Fix wrapping/modifying of single line JS library functions (#20265)

Fixes: #20264

Author: sbc100

src/library.js

@@ -1813,9 +1813,7 @@ addToLibrary({
 
   gethostbyname__deps: ['$getHostByName'],
   gethostbyname__proxy: 'sync',
-  gethostbyname: (name) => {
-    return getHostByName(UTF8ToString(name));
-  },
+  gethostbyname: (name) => getHostByName(UTF8ToString(name)),
 
   $getHostByName__deps: ['malloc', '$stringToNewUTF8', '$DNS', '$inetPton4'],
   $getHostByName: (name) => {

src/parseTools.js

@@ -689,6 +689,7 @@ function modifyJSFunction(text, func) {
   let async_;
   let args;
   let rest;
+  let oneliner = false;
   let match = text.match(/^\s*(async\s+)?function\s+([^(]*)?\s*\(([^)]*)\)/);
   if (match) {
     async_ = match[1] || '';
@@ -701,6 +702,8 @@ function modifyJSFunction(text, func) {
       async_ = match[3] || '';
       args = match[4];
       rest = text.substr(match[0].length);
+      rest = rest.trim();
+      oneliner = rest[0] != '{';
     } else {
       // Match a function without a name (we could probably use a single regex
       // for both, but it would be more complex).
@@ -712,9 +715,8 @@ function modifyJSFunction(text, func) {
     }
   }
   let body = rest;
-  const bodyStart = rest.indexOf('{');
-  let oneliner = bodyStart < 0;
   if (!oneliner) {
+    const bodyStart = rest.indexOf('{');
     const bodyEnd = rest.lastIndexOf('}');
     assert(bodyEnd > 0);
     body = rest.substring(bodyStart + 1, bodyEnd);

test/test_other.py

@@ -3884,6 +3884,30 @@ def test_js_lib_quoted_key(self):
 
     self.do_runf(test_file('hello_world.c'), 'hello, world!', emcc_args=['--js-library', 'lib.js'])
 
+  def test_js_lib_proxying(self):
+    # Regression test for a bug we had where jsifier would find and use
+    # the inner function in a library function consisting of a single
+    # line arrow function.
+    # See https://github.com/emscripten-core/emscripten/issues/20264
+    create_file('lib.js', r'''
+addToLibrary({
+  foo__proxy: 'sync',
+  foo: () => setTimeout(() => {
+    console.log('should not see this before "done"');
+  }),
+});
+''')
+    create_file('src.c', r'''
+    #include <stdio.h>
+    void foo();
+    int main() {
+      printf("main\n");
+      foo();
+      printf("done\n");
+    }
+    ''')
+    self.do_runf('src.c', 'main\ndone\n', emcc_args=['-sEXIT_RUNTIME', '-pthread', '-sPROXY_TO_PTHREAD', '--js-library', 'lib.js'])
+
   def test_js_lib_exported(self):
     create_file('lib.js', r'''
 addToLibrary({