giomodule: Don’t mandatorily cache GIOModule implementations

While all of the current callers of _g_io_module_get_default() want to
cache the returned GObject for the lifetime of the process, that doesn’t
necessarily have to be the case, so let callers make that decision on a
case-by-case basis.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

gio/giomodule.c

@@ -884,6 +884,13 @@ try_implementation (const char           *extension_point,
     }
 }
 
+static void
+weak_ref_free (GWeakRef *weak_ref)
+{
+  g_weak_ref_clear (weak_ref);
+  g_free (weak_ref);
+}
+
 /**
  * _g_io_module_get_default:
  * @extension_point: the name of an extension point
@@ -905,10 +912,11 @@ try_implementation (const char           *extension_point,
  * be called on each candidate implementation after construction, to
  * check if it is actually usable or not.
  *
- * The result is cached after it is generated the first time, and
+ * The result is cached after it is generated the first time (but the cache does
+ * not keep a strong reference to the object), and
  * the function is thread-safe.
  *
- * Returns: (transfer none): an object implementing
+ * Returns: (transfer full) (nullable): an object implementing
  *     @extension_point, or %NULL if there are no usable
  *     implementations.
  */
@@ -923,25 +931,33 @@ _g_io_module_get_default (const gchar         *extension_point,
   GList *l;
   GIOExtensionPoint *ep;
   GIOExtension *extension = NULL, *preferred;
-  gpointer impl;
+  gpointer impl, value;
+  GWeakRef *impl_weak_ref = NULL;
 
   g_rec_mutex_lock (&default_modules_lock);
   if (default_modules)
     {
-      gpointer key;
-
       if (g_hash_table_lookup_extended (default_modules, extension_point,
-					&key, &impl))
+                                        NULL, &value))
         {
           /* Don’t debug here, since we’re returning a cached object which was
            * already printed earlier. */
+          impl_weak_ref = value;
+          impl = g_weak_ref_get (impl_weak_ref);
+
+          /* If the object has been finalised (impl == NULL), fall through and
+           * instantiate a new one. */
+          if (impl != NULL)
+            {
               g_rec_mutex_unlock (&default_modules_lock);
-	  return impl;
+              return g_steal_pointer (&impl);
+            }
         }
     }
   else
     {
-      default_modules = g_hash_table_new (g_str_hash, g_str_equal);
+      default_modules = g_hash_table_new_full (g_str_hash, g_str_equal,
+                                               g_free, (GDestroyNotify) weak_ref_free);
     }
 
   _g_io_modules_ensure_loaded ();
@@ -993,9 +1009,18 @@ _g_io_module_get_default (const gchar         *extension_point,
   impl = NULL;
 
  done:
-  g_hash_table_insert (default_modules,
+  if (impl_weak_ref == NULL)
-		       g_strdup (extension_point),
+    {
-		       impl ? g_object_ref (impl) : NULL);
+      impl_weak_ref = g_new0 (GWeakRef, 1);
+      g_weak_ref_init (impl_weak_ref, impl);
+      g_hash_table_insert (default_modules, g_strdup (extension_point),
+                           g_steal_pointer (&impl_weak_ref));
+    }
+  else
+    {
+      g_weak_ref_set (impl_weak_ref, impl);
+    }
+
   g_rec_mutex_unlock (&default_modules_lock);
 
   if (impl != NULL)
@@ -1009,7 +1034,7 @@ _g_io_module_get_default (const gchar         *extension_point,
     g_debug ("%s: Failed to find default implementation for ‘%s’",
              G_STRFUNC, extension_point);
 
-  return impl;
+  return g_steal_pointer (&impl);
 }
 
 G_LOCK_DEFINE_STATIC (registered_extensions);

gio/gnetworkmonitor.c

@@ -77,6 +77,7 @@ enum {
 };
 
 static guint signals[LAST_SIGNAL] = { 0 };
+static GNetworkMonitor *network_monitor_default_singleton = NULL;  /* (owned) (atomic) */
 
 /**
  * g_network_monitor_get_default:
@@ -91,9 +92,18 @@ static guint signals[LAST_SIGNAL] = { 0 };
 GNetworkMonitor *
 g_network_monitor_get_default (void)
 {
-  return _g_io_module_get_default (G_NETWORK_MONITOR_EXTENSION_POINT_NAME,
+  if (g_once_init_enter (&network_monitor_default_singleton))
+    {
+      GNetworkMonitor *singleton;
+
+      singleton = _g_io_module_get_default (G_NETWORK_MONITOR_EXTENSION_POINT_NAME,
                                             "GIO_USE_NETWORK_MONITOR",
                                             NULL);
+
+      g_once_init_leave (&network_monitor_default_singleton, singleton);
+    }
+
+  return network_monitor_default_singleton;
 }
 
 /**

gio/gproxyresolver.c

@@ -67,6 +67,8 @@ g_proxy_resolver_default_init (GProxyResolverInterface *iface)
 {
 }
 
