[esm-integration] Allow modules to be runnable even without main (#24321)

Also, ensure the output format defaults to `MJS` in esm integration and
modularize=instance modes.

Author: sbc100

.circleci/config.yml

@@ -802,7 +802,6 @@ jobs:
             core0.test_pthread_join_and_asyncify
             core0.test_async_ccall_promise_jspi*
             core0.test_cubescript_jspi
-            esm_integration.test_fs_js_api*
             instance.test_hello_world
             instance.test_dylink_basics
             instance.test_cube2hash*
@@ -812,8 +811,10 @@ jobs:
             instance.test_iostream_and_determinism
             instance.test_fannkuch
             instance.test_fasta
+            esm_integration.test_fs_js_api*
             esm_integration.test_inlinejs3
             esm_integration.test_embind_val_basics
+            esm_integration.test_undefined_main
             "
       # Run some basic tests with the minimum version of node that we currently
       # support in the generated code.

test/core/test_esm_integration.expected.mjs

@@ -3,7 +3,8 @@
 import * as unused from './hello_world.wasm';
 export { default, _foo, _main, err, stringToNewUTF8 } from './hello_world.support.mjs';
 
-// When run as the main module under node, execute main directly here
+// When run as the main module under node, create the module directly.  This will
+// execute any startup code along with main (if it exists).
 import init from './hello_world.support.mjs';
 const isNode = typeof process == 'object' && typeof process.versions == 'object' && typeof process.versions.node == 'string' && process.type != 'renderer';
 if (isNode) {

tools/link.py

@@ -749,6 +749,11 @@ def phase_linker_setup(options, linker_args):  # noqa: C901, PLR0912, PLR0915
     if s in user_settings:
       diagnostics.warning('deprecated', f'{s} is deprecated ({reason}). Please open a bug if you have a continuing need for this setting')
 
+  # Set the EXPORT_ES6 default early since it affects the setting of the
+  # default oformat below.
+  if settings.WASM_ESM_INTEGRATION or settings.MODULARIZE == 'instance':
+    default_setting('EXPORT_ES6', 1)
+
   # If no output format was specified we try to deduce the format based on
   # the output filename extension
   if not options.oformat and (options.relocatable or (options.shared and not settings.SIDE_MODULE)):
@@ -766,10 +771,10 @@ def phase_linker_setup(options, linker_args):  # noqa: C901, PLR0912, PLR0915
   if not options.oformat:
     if settings.SIDE_MODULE or final_suffix == '.wasm':
       options.oformat = OFormat.WASM
-    elif final_suffix == '.mjs':
-      options.oformat = OFormat.MJS
     elif final_suffix == '.html':
       options.oformat = OFormat.HTML
+    elif final_suffix == '.mjs' or settings.EXPORT_ES6:
+      options.oformat = OFormat.MJS
     else:
       options.oformat = OFormat.JS
 
@@ -786,10 +791,11 @@ def phase_linker_setup(options, linker_args):  # noqa: C901, PLR0912, PLR0915
 
   if settings.WASM_ESM_INTEGRATION:
     diagnostics.warning('experimental', '-sWASM_ESM_INTEGRATION is still experimental and not yet supported in browsers')
-    default_setting('EXPORT_ES6', 1)
     default_setting('MODULARIZE', 'instance')
-    if not settings.EXPORT_ES6 or settings.MODULARIZE != 'instance':
-      exit_with_error('WASM_ESM_INTEGRATION requires EXPORT_ES6 and MODULARIZE=instance')
+    if options.oformat != OFormat.MJS:
+      exit_with_error('WASM_ESM_INTEGRATION is only compatible with EM module output format')
+    if settings.MODULARIZE != 'instance':
+      exit_with_error('WASM_ESM_INTEGRATION requires MODULARIZE=instance')
     if settings.RELOCATABLE:
       exit_with_error('WASM_ESM_INTEGRATION is not compatible with dynamic linking')
 
@@ -806,11 +812,11 @@ def limit_incoming_module_api():
   if settings.MODULARIZE == 'instance':
     diagnostics.warning('experimental', '-sMODULARIZE=instance is still experimental. Many features may not work or will change.')
     if options.oformat != OFormat.MJS:
-      exit_with_error('emcc: MODULARIZE instance is only compatible with .mjs output files')
+      exit_with_error('MODULARIZE instance is only compatible with ES module output format')
     limit_incoming_module_api()
     for s in ['wasmMemory', 'INITIAL_MEMORY']:
       if s in settings.INCOMING_MODULE_JS_API:
-        exit_with_error(f'emcc: {s} cannot be in INCOMING_MODULE_JS_API in MODULARIZE=instance mode')
+        exit_with_error(f'{s} cannot be in INCOMING_MODULE_JS_API in MODULARIZE=instance mode')
 
   if options.oformat in (OFormat.WASM, OFormat.BARE):
     if options.emit_tsd:
@@ -826,7 +832,7 @@ def limit_incoming_module_api():
     wasm_target = get_secondary_target(target, '.wasm')
 
   if settings.SAFE_HEAP not in [0, 1, 2]:
-    exit_with_error('emcc: SAFE_HEAP must be 0, 1 or 2')
+    exit_with_error('SAFE_HEAP must be 0, 1 or 2')
 
   if not settings.WASM:
     # When the user requests non-wasm output, we enable wasm2js. that is,
@@ -2130,9 +2136,10 @@ def create_esm_wrapper(wrapper_file, support_target, wasm_target):
   else:
     wrapper.append(f"export {{ default }} from '{support_url}';")
 
-  if settings.ENVIRONMENT_MAY_BE_NODE and settings.INVOKE_RUN and settings.EXPECT_MAIN:
+  if settings.ENVIRONMENT_MAY_BE_NODE:
     wrapper.append(f'''
-// When run as the main module under node, execute main directly here
+// When run as the main module under node, create the module directly.  This will
+// execute any startup code along with main (if it exists).
 import init from '{support_url}';
 const isNode = typeof process == 'object' && typeof process.versions == 'object' && typeof process.versions.node == 'string' && process.type != 'renderer';
 if (isNode) {{