async_scheduler_hook: make php_async_starting thread-local

It is the only channel handing a fresh fiber context its coroutine handle;
as a plain static, one ZTS thread's write could be clobbered by another's
before the first entry consumed it, starting a coroutine with a foreign
handle.

Author: EdmondDantes

ext/async_scheduler_hook/async_scheduler_hook.c

@@ -413,8 +413,9 @@ static void php_async_record_current(bool ok, zval *retval)
 static zend_function php_async_root_function = { ZEND_INTERNAL_FUNCTION };
 
 /* Set by the switcher before a first entry: a fresh context has no other way
- * to reach its handle. */
-static php_coroutine_t *php_async_starting = NULL;
+ * to reach its handle. Thread-local: the write and the first-entry read must
+ * stay on the same thread. */
+ZEND_TLS php_coroutine_t *php_async_starting = NULL;
 
 /* First-run trampoline: install a VM stack, run the body, return to the last
  * switcher (the engine's trampoline marks the context DEAD). */