+static GProxyResolver *proxy_resolver_default_singleton = NULL;  /* (owned) (atomic) */
+
 /**
  * g_proxy_resolver_get_default:
  *
@@ -80,9 +82,18 @@ g_proxy_resolver_default_init (GProxyResolverInterface *iface)
 GProxyResolver *
 g_proxy_resolver_get_default (void)
 {
-  return _g_io_module_get_default (G_PROXY_RESOLVER_EXTENSION_POINT_NAME,
+  if (g_once_init_enter (&proxy_resolver_default_singleton))
+    {
+      GProxyResolver *singleton;
+
+      singleton = _g_io_module_get_default (G_PROXY_RESOLVER_EXTENSION_POINT_NAME,
                                             "GIO_USE_PROXY_RESOLVER",
                                             (GIOModuleVerifyFunc) g_proxy_resolver_is_supported);
+
+      g_once_init_leave (&proxy_resolver_default_singleton, singleton);
+    }
+
+  return proxy_resolver_default_singleton;
 }
 
 /**

gio/gsettingsbackend.c

@@ -992,6 +992,12 @@ g_settings_backend_verify (gpointer impl)
   return TRUE;
 }
 
+/* We need to cache the default #GSettingsBackend for the entire process
+ * lifetime, especially if the backend is #GMemorySettingsBackend: it needs to
+ * keep the in-memory settings around even while there are no #GSettings
+ * instances alive. */
+static GSettingsBackend *settings_backend_default_singleton = NULL;  /* (owned) (atomic) */
+
 /**
  * g_settings_backend_get_default:
  *
@@ -1010,12 +1016,18 @@ g_settings_backend_verify (gpointer impl)
 GSettingsBackend *
 g_settings_backend_get_default (void)
 {
-  GSettingsBackend *backend;
+  if (g_once_init_enter (&settings_backend_default_singleton))
+    {
+      GSettingsBackend *singleton;
 
-  backend = _g_io_module_get_default (G_SETTINGS_BACKEND_EXTENSION_POINT_NAME,
+      singleton = _g_io_module_get_default (G_SETTINGS_BACKEND_EXTENSION_POINT_NAME,
                                             "GSETTINGS_BACKEND",
                                             g_settings_backend_verify);
-  return g_object_ref (backend);
+
+      g_once_init_leave (&settings_backend_default_singleton, singleton);
+    }
+
+  return g_object_ref (settings_backend_default_singleton);
 }
 
 /*< private >

gio/gtlsbackend.c

@@ -93,6 +93,8 @@ g_tls_backend_default_init (GTlsBackendInterface *iface)
 {
 }
 
+static GTlsBackend *tls_backend_default_singleton = NULL;  /* (owned) (atomic) */
+
 /**
  * g_tls_backend_get_default:
  *
@@ -106,8 +108,18 @@ g_tls_backend_default_init (GTlsBackendInterface *iface)
 GTlsBackend *
 g_tls_backend_get_default (void)
 {
-  return _g_io_module_get_default (G_TLS_BACKEND_EXTENSION_POINT_NAME,
+  if (g_once_init_enter (&tls_backend_default_singleton))
-				   "GIO_USE_TLS", NULL);
+    {
+      GTlsBackend *singleton;
+
+      singleton = _g_io_module_get_default (G_TLS_BACKEND_EXTENSION_POINT_NAME,
+                                            "GIO_USE_TLS",
+                                            NULL);
+
+      g_once_init_leave (&tls_backend_default_singleton, singleton);
+    }
+
+  return tls_backend_default_singleton;
 }
 
 /**

gio/gvfs.c

@@ -337,6 +337,8 @@ g_vfs_parse_name (GVfs       *vfs,
   return (* class->parse_name) (vfs, parse_name);
 }
 
+static GVfs *vfs_default_singleton = NULL;  /* (owned) (atomic) */
+
 /**
  * g_vfs_get_default:
  *
@@ -350,9 +352,19 @@ g_vfs_get_default (void)
 {
   if (GLIB_PRIVATE_CALL (g_check_setuid) ())
     return g_vfs_get_local ();
-  return _g_io_module_get_default (G_VFS_EXTENSION_POINT_NAME,
+
+  if (g_once_init_enter (&vfs_default_singleton))
+    {
+      GVfs *singleton;
+
+      singleton = _g_io_module_get_default (G_VFS_EXTENSION_POINT_NAME,
                                             "GIO_USE_VFS",
                                             (GIOModuleVerifyFunc) g_vfs_is_active);
+
+      g_once_init_leave (&vfs_default_singleton, singleton);
+    }
+
+  return vfs_default_singleton;
 }
 
 /**

glib.supp

@@ -421,6 +421,14 @@
 	fun:_g_io_module_get_default*
 }
 
+{
+	g-io-module-default-singleton-weak-ref
+	Memcheck:Leak
+	fun:calloc
+	...
+	fun:_g_io_module_get_default
+}
+
 {
 	g-get-language-names-malloc
 	Memcheck:Leak