Revert "Increase serialize_lock while decoding session"

This reverts commit b8ef7c35ab.

See bug #79031. The semantics of serialize locking aren't quite
correct right now, and the use of the lock in this particular
place makes us hit the issue in a common case. I'm reverting this
commit for PHP 7.4 and will try to fix this properly for PHP 8,
as I believe it will require ABI breakage.

ext/session/session.c

@@ -244,18 +244,11 @@ 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;
 	}
-	/* 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) {
+	if (PS(serializer)->decode(ZSTR_VAL(data), ZSTR_LEN(data)) == 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

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