Don't execute the proxy queue while adding to it from same thread.

system/lib/pthread/proxying.c

@@ -35,6 +35,8 @@ static em_proxying_queue system_proxying_queue = {
   .capacity = 0,
 };
 
+static _Thread_local int system_queue_in_use = 0;
+
 em_proxying_queue* emscripten_proxy_get_system_queue(void) {
   return &system_proxying_queue;
 }
@@ -110,17 +112,24 @@ void emscripten_proxy_execute_queue(em_proxying_queue* q) {
   assert(q != NULL);
   assert(pthread_self());
 
-  // Recursion guard to avoid infinite recursion when we arrive here from the
-  // pthread_lock call below that executes the system queue. The per-task_queue
-  // recursion lock can't catch these recursions because it can only be checked
-  // after the lock has been acquired.
-  static _Thread_local int executing_system_queue = 0;
+  // Below is a recursion and deadlock guard:
+  // The recursion guard is to avoid infinite recursion when we arrive here from
+  // the pthread_lock call below that executes the system queue. The
+  // per-task_queue recursion lock can't catch these recursions because it can
+  // only be checked after the lock has been acquired.
+  // 
+  // This also guards against deadlocks when adding to the system queue. When
+  // the current thread is adding tasks, it locks the queue, but we can
+  // potentially try to execute the queue during the add (from
+  // emscripten_yield). This will deadlock the thread, so only try to take the
+  // lock if the current thread is not using the queue. We then hope the
+  // queue is executed later when it is unlocked.
   int is_system_queue = q == &system_proxying_queue;
   if (is_system_queue) {
-    if (executing_system_queue) {
+    if (system_queue_in_use) {
       return;
     }
-    executing_system_queue = 1;
+    system_queue_in_use = 1;
   }
 
   pthread_mutex_lock(&q->mutex);
@@ -133,14 +142,21 @@ void emscripten_proxy_execute_queue(em_proxying_queue* q) {
   }
 
   if (is_system_queue) {
-    executing_system_queue = 0;
+    system_queue_in_use = 0;
   }
 }
 
 static int do_proxy(em_proxying_queue* q, pthread_t target_thread, task t) {
   assert(q != NULL);
   pthread_mutex_lock(&q->mutex);
+  int is_system_queue = q == &system_proxying_queue;
+  if (is_system_queue) {
+    system_queue_in_use = 1;
+  }
   em_task_queue* tasks = get_or_add_tasks_for_thread(q, target_thread);
+  if (is_system_queue) {
+    system_queue_in_use = 0;
+  }
   pthread_mutex_unlock(&q->mutex);
   if (tasks == NULL) {
     return 0;

test/other/codesize/test_codesize_minimal_pthreads.funcs

@@ -72,6 +72,7 @@ $emscripten_futex_wake
 $emscripten_stack_get_current
 $emscripten_stack_set_limits
 $free_ctx
+$get_or_add_tasks_for_thread
 $get_tasks_for_thread
 $init_active_ctxs
 $init_file_lock

test/other/codesize/test_codesize_minimal_pthreads.size

@@ -1 +1 @@
-19396
+19446

test/other/codesize/test_codesize_minimal_pthreads_memgrowth.funcs

@@ -72,6 +72,7 @@ $emscripten_futex_wake
 $emscripten_stack_get_current
 $emscripten_stack_set_limits
 $free_ctx
+$get_or_add_tasks_for_thread
 $get_tasks_for_thread
 $init_active_ctxs
 $init_file_lock

test/other/codesize/test_codesize_minimal_pthreads_memgrowth.size

@@ -1 +1 @@
-19397
+19447