Fix EM_CALLBACK_THREAD_CONTEXT_CALLING_THREAD when used on the main thread. (#19943)

Back in #19691 I tried to fix EM_CALLBACK_THREAD_CONTEXT_CALLING_THREAD
but I broken the case where it was used in pthread applications but
on the main thread itself.

We were not previously testing this case because most of our testing
was don't using PROXY_TO_PTHREAD, so the calls we originating from
a non-main thread.

Fixes: #19921

Author: sbc100

src/library_html5.js

@@ -225,11 +225,9 @@ var LibraryHTML5 = {
         case {{{ cDefs.EM_CALLBACK_THREAD_CONTEXT_CALLING_THREAD }}}:
           // The event callback for the current event should be backproxied to
           // the thread that is registering the event.
-#if ASSERTIONS
-          // If we get here PThread.currentProxiedOperationCallerThread should
-          // be set to the calling thread.
-          assert(PThread.currentProxiedOperationCallerThread);
-#endif
+          // This can be 0 in the case that the caller uses
+          // EM_CALLBACK_THREAD_CONTEXT_CALLING_THREAD but on the main thread
+          // itself.
           return PThread.currentProxiedOperationCallerThread;
         default:
           // The event callback for the current event should be proxied to the

src/library_pthread.js

@@ -1013,7 +1013,6 @@ var LibraryPThread = {
     // HTML5 DOM events handlers such as
     // emscripten_set_mousemove_callback()), so keep track in a globally
     // accessible variable about the thread that initiated the proxying.
-    PThread.currentProxiedOperationCallerThread = callingThread;
 #if WASM_BIGINT
     numCallArgs /= 2;
 #endif
@@ -1043,7 +1042,10 @@ var LibraryPThread = {
 #if ASSERTIONS
     assert(func.length == numCallArgs, 'Call args mismatch in _emscripten_receive_on_main_thread_js');
 #endif
-    return func.apply(null, proxiedJSCallArgs);
+    PThread.currentProxiedOperationCallerThread = callingThread;
+    var rtn = func.apply(null, proxiedFunctionTable);
+    PThread.currentProxiedOperationCallerThread = 0;
+    return rtn;
   },
 
   $establishStackSpace__internal: true,

test/other/metadce/test_metadce_minimal_pthreads.jssize

@@ -1 +1 @@
-14374
+14391

test/test_browser.py

@@ -2636,7 +2636,8 @@ def test_doublestart_bug(self):
   @parameterized({
     '': ([],),
     'closure': (['-O2', '-g1', '--closure=1', '-sHTML5_SUPPORT_DEFERRING_USER_SENSITIVE_REQUESTS=0'],),
-    'pthread': (['-pthread', '-sPROXY_TO_PTHREAD'],),
+    'pthread': (['-pthread'],),
+    'proxy_to_pthread': (['-pthread', '-sPROXY_TO_PTHREAD'],),
     'legacy': (['-sMIN_FIREFOX_VERSION=0', '-sMIN_SAFARI_VERSION=0', '-sMIN_IE_VERSION=0', '-sMIN_EDGE_VERSION=0', '-sMIN_CHROME_VERSION=0', '-Wno-transpile'],)
   })
   @requires_threads

test/test_interactive.py

@@ -237,13 +237,17 @@ def test_html5_callbacks_on_calling_thread(self):
     for args in [[], ['-DTEST_SYNC_BLOCKING_LOOP=1']]:
       self.btest('html5_callbacks_on_calling_thread.c', expected='1', args=args + ['-sDISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR', '-pthread', '-sPROXY_TO_PTHREAD'])
 
-  # Test that it is possible to register HTML5 event callbacks on either main browser thread, or application main thread,
-  # and that the application can manually proxy the event from main browser thread to the application main thread, to
-  # implement event suppression capabilities.
-  def test_html5_callback_on_two_threads(self):
+  # Test that it is possible to register HTML5 event callbacks on either main browser thread, or
+  # application main thread, and that the application can manually proxy the event from main browser
+  # thread to the application main thread, to implement event suppression capabilities.
+  @parameterized({
+    '': ([],),
+    'pthread': (['-pthread'],),
+    'proxy_to_pthread': (['-pthread', '-sPROXY_TO_PTHREAD'],),
+  })
+  def test_html5_event_callback_in_two_threads(self, args):
     # TODO: Make this automatic by injecting enter key press in e.g. shell html file.
-    for args in [[], ['-pthread', '-sPROXY_TO_PTHREAD']]:
-      self.btest('html5_event_callback_in_two_threads.c', expected='1', args=args)
+    self.btest('html5_event_callback_in_two_threads.c', expected='1', args=args)
 
   # Test that emscripten_hide_mouse() is callable from pthreads (and proxies to main thread to obtain the proper window.devicePixelRatio value).
   def test_emscripten_hide_mouse(self):