Revert cancelDone removal

Author: kleisauke

src/library_pthread.js

@@ -280,6 +280,8 @@ var LibraryPThread = {
           assert(worker.pthread);
 #endif
           PThread.returnWorkerToPool(worker);
+        } else if (cmd === 'cancelDone') {
+          PThread.returnWorkerToPool(worker);
         } else if (d.target === 'setimmediate') {
           // Worker wants to postMessage() to itself to implement setImmediate()
           // emulation.
@@ -458,17 +460,19 @@ var LibraryPThread = {
 #endif
     var pthread = PThread.pthreads[pthread_ptr];
     // If pthread has been removed from this map this also means that pthread_ptr points
-    // to already freed data. Such situation may occur in following circumstance:
-    // Joining thread from non-main browser thread (this also includes thread running main()
-    // when compiled with `PROXY_TO_PTHREAD`) - in such situation it may happen that following
-    // code flow occur (MB - Main Browser Thread, S1, S2 - Worker Threads):
-    // S2: thread ends, 'exit' message is sent to MB
-    // S1: calls pthread_join(S2), this causes:
-    //     a. S2 is marked as detached,
-    //     b. 'cleanupThread' message is sent to MB.
-    // MB: handles 'exit' message, as thread is detached, so returnWorkerToPool()
-    //     is called and all thread related structs are freed/released.
-    // MB: handles 'cleanupThread' message which calls this function.
+    // to already freed data. Such situation may occur in following circumstances:
+    // 1. Joining cancelled thread - in such situation it may happen that pthread data will
+    //    already be removed by handling 'cancelDone' message.
+    // 2. Joining thread from non-main browser thread (this also includes thread running main()
+    //    when compiled with `PROXY_TO_PTHREAD`) - in such situation it may happen that following
+    //    code flow occur (MB - Main Browser Thread, S1, S2 - Worker Threads):
+    //    S2: thread ends, 'exit' message is sent to MB
+    //    S1: calls pthread_join(S2), this causes:
+    //        a. S2 is marked as detached,
+    //        b. 'cleanupThread' message is sent to MB.
+    //    MB: handles 'exit' message, as thread is detached, so returnWorkerToPool()
+    //        is called and all thread related structs are freed/released.
+    //    MB: handles 'cleanupThread' message which calls this function.
     if (pthread) {
       {{{ makeSetValue('pthread_ptr', C_STRUCTS.pthread.self, 0, 'i32') }}};
       var worker = pthread.worker;

src/worker.js

@@ -280,6 +280,7 @@ self.onmessage = function(e) {
       if (Module['_pthread_self']()) {
         Module['__emscripten_thread_exit'](-1/*PTHREAD_CANCELED*/);
       }
+      postMessage({ 'cmd': 'cancelDone' });
     } else if (e.data.target === 'setimmediate') {
       // no-op
     } else if (e.data.cmd === 'processThreadQueue') {