Add _emscripten_thread_is_valid helper. NFC (#17020)

Author: sbc100

system/lib/libc/musl/src/thread/pthread_detach.c

@@ -3,10 +3,12 @@
 
 static int __pthread_detach(pthread_t t)
 {
+#ifdef __EMSCRIPTEN__
 	// XXX EMSCRIPTEN: Add check for invalid (already joined) thread.  Again
 	// for the benefit of the conformance tests.
-	if (t->self != t)
+	if (!_emscripten_thread_is_valid(t))
 		return ESRCH;
+#endif
 	/* If the cas fails, detach state is either already-detached
 	 * or exiting/exited, and pthread_join will trap or cleanup. */
 	if (a_cas(&t->detach_state, DT_JOINABLE, DT_DETACHED) != DT_JOINABLE)

system/lib/libc/musl/src/thread/pthread_join.c

@@ -16,10 +16,11 @@ static int __pthread_timedjoin_np(pthread_t t, void **res, const struct timespec
 #ifdef __EMSCRIPTEN__
 	// Attempt to join a thread which does not point to a valid thread, or
 	// does not exist anymore.
-	if (t->self != t) return ESRCH;
+	if (!_emscripten_thread_is_valid(t)) return ESRCH;
 	// Thread is attempting to join to itself.  Already detached threads are
 	// handled below by returning EINVAL instead.
-	// TODO: The detached check here is just to satisfy the `other.test_{proxy,main}_pthread_join_detach` tests.
+	// TODO: The detached check here is just to satisfy the
+	// `other.test_{proxy,main}_pthread_join_detach` tests.
 	if (t->detach_state != DT_DETACHED && __pthread_self() == t) return EDEADLK;
 #endif
 	int state, cs, r = 0;

system/lib/pthread/library_pthread.c

@@ -657,6 +657,10 @@ int emscripten_dispatch_to_thread_async_(pthread_t target_thread,
   return ret;
 }
 
+int _emscripten_thread_is_valid(pthread_t thread) {
+  return thread->self == thread;
+}
+
 static void *dummy_tsd[1] = { 0 };
 weak_alias(dummy_tsd, __pthread_tsd_main);
 

system/lib/pthread/threading_internal.h

@@ -99,6 +99,12 @@ typedef struct thread_profiler_block {
 void __emscripten_init_main_thread_js(void* tb);
 void _emscripten_thread_profiler_enable();
 
+// Checks certain structural invariants.  This allows us to detect when
+// already-freed threads are used in some APIs.  Technically this is undefined
+// behaviour, but we have a couple of places where we add these checks so that
+// we can pass more of the posixtest suite that vanilla musl.
+int _emscripten_thread_is_valid(pthread_t thread);
+
 #ifdef NDEBUG
 #define emscripten_set_current_thread_status(newStatus)
 #define emscripten_conditional_set_current_thread_status(expectedStatus, newStatus)