zend_async_API: honor the NULL-coroutine contract in context_destroy, lock key_name

zend_async_context_destroy is documented with its siblings as resolving a
NULL coroutine to the current one but dereferenced the argument directly.
zend_async_internal_context_key_name read the process-wide key registry
without the mutex every writer takes, racing a concurrent rehash.

Author: EdmondDantes

Zend/zend_async_API.c

@@ -112,11 +112,18 @@ ZEND_API uint32_t zend_async_internal_context_key_alloc(const char *key_name)
 
 ZEND_API const char *zend_async_internal_context_key_name(uint32_t key)
 {
+	/* A non-NULL table implies the mutex exists: key_alloc creates the mutex
+	 * before the table. The lock shields the read from a concurrent alloc
+	 * rehashing the bucket array. */
 	if (internal_context_key_names == NULL) {
 		return NULL;
 	}
 
-	return zend_hash_index_find_ptr(internal_context_key_names, key);
+	tsrm_mutex_lock(internal_context_mutex);
+	const char *name = zend_hash_index_find_ptr(internal_context_key_names, key);
+	tsrm_mutex_unlock(internal_context_mutex);
+
+	return name;
 }
 
 static void internal_context_keys_shutdown(void)
@@ -383,6 +390,14 @@ ZEND_API bool zend_async_context_unset(zend_coroutine_t *coroutine, zval *key)
 
 ZEND_API void zend_async_context_destroy(zend_coroutine_t *coroutine)
 {
+	if (coroutine == NULL) {
+		coroutine = ZEND_ASYNC_CURRENT_COROUTINE;
+
+		if (coroutine == NULL) {
+			return;
+		}
+	}
+
 	if (coroutine->context != NULL) {
 		OBJ_RELEASE(coroutine->context);
 		coroutine->context = NULL;