Fix dropping of arguments in JS library functions

src/jsifier.js

@@ -137,19 +137,22 @@ function runJSify() {
       const newArgs = [];
       let innerArgs = [];
       let argConvertions = '';
-      for (let i = 1; i < sig.length; i++) {
-        const name = argNames[i - 1];
-        if (!name) {
-          error(`handleI64Signatures: missing name for argument ${i} in ${symbol}`);
-          return snippet;
-        }
-        if (WASM_BIGINT && ((MEMORY64 && sig[i] == 'p') || (i53abi && sig[i] == 'j'))) {
+      if (sig.length > argNames.length + 1) {
+        error(`handleI64Signatures: signature too long for ${symbol}`);
+        return snippet;
+      }
+      for (let i = 0; i < argNames.length; i++) {
+        const name = argNames[i];
+        // If sig is shorter than argNames list then argType will be undefined
+        // here, which will result in the default case below.
+        const argType = sig[i + 1];
+        if (WASM_BIGINT && ((MEMORY64 && argType == 'p') || (i53abi && argType == 'j'))) {
           argConvertions += `  ${receiveI64ParamAsI53(name, undefined, false)}\n`;
         } else {
-          if (sig[i] == 'j' && i53abi) {
+          if (argType == 'j' && i53abi) {
             argConvertions += `  ${receiveI64ParamAsI53(name, undefined, false)}\n`;
             newArgs.push(defineI64Param(name));
-          } else if (sig[i] == 'p' && CAN_ADDRESS_2GB) {
+          } else if (argType == 'p' && CAN_ADDRESS_2GB) {
             argConvertions += `  ${name} >>>= 0;\n`;
             newArgs.push(name);
           } else {

test/test_other.py

@@ -4029,6 +4029,28 @@ def test_js_lib_missing_sig(self):
     err = self.expect_fail([EMCC, 'some_func.c'] + self.get_emcc_args())
     self.assertContained('some_func.js: __sig is missing for function: someFunc. Do not use checkSig if this is intended', err)
 
+  def test_js_lib_extra_args(self):
+    # Verify that extra arguments in addition to those listed in `__sig` are still present
+    # in the generated JS library function.
+    # See https://github.com/emscripten-core/emscripten/issues/21056
+    create_file('some_func.js', '''
+      addToLibrary({
+        someFunc: (arg1, arg2) => {
+          err('arg1:' + arg1);
+          err('arg2:' + arg2);
+        },
+        someFunc__sig: 'pp',
+      });
+    ''')
+    create_file('test.c', '''
+    void someFunc(long p);
+    int main() {
+      someFunc(42);
+    }
+    ''')
+    self.emcc_args += ['--js-library', 'some_func.js', '-sALLOW_MEMORY_GROWTH', '-sMAXIMUM_MEMORY=4Gb']
+    self.do_runf('test.c', 'arg1:42\narg2:undefined\n')
+
   def test_js_lib_quoted_key(self):
     create_file('lib.js', r'''
 addToLibrary({