Fix emrun arg handling (#21884)

Fixes: #21883

Author: sbc100

emrun.py

@@ -1568,15 +1568,14 @@ def parse_args(args):
   parser.add_argument('cmdlineparams', nargs='*')
 
   # Support legacy argument names with `_` in them (but don't
-  # advertize these in the --help message.
-  newargs = []
-  for a in args:
+  # advertize these in the --help message).
+  for i, a in enumerate(args):
+    if a == '--':
+      break
     if a.startswith('--') and '_' in a:
-      newargs.append(a.replace('_', '-'))
-    else:
-      newargs.append(a)
+      args[i] = a.replace('_', '-')
 
-  return parser.parse_args(newargs)
+  return parser.parse_args(args)
 
 
 def run(args):

test/test_browser.py

@@ -5594,7 +5594,7 @@ def test_emrun(self):
         args_base + ['--private_browsing', '--port', '6941'],
         args_base + ['--dump_out_directory', 'other dir/multiple', '--port', '6942']
     ]:
-      args += [self.in_dir('hello_world.html'), '--', '1', '2', '--3', 'escaped space']
+      args += [self.in_dir('hello_world.html'), '--', '1', '2', '--3', 'escaped space', 'with_underscore']
       print(shared.shlex_join(args))
       proc = self.run_process(args, check=False)
       self.assertEqual(proc.returncode, 100)
@@ -5607,6 +5607,7 @@ def test_emrun(self):
       self.assertContained('argc: 5', stdout)
       self.assertContained('argv[3]: --3', stdout)
       self.assertContained('argv[4]: escaped space', stdout)
+      self.assertContained('argv[5]: with_underscore', stdout)
       self.assertContained('hello, world!', stdout)
       self.assertContained('Testing ASCII characters: !"$%&\'()*+,-./:;<=>?@[\\]^_`{|}~', stdout)
       self.assertContained('Testing char sequences: %20%21 &auml;', stdout)