gio: Add GMemoryMonitor to monitor for low-memory

Add a memory monitor object, with D-Bus and Portal based
implementations. The D-Bus implementation uses the Linux-only
low-memory-monitor Freedesktop project.

Low Memory Monitor D-Bus API:
https://hadess.pages.freedesktop.org/low-memory-monitor/

Android API:
https://developer.android.com/reference/android/content/ComponentCallbacks2.html#onTrimMemory(int)

iOS API:
https://developer.apple.com/documentation/uikit/app_and_environment/managing_your_app_s_life_cycle/responding_to_memory_warnings

Win32 API:
https://docs.microsoft.com/en-us/dotnet/api/microsoft.win32.systemevents.lowmemory?view=netframework-4.8

Tizen API:
https://samsung.github.io/TizenFX/master/api/Tizen.Applications.EventManager.SystemEvents.LowMemory.html

docs/reference/gio/gio-docs.xml

@@ -232,6 +232,7 @@
         <xi:include href="xml/gremoteactiongroup.xml"/>
         <xi:include href="xml/gactiongroupexporter.xml"/>
         <xi:include href="xml/gdbusactiongroup.xml"/>
+        <xi:include href="xml/gmemorymonitor.xml"/>
         <xi:include href="xml/gmenumodel.xml"/>
         <xi:include href="xml/gmenu.xml"/>
         <xi:include href="xml/gmenuexporter.xml"/>

docs/reference/gio/gio-sections-common.txt

@@ -4170,6 +4170,23 @@ G_DBUS_OBJECT_MANAGER_SERVER_GET_CLASS
 GDBusObjectManagerServerPrivate
 </SECTION>
 
+<SECTION>
+<FILE>gmemorymonitor</FILE>
+<TITLE>GMemoryMonitor</TITLE>
+GMemoryMonitor
+GMemoryMonitorFlags
+GMemoryMonitorInterface
+GMemoryMonitorWarningLevel
+G_MEMORY_MONITOR_EXTENSION_POINT_NAME
+g_memory_monitor_dup_default
+<SUBSECTION Standard>
+g_memory_monitor_get_type
+G_TYPE_MEMORY_MONITOR
+G_MEMORY_MONITOR
+G_IS_MEMORY_MONITOR
+G_MEMORY_MONITOR_GET_INTERFACE
+</SECTION>
+
 <SECTION>
 <FILE>gnetworkmonitor</FILE>
 <TITLE>GNetworkMonitor</TITLE>

docs/reference/gio/meson.build

@@ -50,6 +50,8 @@ if get_option('gtk_doc')
     'glocalfilemonitor.h',
     'glocalfileoutputstream.h',
     'glocalvfs.h',
+    'gmemorymonitordbus.h',
+    'gmemorymonitorportal.h',
     'gmountprivate.h',
     'gnativevolumemonitor.h',
     'gnetworkingprivate.h',

gio/gio.h

@@ -91,6 +91,7 @@
 #include <gio/gloadableicon.h>
 #include <gio/gmemoryinputstream.h>
 #include <gio/gmemoryoutputstream.h>
+#include <gio/gmemorymonitor.h>
 #include <gio/gmount.h>
 #include <gio/gmountoperation.h>
 #include <gio/gnativesocketaddress.h>

gio/gioenums.h

@@ -1964,6 +1964,38 @@ typedef enum {
   G_POLLABLE_RETURN_WOULD_BLOCK  = -G_IO_ERROR_WOULD_BLOCK
 } GPollableReturn;
 
