	$getExceptionMessage__deps: ['__get_exception_message', 'free'],
  	$getExceptionMessage: function(ptr) {
  	var utf8_addr = ___get_exception_message(ptr);
  	var result = UTF8ToString(utf8_addr);
  	_free(utf8_addr);
  	return result;
  	},
  	#endif

Note that getExceptionMessage uses getCppExceptionThrownValue and getCppExceptionThrownValue uses getCppExceptionTag, so in case you don't need direct access to the tag and thrown values and the only thing you want is to print the message, you only need to call getExceptionMessage.