[JS API] Throw useful exceptions on parse errors

CMakeLists.txt

@@ -580,6 +580,11 @@ if(EMSCRIPTEN)
   # Avoid catching exit as that can confuse error reporting in Node,
   # see https://github.com/emscripten-core/emscripten/issues/17228
   target_link_libraries(binaryen_wasm PRIVATE "-sNODEJS_CATCH_EXIT=0")
+  # Don't exit the process on a fatal error, instead throw, so that JS can
+  # catch.
+  add_compile_flag("-DTHROW_ON_FATAL")
+  # Add support for printing C++ exceptions from JS.
+  target_link_libraries(binaryen_wasm PRIVATE "-sEXPORT_EXCEPTION_HANDLING_HELPERS")
   install(TARGETS binaryen_wasm DESTINATION ${CMAKE_INSTALL_BINDIR})
 
   # binaryen.js JavaScript variant
@@ -632,6 +637,8 @@ if(EMSCRIPTEN)
   # Avoid catching exit as that can confuse error reporting in Node,
   # see https://github.com/emscripten-core/emscripten/issues/17228
   target_link_libraries(binaryen_js PRIVATE "-sNODEJS_CATCH_EXIT=0")
+  add_compile_flag("-DTHROW_ON_FATAL")
+  target_link_libraries(binaryen_js PRIVATE "-sEXPORT_EXCEPTION_HANDLING_HELPERS")
   install(TARGETS binaryen_js DESTINATION ${CMAKE_INSTALL_BINDIR})
 endif()
 

src/js/binaryen.js-post.js

@@ -3254,6 +3254,27 @@ Module['emitText'] = function(expr) {
   return ret;
 };
 
+// Calls a function, wrapping it in error handling code so that if it hits a
+// fatal error, we throw a JS exception (which JS can handle) rather than
+// abort the entire process (which would not be a friendly behavior for a
+// library like binaryen.js).
+function handleFatalError(func) {
+  try {
+    return func();
+  } catch (e) {
+    // C++ exceptions are thrown as pointers (numbers).
+    if (typeof e === 'number') {
+      // Fatal errors begin with that prefix. Strip it out, and the newline.
+      var [_, message] = getExceptionMessage(e);
+      if (message?.startsWith('Fatal: ')) {
+        throw new Error(message.substr(7).trim());
+      }
+    }
+    // Rethrow anything else.
+    throw e;
+  }
+}
+
 // Parses a binary to a module
 
 // If building with Emscripten ASSERTIONS, there is a property added to
@@ -3264,7 +3285,7 @@ Object.defineProperty(Module, 'readBinary', { writable: true });
 Module['readBinary'] = function(data) {
   const buffer = _malloc(data.length);
   HEAP8.set(data, buffer);
-  const ptr = Module['_BinaryenModuleRead'](buffer, data.length);
+  const ptr = handleFatalError(() => Module['_BinaryenModuleRead'](buffer, data.length));
   _free(buffer);
   return wrapModule(ptr);
 };
@@ -3273,7 +3294,7 @@ Module['readBinary'] = function(data) {
 Module['parseText'] = function(text) {
   const buffer = _malloc(text.length + 1);
   stringToAscii(text, buffer);
-  const ptr = Module['_BinaryenModuleParse'](buffer);
+  const ptr = handleFatalError(() => Module['_BinaryenModuleParse'](buffer));
   _free(buffer);
   return wrapModule(ptr);
 };

test/binaryen.js/errors.js

@@ -0,0 +1,17 @@
+// Parse invalid text and get a JS exception.
+var caughtAsExpected = false;
+try {
+  console.log('parsing invalid text...');
+  binaryen.parseText(`(module
+    ;; error on next line
+    (func $foo (param__error $x i32))
+  )`)
+  console.log('no error - invalid');
+} catch (e) {
+  assert(e.message === '3:16: error: unrecognized instruction');
+  caughtAsExpected = true;
+}
+assert(caughtAsExpected);
+
+console.log('success.');
+

test/binaryen.js/errors.js.txt

@@ -0,0 +1 @@
+success.