Avoid exporting/defining main when we try to detect an unexported main (#20971)

In debug builds we attempt to detect and unexported main by adding it to
EXPORT_IF_DEFINED, but then ignoring it.  However we were not fully
ignoring it which resulted in `main` getting run in debug builds but not
in release builds.

Author: sbc100

ChangeLog.md

@@ -20,6 +20,8 @@ See docs/process.md for more on how version tagging works.
 
 3.1.52 (in development)
 -----------------------
+- Fix bug where `main` was mistakenly included in debug builds but not in
+  release builds. (#20971)
 - Remove JAVA from the list of `.emscripten` config file settings.  In the
   past we used this to run the java version of closure compiler.  If there are
   folks who prefer to use the java version of closure compiler for some reason

test/test_other.py

@@ -7923,11 +7923,24 @@ def test(contents, expected, args=[], assert_returncode=0):  # noqa
     # test backwards compatibility
     test("Module['print']('print'); Module['printErr']('err'); ", 'print\nerr', ['-sEXPORTED_RUNTIME_METHODS=print,printErr', '-Wno-js-compiler'])
 
-  def test_warn_unexported_main(self):
-    warning = 'emcc: warning: `main` is defined in the input files, but `_main` is not in `EXPORTED_FUNCTIONS`, which means it may be eliminated as dead code. Export it if you want `main` to run. [-Wunused-main]'
+  @parameterized({
+    '': ([],),
+    'O2': (['-O2'],),
+  })
+  def test_warn_unexported_main(self, args):
+    warning = 'emcc: warning: `main` is defined in the input files, but `_main` is not in `EXPORTED_FUNCTIONS`. Add it to this list if you want `main` to run. [-Wunused-main]'
+
+    proc = self.run_process([EMCC, test_file('hello_world.c'), '-sEXPORTED_FUNCTIONS=[]'] + args, stderr=PIPE)
+    # This warning only shows up when ASSERTIONS are enabled.
+    # We run both ways those to ensure that main doesn't get run in either case.
+    if '-O2' in args:
+      self.assertNotContained(warning, proc.stderr)
+    else:
+      self.assertContained(warning, proc.stderr)
 
-    proc = self.run_process([EMCC, test_file('hello_world.c'), '-sEXPORTED_FUNCTIONS=[]'], stderr=PIPE)
-    self.assertContained(warning, proc.stderr)
+    # Verify that main indeed does not run
+    output = self.run_js('a.out.js')
+    self.assertEqual('', output)
 
   def test_source_file_with_fixed_language_mode(self):
     create_file('src_tmp_fixed_lang', '''

tools/emscripten.py

@@ -568,9 +568,16 @@ def finalize_wasm(infile, outfile, js_syms):
   unexpected_exports = [e for e in metadata.all_exports if treat_as_user_export(e)]
   unexpected_exports = [asmjs_mangle(e) for e in unexpected_exports]
   unexpected_exports = [e for e in unexpected_exports if e not in expected_exports]
+
+  # If `_main` was unexpectedly exported we assume it was added to
+  # EXPORT_IF_DEFINED by emcc.py in order that we can detect it and
+  # report this warning.  In this case we explicitly ignore the export
+  # and run as if there was no main function since that is defined is
+  # behaviour for programs that don't include `_main` in EXPORTED_FUNCTIONS.
   if not settings.STANDALONE_WASM and '_main' in unexpected_exports:
-    diagnostics.warning('unused-main', '`main` is defined in the input files, but `_main` is not in `EXPORTED_FUNCTIONS`, which means it may be eliminated as dead code. Export it if you want `main` to run.')
+    diagnostics.warning('unused-main', '`main` is defined in the input files, but `_main` is not in `EXPORTED_FUNCTIONS`. Add it to this list if you want `main` to run.')
     unexpected_exports.remove('_main')
+    metadata.all_exports.remove('main')
 
   building.user_requested_exports.update(unexpected_exports)
   settings.EXPORTED_FUNCTIONS.extend(unexpected_exports)