Fix GH-8979: Possible Memory Leak with SSL-enabled MySQL connections

The stream context inside `mysqlnd_vio::enable_ssl()` is leaking.
In particular: when `php_stream_context_set()` get called the refcount
of `context` is increased by 1, which means that `context` will now
have a refcount of 2. Later on we remove the context from the stream
by calling `php_stream_context_set(stream, NULL)` but that leaves our
`context` with a refcount of 1, and therefore it's never destroyed.
In my test case this yielded a leak of 1456 bytes per connection
(but could be more depending on your settings ofc).

Annoyingly, Valgrind doesn't find it because the context is still
in the `EG(regular_list)` and will thus be destroyed at the end of
the request. However, I still think this bug needs to be fixed because
as the users in the issue report already mentioned:
there can be long-running PHP scripts.

Fix it by decreasing the refcount to transfer the ownership.

Closes GH-10909.

Author: nielsdos

NEWS

@@ -31,6 +31,10 @@ PHP                                                                        NEWS
   . Fixed bug GH-10521 (ftp_get/ftp_nb_get resumepos offset is maximum 10GB).
     (nielsdos)
 
+- MySQLnd:
+  . Fixed bug GH-8979 (Possible Memory Leak with SSL-enabled MySQL
+    connections). (nielsdos)
+
 - Opcache:
   . Fixed build for macOS to cater with pkg-config settings. (David Carlier)
   . Fixed bug GH-8065 (opcache.consistency_checks > 0 causes segfaults in

ext/mysqlnd/mysqlnd_vio.c

@@ -561,6 +561,10 @@ MYSQLND_METHOD(mysqlnd_vio, enable_ssl)(MYSQLND_VIO * const net)
 		}
 	}
 	php_stream_context_set(net_stream, context);
+	/* php_stream_context_set() increases the refcount of context, but we just want to transfer ownership
+	 * hence the need to decrease the refcount so the refcount will be equal to 1. */
+	ZEND_ASSERT(GC_REFCOUNT(context->res) == 2);
+	GC_DELREF(context->res);
 	if (php_stream_xport_crypto_setup(net_stream, STREAM_CRYPTO_METHOD_TLS_CLIENT, NULL) < 0 ||
 	    php_stream_xport_crypto_enable(net_stream, 1) < 0)
 	{