Revert ->tid simplification

Author: kleisauke

system/lib/pthread/library_pthread.c

@@ -101,7 +101,7 @@ void emscripten_main_thread_process_queued_calls() {
 }
 
 int _emscripten_thread_is_valid(pthread_t thread) {
-  return thread->tid;
+  return thread->self == thread;
 }
 
 static void *dummy_tsd[1] = { 0 };
@@ -110,7 +110,8 @@ weak_alias(dummy_tsd, __pthread_tsd_main);
 // See system/lib/README.md for static constructor ordering.
 __attribute__((constructor(48)))
 void _emscripten_init_main_thread(void) {
-  // The pthread struct has a field that points to itself.
+  // The pthread struct has a field that points to itself - this is used as
+  // a magic ID to detect whether the pthread_t structure is 'alive'.
   __main_pthread.self = &__main_pthread;
   __main_pthread.detach_state = DT_JOINABLE;
   // pthread struct robust_list head should point to itself.

system/lib/pthread/pthread_create.c

@@ -160,10 +160,9 @@ int __pthread_create(pthread_t* restrict res,
   new->map_base = block;
   new->map_size = size;
 
-  // The pthread struct has a field that points to itself.
+  // The pthread struct has a field that points to itself - this is used as a
+  // magic ID to detect whether the pthread_t structure is 'alive'.
   new->self = new;
-
-  // Thread ID, this becomes zero when the thread is no longer available.
   new->tid = _emscripten_get_next_tid();
 
   // pthread struct robust_list head should point to itself.
@@ -280,10 +279,10 @@ void _emscripten_thread_free_data(pthread_t t) {
     emscripten_builtin_free(t->profilerBlock);
   }
 #endif
-  // The tid may be reused. Clear it to prevent inadvertent use
-  // and inform functions that would use it that it's no longer
-  // available.
-  t->tid = 0;
+  // Clear the self-reference to prevent inadvertent use and
+  // inform functions that validate it that the thread is no
+  // longer available.
+  t->self = NULL;
 
   // Free the entire thread block (called map_base because
   // musl normally allocates this using mmap).  This region