+/**
+ * GMemoryMonitorWarningLevel:
+ * @G_MEMORY_MONITOR_WARNING_LEVEL_LOW: Memory on the device is low, processes
+ *   should free up unneeded resources (for example, in-memory caches) so they can
+ *   be used elsewhere.
+ * @G_MEMORY_MONITOR_WARNING_LEVEL_MEDIUM: Same as @G_MEMORY_MONITOR_WARNING_LEVEL_LOW
+ *   but the device has even less free memory, so processes should try harder to free
+ *   up unneeded resources. If your process does not need to stay running, it is a
+ *   good time for it to quit.
+ * @G_MEMORY_MONITOR_WARNING_LEVEL_CRITICAL: The system will soon start terminating
+ *   processes to reclaim memory, including background processes.
+ *
+ * Memory availability warning levels.
+ *
+ * Note that because new values might be added, it is recommended that applications check
+ * #GMemoryMonitorWarningLevel as ranges, for example:
+ * <example>
+ *  <title>Comparing memory warning levels</title>
+ *  <programlisting>
+ * if (warning_level > G_MEMORY_MONITOR_WARNING_LEVEL_LOW)
+ *   drop_caches ();
+ *  </programlisting>
+ * </example>
+ *
+ * Since: 2.64
+ */
+typedef enum {
+  G_MEMORY_MONITOR_WARNING_LEVEL_LOW      = 50,
+  G_MEMORY_MONITOR_WARNING_LEVEL_MEDIUM   = 100,
+  G_MEMORY_MONITOR_WARNING_LEVEL_CRITICAL = 255
+} GMemoryMonitorWarningLevel;
+
 G_END_DECLS
 
 #endif /* __GIO_ENUMS_H__ */

gio/giomodule.c

@@ -42,6 +42,9 @@
 #include "gnotificationbackend.h"
 #include "ginitable.h"
 #include "gnetworkmonitor.h"
+#include "gmemorymonitor.h"
+#include "gmemorymonitorportal.h"
+#include "gmemorymonitordbus.h"
 #ifdef G_OS_WIN32
 #include "gregistrysettingsbackend.h"
 #endif
@@ -1025,6 +1028,9 @@ extern GType _g_network_monitor_netlink_get_type (void);
 extern GType _g_network_monitor_nm_get_type (void);
 #endif
 
+extern GType g_memory_monitor_dbus_get_type (void);
+extern GType g_memory_monitor_portal_get_type (void);
+
 #ifdef G_OS_UNIX
 extern GType g_fdo_notification_backend_get_type (void);
 extern GType g_gtk_notification_backend_get_type (void);
@@ -1127,6 +1133,9 @@ _g_io_modules_ensure_extension_points_registered (void)
 
       ep = g_io_extension_point_register (G_NOTIFICATION_BACKEND_EXTENSION_POINT_NAME);
       g_io_extension_point_set_required_type (ep, G_TYPE_NOTIFICATION_BACKEND);
+
+      ep = g_io_extension_point_register (G_MEMORY_MONITOR_EXTENSION_POINT_NAME);
+      g_io_extension_point_set_required_type (ep, G_TYPE_MEMORY_MONITOR);
     }
   
   G_UNLOCK (registered_extensions);
@@ -1235,6 +1244,8 @@ _g_io_modules_ensure_loaded (void)
       g_type_ensure (g_fdo_notification_backend_get_type ());
       g_type_ensure (g_gtk_notification_backend_get_type ());
       g_type_ensure (g_portal_notification_backend_get_type ());
+      g_type_ensure (g_memory_monitor_dbus_get_type ());
+      g_type_ensure (g_memory_monitor_portal_get_type ());
       g_type_ensure (g_network_monitor_portal_get_type ());
       g_type_ensure (g_proxy_resolver_portal_get_type ());
 #endif

gio/giotypes.h

@@ -122,6 +122,7 @@ typedef struct _GLoadableIcon                 GLoadableIcon; /* Dummy typedef */
 typedef struct _GBytesIcon                    GBytesIcon;
 typedef struct _GMemoryInputStream            GMemoryInputStream;
 typedef struct _GMemoryOutputStream           GMemoryOutputStream;
+typedef struct _GMemoryMonitor                GMemoryMonitor;
 
 /**
  * GMount:

