Allow pthread-based programs to be loaded cross-origin

ChangeLog.md

@@ -22,6 +22,8 @@ See docs/process.md for more on how version tagging works.
 -----------------------
 - `-sUSE_WEBGPU` was removed in favor of the external port Emdawnwebgpu which
   are used via `--use-port=emdawnwebgpu`. See 4.0.10 release notes for details.
+- A new `CROSS_ORIGIN` setting was added in order to work around issues hosting
+  emscripten programs across different origins (#25581)
 
 4.0.17 - 10/17/25
 -----------------

site/source/docs/tools_reference/settings_reference.rst

@@ -3341,3 +3341,15 @@ It's currently only available under a flag in certain browsers,
 so we disable it by default to save on code size.
 
 Default value: false
+
+.. _cross_origin:
+
+CROSS_ORIGIN
+============
+
+If the emscripten-generated program is hosted on separate origin then
+starting new pthread worker can violate CSP rules.  Enabling
+CROSS_ORIGIN uses an inline worker to instead load the worker script
+indirectly using `importScripts`
+
+Default value: false

src/lib/libpthread.js

@@ -454,6 +454,16 @@ var LibraryPThread = {
       worker = new Worker(new URL('{{{ pthreadWorkerScript }}}', import.meta.url), {{{ pthreadWorkerOptions }}});
 #else // EXPORT_ES6
       var pthreadMainJs = _scriptName;
+#if CROSS_ORIGIN && ENVIRONMENT_MAY_BE_WEB
+      // In order to support cross origin loading of worker threads load the
+      // worker via a tiny inline `importScripts` call.   For some reason its
+      // fine to `importScripts` across origins, in cases where new Worker
+      // itself does not allow this.
+      // https://github.com/emscripten-core/emscripten/issues/21937
+      if (ENVIRONMENT_IS_WEB) {
+        pthreadMainJs = URL.createObjectURL(new Blob([`importScripts('${_scriptName}')`], { type: 'application/javascript' }));
+      }
+#endif
 #if expectToReceiveOnModule('mainScriptUrlOrBlob')
       // We can't use makeModuleReceiveWithVar here since we want to also
       // call URL.createObjectURL on the mainScriptUrlOrBlob.

src/settings.js

@@ -2187,6 +2187,12 @@ var GROWABLE_ARRAYBUFFERS = false;
 // so we disable it by default to save on code size.
 var WASM_JS_TYPES = false;
 
+// If the emscripten-generated program is hosted on separate origin then
+// starting new pthread worker can violate CSP rules.  Enabling
+// CROSS_ORIGIN uses an inline worker to instead load the worker script
+// indirectly using `importScripts`
+var CROSS_ORIGIN = false;
+
 // For renamed settings the format is:
 // [OLD_NAME, NEW_NAME]
 // For removed settings (which now effectively have a fixed value and can no

test/test_browser.py

@@ -15,7 +15,7 @@
 import unittest
 import zlib
 from functools import wraps
-from http.server import BaseHTTPRequestHandler, HTTPServer
+from http.server import BaseHTTPRequestHandler, HTTPServer, ThreadingHTTPServer, SimpleHTTPRequestHandler
 from pathlib import Path
 from urllib.request import urlopen
 
@@ -5631,6 +5631,46 @@ def test_rollup(self):
     shutil.copy('hello.wasm', 'dist/')
     self.run_browser('index.html', '/report_result?exit:0')
 
+  def test_cross_origin(self):
+    # Verfies that the emscripten-generted JS and Wasm can be hosted on a different origin.
+    # This test create a second HTTP server running on port 9999 that servers files from `subdir`.
+    # The main html is the servers from the normal 8888 server while the JS and Wasm are hosted
+    # on at 9999.
+    os.mkdir('subdir')
+    create_file('subdir/foo.txt', 'hello')
+    self.compile_btest('hello_world.c', ['-o', 'subdir/hello.js', '-sCROSS_ORIGIN', '-sPROXY_TO_PTHREAD', '-pthread', '-sEXIT_RUNTIME'])
+
+    class MyReqestHandler(SimpleHTTPRequestHandler):
+      def __init__(self, *args, **kwargs):
+        super().__init__(*args, directory='subdir', **kwargs)
+
+      # Add COOP, COEP, CORP, and no-caching headers
+      def end_headers(self):
+        self.send_header('Accept-Ranges', 'bytes')
+        self.send_header('Access-Control-Allow-Origin', '*')
+        self.send_header('Cross-Origin-Opener-Policy', 'same-origin')
+        self.send_header('Cross-Origin-Embedder-Policy', 'require-corp')
+        self.send_header('Cross-Origin-Resource-Policy', 'cross-origin')
+
+        self.send_header('Cache-Control', 'no-cache, no-store, must-revalidate, private, max-age=0')
+        self.send_header('Expires', '0')
+        self.send_header('Pragma', 'no-cache')
+        self.send_header('Vary', '*') # Safari insists on caching if this header is not present in addition to the above
+
+        return SimpleHTTPRequestHandler.end_headers(self)
+
+    create_file('test.html', '''
+      <script src="http://localhost:9999/hello.js"></script>
+    ''')
+
+    server = HttpServerThread(ThreadingHTTPServer(('localhost', 9999), MyReqestHandler))
+    server.start()
+    try:
+      self.run_browser('test.html', '/report_result?exit:0')
+    finally:
+      server.stop()
+      server.join()
+
 
 class emrun(RunnerCore):
   def test_emrun_info(self):