Fix a use-after-free bug for detached threads (#420)

* Fix a use-after-free bug for detached threads

the issue was pointed out by @alexcrichton in
#405

* Rename map_base_lazy_free_queue as it only keeps a single item

Also, align the comment style with musl.

libc-top-half/musl/src/thread/pthread_create.c

@@ -60,6 +60,17 @@ void __tl_sync(pthread_t td)
 	if (tl_lock_waiters) __wake(&__thread_list_lock, 1, 0);
 }
 
+#ifndef __wasilibc_unmodified_upstream
+static void *map_base_deferred_free;
+
+static void process_map_base_deferred_free()
+{
+	/* called with __tl_lock held */
+	free(map_base_deferred_free);
+	map_base_deferred_free = NULL;
+}
+#endif
+
 #ifdef __wasilibc_unmodified_upstream
 _Noreturn void __pthread_exit(void *result)
 #else
@@ -182,7 +193,17 @@ static void __pthread_exit(void *result)
 	}
 #else
 	if (state==DT_DETACHED && self->map_base) {
-		free(self->map_base);
+		/* As we use malloc/free which is considerably more complex
+		 * than mmap/munmap to call and can even require a valid
+		 * thread context, it's difficult to implement __unmapself.
+		 *
+		 * Here we take an alternative approach which simply defers
+		 * the deallocation. An obvious downside of this approach is
+		 * that it keeps the stack longer. (possibly forever.)
+		 * To avoid wasting too much memory, we only defer a single
+		 * item at most. */
+		process_map_base_deferred_free();
+		map_base_deferred_free = self->map_base;
 		// Can't use `exit()` here, because it is too high level
 		return;
 	}
@@ -424,6 +445,14 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 			if (map == MAP_FAILED) goto fail;
 		}
 #else
+		/* Process the deferred free request if any before
+		 * allocationg a new one. Hopefully it enables a reuse of the memory.
+		 *
+		 * Note: We can't perform a simple "handoff" becasue allocation
+		 * sizes might be different. (eg. the stack size might differ) */
+		__tl_lock();
+		process_map_base_deferred_free();
+		__tl_unlock();
 		map = malloc(size);
 		if (!map) goto fail;
 #endif