Add AUTO_DETECT_MAIN setting

This settings, when enabled, forces the developer to be explicit
about the presence or absence of `main` their program.  It is disabled
by default but enabled in STRICT mode.

In this mode, if you want to build a program without a `main` function
you must either build with `--no-entry` or pass a list of
EXPORTED_FUNCTIONS the doesn't include `_main`.

The reason for adding the extra `--no-entry` option is that this
is what `wasm-ld` recommends in its corresponding error message.  Its
also a lot easier to type than `-s EXPORTED_FUNCTIONS=[]`.

This change is a step towards enabling better error reporting from lld
by enabling the removal of the `--allow-undefined` option.

emcc.py

@@ -260,6 +260,7 @@ def __init__(self):
     # Whether we will expand the full path of any input files to remove any
     # symlinks.
     self.expand_symlinks = True
+    self.no_entry = False
 
 
 def use_source_map(options):
@@ -1180,6 +1181,9 @@ def get_last_setting_change(setting):
     # Apply -s settings in newargs here (after optimization levels, so they can override them)
     apply_settings(settings_changes)
 
+    if options.no_entry and '_main' in shared.Settings.EXPORTED_FUNCTIONS:
+      shared.Settings.EXPORTED_FUNCTIONS.remove('_main')
+
     shared.verify_settings()
 
     def filter_out_dynamic_libs(inputs):
@@ -1214,6 +1218,7 @@ def check(input_file):
       shared.Settings.STRICT_JS = 1
       shared.Settings.AUTO_JS_LIBRARIES = 0
       shared.Settings.AUTO_ARCHIVE_INDEXES = 0
+      shared.Settings.IGNORE_MISSING_MAIN = 0
 
     # If set to 1, we will run the autodebugger (the automatic debugging tool, see
     # tools/autodebugger).  Note that this will disable inclusion of libraries. This
@@ -2914,6 +2919,9 @@ def consume_arg():
       options.shell_path = consume_arg()
     elif check_arg('--source-map-base'):
       options.source_map_base = consume_arg()
+    elif newargs[i] == '--no-entry':
+      options.no_entry = True
+      newargs[i] = ''
     elif check_arg('--js-library'):
       shared.Settings.SYSTEM_JS_LIBRARIES.append(os.path.abspath(consume_arg()))
     elif newargs[i] == '--remove-duplicates':

emscripten.py

@@ -920,6 +920,11 @@ def report_missing_symbols(all_implemented, pre):
       continue
     diagnostics.warning('undefined', 'undefined exported function: "%s"', requested)
 
+  # Handle main specially, unless IGNORE_MISSING_MAIN is set
+  if not shared.Settings.IGNORE_MISSING_MAIN:
+    if '_main' in shared.Settings.EXPORTED_FUNCTIONS and '_main' not in all_implemented:
+      exit_with_error('entry symbol not defined (pass --no-entry to suppress): main')
+
 
 def get_exported_implemented_functions(all_exported_functions, all_implemented, metadata):
   funcs = set(metadata['exports'])

src/settings.js

@@ -941,11 +941,18 @@ var LINKABLE = 0;
 //   * The C define EMSCRIPTEN is not defined (__EMSCRIPTEN__ always is, and
 //     is the correct thing to use).
 //   * STRICT_JS is enabled.
+//   * IGNORE_MISSING_MAIN is disabled.
 //   * AUTO_JS_LIBRARIES is disabled.
 //   * AUTO_ARCHIVE_INDEXES is disabled.
 // [compile+link]
 var STRICT = 0;
 
+// Allow program to link with or without `main` symbol.
+// If this is disabled then one must provide a `main` symbol or expclictly
+// opt out by passing `--no-entry` or an EXPORTED_FUNCTIONS list that doesn't
+// include `_main`.
+var IGNORE_MISSING_MAIN = 1;
+
 // Automatically attempt to add archive indexes at link time to archives that 
 // don't already have them.  This can happen when GNU ar or GNU ranlib is used
 // rather than `llvm-ar` or `emar` since the former don't understand the wasm

tests/test_core.py

@@ -1613,17 +1613,17 @@ def test_inherit(self):
     self.do_run_in_out_file_test('tests', 'core', 'test_inherit')
 
   def test_isdigit_l(self):
-      # needs to flush stdio streams
-      self.set_setting('EXIT_RUNTIME', 1)
-      self.do_run_in_out_file_test('tests', 'core', 'test_isdigit_l')
+    # needs to flush stdio streams
+    self.set_setting('EXIT_RUNTIME', 1)
+    self.do_run_in_out_file_test('tests', 'core', 'test_isdigit_l')
 
   def test_iswdigit(self):
-      # needs to flush stdio streams
-      self.set_setting('EXIT_RUNTIME', 1)
-      self.do_run_in_out_file_test('tests', 'core', 'test_iswdigit')
+    # needs to flush stdio streams
+    self.set_setting('EXIT_RUNTIME', 1)
+    self.do_run_in_out_file_test('tests', 'core', 'test_iswdigit')
 
   def test_polymorph(self):
-      self.do_run_in_out_file_test('tests', 'core', 'test_polymorph')
+    self.do_run_in_out_file_test('tests', 'core', 'test_polymorph')
 
   def test_complex(self):
     self.do_run_in_out_file_test('tests', 'core', 'test_complex')
@@ -8862,6 +8862,22 @@ def test_minimal_runtime_emscripten_get_exported_function(self):
     self.emcc_args += ['-lexports.js', '-s', 'MINIMAL_RUNTIME=1']
     self.do_run_in_out_file_test('tests', 'core', 'test_get_exported_function')
 
+  def test_auto_detect_main(self):
+    self.do_run_in_out_file_test('tests', 'core', 'test_ctors_no_main')
+
+    # Disabling IGNORE_MISSING_MAIN should cause link to fail due to missing main
+    self.set_setting('IGNORE_MISSING_MAIN', 0)
+    err = self.expect_fail([PYTHON, EMCC, path_from_root('tests', 'core', 'test_ctors_no_main.cpp')] + self.get_emcc_args())
+    self.assertContained('error: entry symbol not defined (pass --no-entry to suppress): main', err)
+
+    # We can fix the error either by adding --no-entry or by setting EXPORTED_FUNCTIONS to empty
+    self.emcc_args.append('--no-entry')
+    self.do_run_in_out_file_test('tests', 'core', 'test_ctors_no_main')
+
+    self.emcc_args.remove('--no-entry')
+    self.set_setting('EXPORTED_FUNCTIONS', [])
+    self.do_run_in_out_file_test('tests', 'core', 'test_ctors_no_main')
+
 
 # Generate tests for everything
 def make_run(name, emcc_args, settings=None, env=None):

tools/shared.py

@@ -1812,8 +1812,13 @@ def link_lld(args, target, opts=[], all_external_symbols=None):
         '--initial-memory=%d' % Settings.INITIAL_MEMORY,
       ]
       use_start_function = Settings.STANDALONE_WASM
+
       if not use_start_function:
-        cmd += ['--no-entry']
+        has_main = '_main' in Settings.EXPORTED_FUNCTIONS
+        if has_main and not Settings.IGNORE_MISSING_MAIN:
+          cmd += ['--entry=main']
+        else:
+          cmd += ['--no-entry']
       if not Settings.ALLOW_MEMORY_GROWTH:
         cmd.append('--max-memory=%d' % Settings.INITIAL_MEMORY)
       elif Settings.MAXIMUM_MEMORY != -1: