Re-enable extern-pre-js and extern-post-js under pthreads (#22013)

In #21701 the running of extern-pre/post-js in pthreads was disabled.
The reason was that we had a couple of tests that seemed to be assuming
this.  However, it turns out that there are important use cases that
depend on this code running in threads.

Instead, fix the two test cases by adding an extra condition.

Fixes: #22012

Author: sbc100

test/test_core.py

@@ -9268,7 +9268,7 @@ def test_pthread_offset_converter_modularize(self):
     self.set_setting('USE_OFFSET_CONVERTER')
     self.set_setting('MODULARIZE')
     self.set_setting('EXPORT_NAME', 'foo')
-    create_file('post.js', 'foo();')
+    create_file('post.js', 'if (!isPthread) foo();')
     self.emcc_args += ['--extern-post-js', 'post.js']
     if '-g' in self.emcc_args:
       self.emcc_args += ['-DDEBUG']

test/test_other.py

@@ -366,7 +366,7 @@ def test_emcc_output_mjs(self, args):
   })
   @node_pthreads
   def test_emcc_output_worker_mjs(self, args):
-    create_file('extern-post.js', 'await Module();')
+    create_file('extern-post.js', 'if (!isPthread) await Module();')
     os.mkdir('subdir')
     self.run_process([EMCC, '-o', 'subdir/hello_world.mjs',
                       '-sEXIT_RUNTIME', '-sPROXY_TO_PTHREAD', '-pthread', '-O1',

tools/link.py

@@ -677,13 +677,6 @@ def phase_linker_setup(options, state, newargs):
   options.extern_pre_js = read_js_files(options.extern_pre_js)
   options.extern_post_js = read_js_files(options.extern_post_js)
 
-  if settings.PTHREADS:
-    # Don't run extern pre/post code on pthreads.
-    if options.extern_pre_js:
-      options.extern_pre_js = 'if (!isPthread) {' + options.extern_pre_js + '}'
-    if options.extern_post_js:
-      options.extern_post_js = 'if (!isPthread) {' + options.extern_post_js + '}'
-
   # TODO: support source maps with js_transform
   if options.js_transform and settings.GENERATE_SOURCE_MAP:
     logger.warning('disabling source maps because a js transform is being done')
@@ -2082,7 +2075,7 @@ def phase_final_emitting(options, state, target, wasm_target):
     create_worker_file('src/audio_worklet.js', target_dir, settings.AUDIO_WORKLET_FILE, options)
 
   if settings.MODULARIZE:
-    modularize(options)
+    modularize()
   elif settings.USE_CLOSURE_COMPILER:
     module_export_name_substitution()
 
@@ -2109,8 +2102,6 @@ def phase_final_emitting(options, state, target, wasm_target):
     src = read_file(final_js)
     final_js += '.epp.js'
     with open(final_js, 'w', encoding='utf-8') as f:
-      if settings.PTHREADS and (options.extern_pre_js or options.extern_post_js):
-        f.write(make_pthread_detection())
       f.write(options.extern_pre_js)
       f.write(src)
       f.write(options.extern_post_js)
@@ -2352,34 +2343,7 @@ def node_pthread_detection():
     return "require('worker_threads').workerData === 'em-pthread'\n"
 
 
-def make_pthread_detection():
-  """Create code for detecting if we are running in a pthread.
-
-  Normally this detection is done when the module is itself is run but there
-  are some cases were we need to do this detection outside of the normal
-  module code.
-
-  1. When running in MODULARIZE mode we need use this to know if we should
-     run the module constructor on startup (true only for pthreads)
-  2. When using `--extern-pre-js` and `--extern-post-js` we need to avoid
-     running this code on pthreads.
-  """
-
-  if settings.ENVIRONMENT_MAY_BE_WEB or settings.ENVIRONMENT_MAY_BE_WORKER:
-    code = "var isPthread = globalThis.self?.name === 'em-pthread';\n"
-    # In order to support both web and node we also need to detect node here.
-    if settings.ENVIRONMENT_MAY_BE_NODE:
-      code += "var isNode = typeof globalThis.process?.versions?.node == 'string';\n"
-      code += f'if (isNode) isPthread = {node_pthread_detection()}\n'
-  elif settings.ENVIRONMENT_MAY_BE_NODE:
-    code = f'var isPthread = {node_pthread_detection()}\n'
-  else:
-    assert False
-
-  return code
-
-
-def modularize(options):
+def modularize():
   global final_js
   logger.debug(f'Modularizing, assigning to var {settings.EXPORT_NAME}')
   src = read_file(final_js)
@@ -2461,8 +2425,18 @@ def modularize(options):
 ''' % {'EXPORT_NAME': settings.EXPORT_NAME}
 
   if settings.PTHREADS:
-    if not options.extern_pre_js and not options.extern_post_js:
-      src += make_pthread_detection()
+    # Create code for detecting if we are running in a pthread.
+    # Normally this detection is done when the module is itself run but
+    # when running in MODULARIZE mode we need use this to know if we should
+    # run the module constructor on startup (true only for pthreads).
+    if settings.ENVIRONMENT_MAY_BE_WEB or settings.ENVIRONMENT_MAY_BE_WORKER:
+      src += "var isPthread = globalThis.self?.name === 'em-pthread';\n"
+      # In order to support both web and node we also need to detect node here.
+      if settings.ENVIRONMENT_MAY_BE_NODE:
+        src += "var isNode = typeof globalThis.process?.versions?.node == 'string';\n"
+        src += f'if (isNode) isPthread = {node_pthread_detection()}\n'
+    elif settings.ENVIRONMENT_MAY_BE_NODE:
+      src += f'var isPthread = {node_pthread_detection()}\n'
     src += '// When running as a pthread, construct a new instance on startup\n'
     src += 'isPthread && %s();\n' % settings.EXPORT_NAME