Convert Exception::getMessage() result to string

We specify that the return type of Exception::getMessage() is a
string. However, we don't currently ensure this, because
Exception::$message is a protected member that can be set to any
type. Fix this by performing an explicit type-cast.

This also requires a temporary refcount increment in the __toString()
object handler, because there is no additional owner of the object,
and it may get released prematurely as part of the __toString() call.

Author: nikic

Zend/tests/exception_delayed_message.phpt

@@ -0,0 +1,41 @@
+--TEST--
+Exception with delayed message computation
+--FILE--
+<?php
+
+class MyException extends Exception {
+    public $message;
+    public $messageCallback;
+
+    public function __construct() {
+        $this->messageCallback = static function() {
+            return "Foobar";
+        };
+        $this->message = new class($this->message, $this->messageCallback) {
+            private $message;
+            private $messageCallback;
+
+            public function __construct(&$message, &$messageCallback)
+            {
+                $this->message = &$message;
+                $this->messageCallback = &$messageCallback;
+            }
+
+            public function __toString(): string
+            {
+                $messageCallback = $this->messageCallback;
+                $this->messageCallback = null;
+                return $this->message = $messageCallback();
+            }
+        };
+    }
+}
+
+throw new MyException;
+
+?>
+--EXPECTF--
+Fatal error: Uncaught MyException: Foobar in %s:%d
+Stack trace:
+#0 {main}
+  thrown in %s on line %d

Zend/zend_exceptions.c

@@ -414,8 +414,7 @@ ZEND_METHOD(Exception, getMessage)
 	ZEND_PARSE_PARAMETERS_NONE();
 
 	prop = GET_PROPERTY(ZEND_THIS, ZEND_STR_MESSAGE);
-	ZVAL_DEREF(prop);
-	ZVAL_COPY(return_value, prop);
+	RETURN_STR(zval_get_string(prop));
 }
 /* }}} */
 

Zend/zend_object_handlers.c

@@ -1800,7 +1800,9 @@ ZEND_API int zend_std_cast_object_tostring(zend_object *readobj, zval *writeobj,
 			zend_class_entry *ce = readobj->ce;
 			if (ce->__tostring) {
 				zval retval;
+				GC_ADDREF(readobj);
 				zend_call_method_with_0_params(readobj, ce, &ce->__tostring, "__tostring", &retval);
+				zend_object_release(readobj);
 				if (EXPECTED(Z_TYPE(retval) == IS_STRING)) {
 					ZVAL_COPY_VALUE(writeobj, &retval);
 					return SUCCESS;