Feedback from emscripten-core#26000. NFC (emscripten-core#26147)

Author: sbc100

ChangeLog.md

@@ -33,7 +33,9 @@ See docs/process.md for more on how version tagging works.
   works with `for...of`/`Array.from()`/spread. (#25993)
 - ASYNCIFY/JSPI functions in JS library files can now be marked as `__async:
   'auto'`, which allows async JS function to be used unmodified with
-  ASYNCIFY/JSPI. (#26130, #26019)
+  ASYNCIFY/JSPI.  In addition, when such function are also marked as `__proxy:
+  'sync'` it is now possible for them to be called from back background threads
+  with the same blocking semantics. (#26130, #26019, #26000)
 - SDL3 port updated to 3.2.30. (#26135)
 
 4.0.23 - 01/10/26

site/source/docs/porting/pthreads.rst

@@ -59,6 +59,10 @@ In addition, Emscripten currently has a simple model of file I/O only happening
 on the main application thread (as we support JS plugin filesystems, which
 cannot share memory); this is another set of operations that are proxied.
 
+When a JS library function is marked as both ``__proxy: 'sync'`` and ``__proxy:
+'async'`` the calling thread will block until the async operation on the
+main thread is completed (i.e. the returned promise is resolved).
+
 Proxying can cause problems in certain cases, see the section on blocking below.
 
 Blocking on the main browser thread

src/lib/libpthread.js

@@ -1224,7 +1224,7 @@ var LibraryPThread = {
   _emscripten_dlsync_threads: () => {
     const callingThread = PThread.currentProxiedOperationCallerThread;
     if (callingThread) {
-      return dlsyncThreadsAsync()
+      return dlsyncThreadsAsync();
     }
     for (const ptr of Object.keys(PThread.pthreads)) {
       const pthread_ptr = Number(ptr);