gio/gmemorymonitor.c

@@ -0,0 +1,127 @@
+/* GIO - GLib Input, Output and Streaming Library
+ *
+ * Copyright 2019 Red Hat, Inc
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "config.h"
+#include "glib.h"
+#include "glibintl.h"
+
+#include "gmemorymonitor.h"
+#include "ginetaddress.h"
+#include "ginetsocketaddress.h"
+#include "ginitable.h"
+#include "gioenumtypes.h"
+#include "giomodule-priv.h"
+#include "gtask.h"
+
+/**
+ * SECTION:gmemorymonitor
+ * @title: GMemoryMonitor
+ * @short_description: Memory usage monitor
+ * @include: gio/gio.h
+ *
+ * #GMemoryMonitor will monitor system memory and suggest to the application
+ * when to free memory so as to leave more room for other applications.
+ * It is implemented on Linux using the [Low Memory Monitor](https://gitlab.freedesktop.org/hadess/low-memory-monitor/)
+ * ([API documentation](https://hadess.pages.freedesktop.org/low-memory-monitor/)).
+ *
+ * There is also an implementation for use inside Flatpak sandboxes.
+ *
+ * Possible actions to take when the signal is received are:
+ * - Free caches
+ * - Save files that haven't been looked at in a while to disk, ready to be reopened when needed
+ * - Run a garbage collection cycle
+ * - Try and compress fragmented allocations
+ * - Exit on idle if the process has no reason to stay around
+ *
+ * See #GMemoryMonitorWarningLevel for details on the various warning levels.
+ *
+ * Since: 2.64
+ */
+
+/**
+ * GMemoryMonitor:
+ *
+ * #GMemoryMonitor monitors system memory and indicates when
+ * the system is low on memory.
+ *
+ * Since: 2.64
+ */
+
+/**
+ * GMemoryMonitorInterface:
+ * @g_iface: The parent interface.
+ * @low_memory_warning: the virtual function pointer for the
+ *  #GMemoryMonitor::low-memory-warning signal.
+ *
+ * The virtual function table for #GMemoryMonitor.
+ *
+ * Since: 2.64
+ */
+
+G_DEFINE_INTERFACE_WITH_CODE (GMemoryMonitor, g_memory_monitor, G_TYPE_OBJECT,
+                              g_type_interface_add_prerequisite (g_define_type_id, G_TYPE_INITABLE))
+
+enum {
+  LOW_MEMORY_WARNING,
+  LAST_SIGNAL
+};
+
+static guint signals[LAST_SIGNAL] = { 0 };
+
+/**
+ * g_memory_monitor_dup_default:
+ *
+ * Gets a reference to the default #GMemoryMonitor for the system.
+ *
+ * Returns: (transfer full): a new reference to the default #GMemoryMonitor
+ *
+ * Since: 2.64
+ */
+GMemoryMonitor *
+g_memory_monitor_dup_default (void)
+{
+  return g_object_ref (_g_io_module_get_default (G_MEMORY_MONITOR_EXTENSION_POINT_NAME,
+                                                 "GIO_USE_MEMORY_MONITOR",
+                                                 NULL));
+}
+
+static void
+g_memory_monitor_default_init (GMemoryMonitorInterface *iface)
+{
+  /**
+   * GMemoryMonitor::low-memory-warning:
+   * @monitor: a #GMemoryMonitor
+   * @level: the #GMemoryMonitorWarningLevel warning level
+   *
+   * Emitted when the system is running low on free memory. The signal
+   * handler should then take the appropriate action depending on the
+   * warning level. See the #GMemoryMonitorWarningLevel documentation for
+   * details.
+   *
+   * Since: 2.64
+   */
+  signals[LOW_MEMORY_WARNING] =
+    g_signal_new (I_("low-memory-warning"),
+                  G_TYPE_MEMORY_MONITOR,
+                  G_SIGNAL_RUN_LAST,
+                  G_STRUCT_OFFSET (GMemoryMonitorInterface, low_memory_warning),
+                  NULL, NULL,
+                  NULL,
+                  G_TYPE_NONE, 1,
+                  G_TYPE_MEMORY_MONITOR_WARNING_LEVEL);
+}

