[EH] Make abort() work with Wasm EH (#16910)

We used to implement `abort()` with throwing a JS exception. #9730
changed it to `RuntimeError`, because it is the class a trap becomes
once it hits a JS frame.

But this turned out to be not working, because Wasm EH currently does
not assume all `RuntimeError`s are from traps; it decides whether a
`RuntimeError` is from a trap or not based on a hidden field within the
object. Relevant past discussion:
WebAssembly/exception-handling#89 (comment)

So at the moment we don't have a way of throwing a trap from JS, which
is inconvenient. I think we eventually want to make JS API for this, but
for the moment, this PR resorts to a wasm function that traps. This at
least makes calling `std::terminate` work. So far calling it exhausted
the call stack and aborted.

emcc.py

@@ -2570,6 +2570,9 @@ def get_full_import_name(name):
     if settings.SUPPORT_LONGJMP == 'wasm':
       settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE.append('__c_longjmp')
 
+  if settings.EXCEPTION_HANDLING:
+    settings.REQUIRED_EXPORTS += ['__trap']
+
   return target, wasm_target
 
 

src/preamble.js

@@ -605,12 +605,20 @@ function abort(what) {
   // Use a wasm runtime error, because a JS error might be seen as a foreign
   // exception, which means we'd run destructors on it. We need the error to
   // simply make the program stop.
+  // FIXME This approach does not work in Wasm EH because it currently does not assume
+  // all RuntimeErrors are from traps; it decides whether a RuntimeError is from
+  // a trap or not based on a hidden field within the object. So at the moment
+  // we don't have a way of throwing a wasm trap from JS. TODO Make a JS API that
+  // allows this in the wasm spec.
 
   // Suppress closure compiler warning here. Closure compiler's builtin extern
   // defintion for WebAssembly.RuntimeError claims it takes no arguments even
   // though it can.
   // TODO(https://github.com/google/closure-compiler/pull/3913): Remove if/when upstream closure gets fixed.
-
+#if EXCEPTION_HANDLING == 1
+  // See above, in the meantime, we resort to wasm code for trapping.
+  ___trap();
+#else
   /** @suppress {checkTypes} */
   var e = new WebAssembly.RuntimeError(what);
 
@@ -621,6 +629,7 @@ function abort(what) {
   // in code paths apart from instantiation where an exception is expected
   // to be thrown when abort is called.
   throw e;
+#endif
 }
 
 // {{MEM_INITIALIZER}}

system/lib/compiler-rt/__trap.c

@@ -0,0 +1,3 @@
+void __trap() {
+  __builtin_trap();
+}

tests/test_core.py

@@ -1655,6 +1655,24 @@ class Polymorphic {virtual void member(){}};
 }
 ''', 'exception caught: std::bad_typeid')
 
+  @with_both_eh_sjlj
+  def test_abort_no_dtors(self):
+    # abort() should not run destructors
+    out = self.do_run(r'''
+#include <stdlib.h>
+#include <iostream>
+
+struct Foo {
+  ~Foo() { std::cout << "Destructing Foo" << std::endl; }
+};
+
+int main() {
+  Foo f;
+  abort();
+}
+''', assert_returncode=NON_ZERO)
+    self.assertNotContained('Destructing Foo', out)
+
   def test_iostream_ctors(self):
     # iostream stuff must be globally constructed before user global
     # constructors, so iostream works in global constructors

tools/system_libs.py

@@ -708,7 +708,8 @@ class libcompiler_rt(MTLibrary, SjLjLibrary):
         'stack_ops.S',
         'stack_limits.S',
         'emscripten_setjmp.c',
-        'emscripten_exception_builtins.c'
+        'emscripten_exception_builtins.c',
+        '__trap.c',
       ])