Increase serialize_lock while decoding session

Avoid leaking state between Serializable::unserialize() and
session_decode().

ext/session/session.c

@@ -244,11 +244,18 @@ static zend_string *php_session_encode(void) /* {{{ */
 
 static int php_session_decode(zend_string *data) /* {{{ */
 {
+	int res;
 	if (!PS(serializer)) {
 		php_error_docref(NULL, E_WARNING, "Unknown session.serialize_handler. Failed to decode session object");
 		return FAILURE;
 	}
-	if (PS(serializer)->decode(ZSTR_VAL(data), ZSTR_LEN(data)) == FAILURE) {
+	/* Make sure that any uses of unserialize() during session decoding do not share
+	 * state with any unserialize() that is already in progress (e.g. because we are
+	 * currently inside Serializable::unserialize(). */
+	BG(serialize_lock)++;
+	res = PS(serializer)->decode(ZSTR_VAL(data), ZSTR_LEN(data));
+	BG(serialize_lock)--;
+	if (res == FAILURE) {
 		php_session_destroy();
 		php_session_track_init();
 		php_error_docref(NULL, E_WARNING, "Failed to decode session object. Session has been destroyed");

ext/standard/tests/serialize/bug70219_1.phpt

@@ -18,6 +18,7 @@ class obj implements Serializable {
     }
     function unserialize($data) {
         session_decode($data);
+        return null;
     }
 }
 
@@ -33,20 +34,18 @@ for ($i = 0; $i < 5; $i++) {
 var_dump($data);
 var_dump($_SESSION);
 ?>
---EXPECTF--
+--EXPECT--
 array(2) {
   [0]=>
-  object(obj)#%d (1) {
-    ["data"]=>
-    NULL
-  }
-  [1]=>
-  object(obj)#%d (1) {
-    ["data"]=>
-    NULL
-  }
-}
   object(obj)#1 (1) {
     ["data"]=>
     NULL
   }
+  [1]=>
+  object(obj)#2 (1) {
+    ["data"]=>
+    NULL
+  }
+}
+array(0) {
+}