Add helper function for C++ exception formatting (#16343)

Author: hoodmane

emcc.py

@@ -2077,6 +2077,11 @@ def phase_linker_setup(options, state, newargs, user_settings):
   else:
     settings.JS_LIBRARIES.append((0, 'library_pthread_stub.js'))
 
+  # TODO: Move this into the library JS file once it becomes possible.
+  # See https://github.com/emscripten-core/emscripten/pull/15982
+  if settings.INCLUDE_FULL_LIBRARY and not settings.DISABLE_EXCEPTION_CATCHING:
+    settings.EXPORTED_FUNCTIONS += ['_emscripten_format_exception', '_free']
+
   if settings.FORCE_FILESYSTEM and not settings.MINIMAL_RUNTIME:
     # when the filesystem is forced, we export by default methods that filesystem usage
     # may need, including filesystem usage from standalone file packager output (i.e.

src/library_exceptions.js

@@ -428,6 +428,16 @@ var LibraryExceptions = {
     catchInfo.free();
     {{{ makeThrow('ptr') }}}
   },
+
+#if !DISABLE_EXCEPTION_CATCHING
+  $formatException__deps: ['emscripten_format_exception', 'free'],
+  $formatException: function(excPtr) {
+    var utf8_addr = _emscripten_format_exception(excPtr);
+    var result = UTF8ToString(utf8_addr);
+    _free(utf8_addr);
+    return result;
+  },
+#endif
 };
 
 // In LLVM, exceptions generate a set of functions of form __cxa_find_matching_catch_1(), __cxa_find_matching_catch_2(), etc.

system/lib/libcxxabi/src/format_exception.cpp

@@ -0,0 +1,46 @@
+#include "cxa_exception.h"
+#include "cxxabi.h"
+#include <stdio.h>
+#include <typeinfo>
+
+#ifdef __USING_EMSCRIPTEN_EXCEPTIONS__
+
+extern "C" {
+
+int __cxa_can_catch(const std::type_info* catchType,
+                    const std::type_info* excpType,
+                    void** thrown);
+
+char* emscripten_format_exception(void* exc_ptr) {
+  __cxxabiv1::__cxa_exception* exc_info =
+    (__cxxabiv1::__cxa_exception*)exc_ptr - 1;
+  std::type_info* exc_type = exc_info->exceptionType;
+  const char* exc_name = exc_type->name();
+
+  int status = 0;
+  char* demangled_buf = __cxxabiv1::__cxa_demangle(exc_name, 0, 0, &status);
+  if (status == 0 && demangled_buf) {
+    exc_name = demangled_buf;
+  }
+
+  int can_catch = __cxa_can_catch(&typeid(std::exception), exc_type, &exc_ptr);
+  char* result = NULL;
+  if (can_catch) {
+    const char* exc_what = ((std::exception*)exc_ptr)->what();
+    asprintf(&result, "Cpp Exception %s: %s", exc_name, exc_what);
+  } else {
+    asprintf(&result,
+             "Cpp Exception: The exception is an object of type '%s' at "
+             "address %p which does not inherit from std::exception",
+             exc_name,
+             exc_ptr);
+  }
+
+  if (demangled_buf) {
+    free(demangled_buf);
+  }
+  return result;
+}
+}
+
+#endif // __USING_EMSCRIPTEN_EXCEPTIONS__

tests/test_core.py

@@ -1543,6 +1543,61 @@ def test_exceptions_rethrow_missing(self):
     create_file('main.cpp', 'int main() { throw; }')
     self.do_runf('main.cpp', None, assert_returncode=NON_ZERO)
 
+  def test_format_exception(self):
+    self.set_setting('DISABLE_EXCEPTION_CATCHING', 0)
+    self.set_setting('DEFAULT_LIBRARY_FUNCS_TO_INCLUDE', ['$formatException'])
+    self.set_setting('EXPORTED_FUNCTIONS', ['_main', 'formatException', '_emscripten_format_exception', '_free'])
+    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 {
+                  Module["_throw_exc"](i);
+              } catch(p) {
+                  console.log(Module["formatException"](p).replace(/0x[0-9a-f]*/, "xxx"));
+              }
+            }
+          });
+      }
+    '''
+    expected = '''\
+Cpp Exception: The exception is an object of type 'int' at address xxx which does not inherit from std::exception
+Cpp Exception: The exception is an object of type 'char' at address xxx which does not inherit from std::exception
+Cpp Exception std::runtime_error: abc
+Cpp Exception myexception: My exception happened
+Cpp Exception: The exception is an object of type 'char const*' at address xxx which does not inherit from std::exception
+'''
+
+    self.do_run(src, expected)
+
   @with_both_eh_sjlj
   def test_bad_typeid(self):
     self.do_run(r'''

tools/system_libs.py

@@ -1127,7 +1127,8 @@ def get_files(self):
       'stdlib_exception.cpp',
       'stdlib_stdexcept.cpp',
       'stdlib_typeinfo.cpp',
-      'private_typeinfo.cpp'
+      'private_typeinfo.cpp',
+      'format_exception.cpp',
     ]
     if self.eh_mode == Exceptions.NONE:
       filenames += ['cxa_noexception.cpp']