Make main symbol optional even in STANDALONE_WASM mode (#9641)

See: #9640
Fixes: #9635

Author: sbc100

system/lib/libc/crt1.c

@@ -16,9 +16,19 @@
 #include <stdio.h>
 
 extern void __wasm_call_ctors(void) __attribute__((weak));
-extern int main(int argc, char** argv);
+
+// TODO(sbc): We shouldn't even link this file if there is no main:
+// https://github.com/emscripten-core/emscripten/issues/9640
+extern int main(int argc, char** argv) __attribute__((weak));
 
 void _start(void) {
+  if (!main) {
+    if (__wasm_call_ctors) {
+      __wasm_call_ctors();
+    }
+    return;
+  }
+
   /* Fill in the arguments from WASI syscalls. */
   size_t argc;
   char **argv;

tests/test_core.py

@@ -8213,6 +8213,18 @@ def test_safe_stack_dylink(self):
       }
     ''', ['abort(stack overflow)', '__handle_stack_overflow'], assert_returncode=None)
 
+  @also_with_standalone_wasm
+  def test_undefined_main(self):
+    # By default in emscripten we allow main to be undefined.  Its used when
+    # building library code that has no main.
+    # TODO(sbc): Simplify the code by making this an opt-in feature.
+    # https://github.com/emscripten-core/emscripten/issues/9640
+    src = '''
+    #include <emscripten.h>
+    EMSCRIPTEN_KEEPALIVE void foo() {}
+    '''
+    self.build(src, self.get_dir(), 'test.c')
+
 
 # Generate tests for everything
 def make_run(name, emcc_args, settings=None, env=None):