gtlsconnection: use a vfunc to implement get_negotiated_protocol()

The current code is unsafe to use from multiple threads at once.
GIOStream functions like this are supposed to be semi-threadsafe. It's
allowed for them to be called on both a reader thread and a writer
thread at the same time. Of course, it's still tricky and dangerous,
because it's only *really* threadsafe if the handshake has finished,
and API users have no plausible way to know that because the API
does not require performing an explicit handshake operation. But that's
a glib-networking problem. We can at least avoid the most obvious
threadsafety issue here in the API layer.

Note that we'll need to implement the new vfunc in glib-networking for
this to actually work.

Fixes #2393

gio/gtlsconnection.c

@@ -55,12 +55,7 @@
  * Since: 2.28
  */
 
-struct _GTlsConnectionPrivate
+G_DEFINE_ABSTRACT_TYPE (GTlsConnection, g_tls_connection, G_TYPE_IO_STREAM)
-{
-  gchar *negotiated_protocol;
-};
-
-G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE (GTlsConnection, g_tls_connection, G_TYPE_IO_STREAM)
 
 static void g_tls_connection_get_property (GObject    *object,
 					   guint       prop_id,
@@ -70,7 +65,6 @@ static void g_tls_connection_set_property (GObject      *object,
 					   guint         prop_id,
 					   const GValue *value,
 					   GParamSpec   *pspec);
-static void g_tls_connection_finalize (GObject *object);
 
 enum {
   ACCEPT_CERTIFICATE,
@@ -104,7 +98,6 @@ g_tls_connection_class_init (GTlsConnectionClass *klass)
 
   gobject_class->get_property = g_tls_connection_get_property;
   gobject_class->set_property = g_tls_connection_set_property;
-  gobject_class->finalize = g_tls_connection_finalize;
 
   /**
    * GTlsConnection:base-io-stream:
@@ -413,17 +406,6 @@ g_tls_connection_set_property (GObject      *object,
   G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
 }
 
-static void
-g_tls_connection_finalize (GObject *object)
-{
-  GTlsConnection *conn = G_TLS_CONNECTION(object);
-  GTlsConnectionPrivate *priv = g_tls_connection_get_instance_private (conn);
-
-  g_clear_pointer (&priv->negotiated_protocol, g_free);
-
-  G_OBJECT_CLASS (g_tls_connection_parent_class)->finalize (object);
-}
-
 /**
  * g_tls_connection_set_use_system_certdb:
  * @conn: a #GTlsConnection
@@ -871,31 +853,15 @@ g_tls_connection_set_advertised_protocols (GTlsConnection      *conn,
 const gchar *
 g_tls_connection_get_negotiated_protocol (GTlsConnection *conn)
 {
-  GTlsConnectionPrivate *priv;
+  GTlsConnectionClass *class;
-  gchar *protocol;
 
   g_return_val_if_fail (G_IS_TLS_CONNECTION (conn), NULL);
 
-  g_object_get (G_OBJECT (conn),
+  class = G_TLS_CONNECTION_GET_CLASS (conn);
-                "negotiated-protocol", &protocol,
+  if (class->get_negotiated_protocol == NULL)
-                NULL);
+    return NULL;
 
-  /*
+  return class->get_negotiated_protocol (conn);
-   * Cache the property internally so we can return a `const` pointer
-   * to the caller.
-   */
-  priv = g_tls_connection_get_instance_private (conn);
-  if (g_strcmp0 (priv->negotiated_protocol, protocol) != 0)
-    {
-      g_free (priv->negotiated_protocol);
-      priv->negotiated_protocol = protocol;
-    }
-  else
-    {
-      g_free (protocol);
-    }
-
-  return priv->negotiated_protocol;
 }
 
 /**

gio/gtlsconnection.h

@@ -73,9 +73,11 @@ G_GNUC_BEGIN_IGNORE_DEPRECATIONS
                                   GError                 **error);
 G_GNUC_END_IGNORE_DEPRECATIONS
 
+  const gchar *(*get_negotiated_protocol) (GTlsConnection *conn);
+
   /*< private >*/
   /* Padding for future expansion */
-  gpointer padding[7];
+  gpointer padding[6];
 };
 
 GLIB_AVAILABLE_IN_ALL