gio/gmemorymonitor.h

@@ -0,0 +1,62 @@
+/* GIO - GLib Input, Output and Streaming Library
+ *
+ * Copyright 2019 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __G_MEMORY_MONITOR_H__
+#define __G_MEMORY_MONITOR_H__
+
+#if !defined (__GIO_GIO_H_INSIDE__) && !defined (GIO_COMPILATION)
+#error "Only <gio/gio.h> can be included directly."
+#endif
+
+#include <gio/giotypes.h>
+
+G_BEGIN_DECLS
+
+/**
+ * G_MEMORY_MONITOR_EXTENSION_POINT_NAME:
+ *
+ * Extension point for memory usage monitoring functionality.
+ * See [Extending GIO][extending-gio].
+ *
+ * Since: 2.64
+ */
+#define G_MEMORY_MONITOR_EXTENSION_POINT_NAME "gio-memory-monitor"
+
+#define G_TYPE_MEMORY_MONITOR             (g_memory_monitor_get_type ())
+GLIB_AVAILABLE_IN_2_64
+G_DECLARE_INTERFACE(GMemoryMonitor, g_memory_monitor, g, memory_monitor, GObject)
+
+#define G_MEMORY_MONITOR(o)               (G_TYPE_CHECK_INSTANCE_CAST ((o), G_TYPE_MEMORY_MONITOR, GMemoryMonitor))
+#define G_IS_MEMORY_MONITOR(o)            (G_TYPE_CHECK_INSTANCE_TYPE ((o), G_TYPE_MEMORY_MONITOR))
+#define G_MEMORY_MONITOR_GET_INTERFACE(o) (G_TYPE_INSTANCE_GET_INTERFACE ((o), G_TYPE_MEMORY_MONITOR, GMemoryMonitorInterface))
+
+struct _GMemoryMonitorInterface {
+  /*< private >*/
+  GTypeInterface g_iface;
+
+  /*< public >*/
+  void     (*low_memory_warning)  (GMemoryMonitor             *monitor,
+                                   GMemoryMonitorWarningLevel  level);
+};
+
+GLIB_AVAILABLE_IN_2_64
+GMemoryMonitor      *g_memory_monitor_dup_default           (void);
+
+G_END_DECLS
+
+#endif /* __G_MEMORY_MONITOR_H__ */

gio/gmemorymonitordbus.c

@@ -0,0 +1,171 @@
+/* GIO - GLib Input, Output and Streaming Library
+ *
+ * Copyright 2019 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "config.h"
+
+#include "gmemorymonitor.h"
+#include "gmemorymonitordbus.h"
+#include "gioerror.h"
+#include "ginitable.h"
+#include "giomodule-priv.h"
+#include "glibintl.h"
+#include "glib/gstdio.h"
+#include "gdbusproxy.h"
+#include "gdbusnamewatching.h"
+
+#define G_MEMORY_MONITOR_DBUS_GET_INITABLE_IFACE(o) (G_TYPE_INSTANCE_GET_INTERFACE ((o), G_TYPE_INITABLE, GInitable))
+
+static void g_memory_monitor_dbus_iface_init (GMemoryMonitorInterface *iface);
+static void g_memory_monitor_dbus_initable_iface_init (GInitableIface *iface);
+
+struct _GMemoryMonitorDBus
+{
+  GObject parent_instance;
+
+  guint watch_id;
+  GDBusProxy *proxy;
+  gulong signal_id;
+};
+
+G_DEFINE_TYPE_WITH_CODE (GMemoryMonitorDBus, g_memory_monitor_dbus, G_TYPE_OBJECT,
+                         G_IMPLEMENT_INTERFACE (G_TYPE_INITABLE,
+                                                g_memory_monitor_dbus_initable_iface_init)
+                         G_IMPLEMENT_INTERFACE (G_TYPE_MEMORY_MONITOR,
+                                                g_memory_monitor_dbus_iface_init)
+                         _g_io_modules_ensure_extension_points_registered ();
+                         g_io_extension_point_implement (G_MEMORY_MONITOR_EXTENSION_POINT_NAME,
+                                                         g_define_type_id,
+                                                         "dbus",
+                                                         30))
+
+static void
+g_memory_monitor_dbus_init (GMemoryMonitorDBus *dbus)
+{
+}
+
+static void
+proxy_signal_cb (GDBusProxy         *proxy,
+                 const gchar        *sender_name,
+                 const gchar        *signal_name,
+                 GVariant           *parameters,
+                 GMemoryMonitorDBus *dbus)
+{
+  guint8 level;
+
+  if (g_strcmp0 (signal_name, "LowMemoryWarning") != 0)
+    return;
+  if (parameters == NULL)
+    return;
+
+  g_variant_get (parameters, "(y)", &level);
+  g_signal_emit_by_name (dbus, "low-memory-warning", level);
+}
+
+static void
+lmm_appeared_cb (GDBusConnection *connection,
+                 const gchar     *name,
+                 const gchar     *name_owner,
+                 gpointer         user_data)
+{
+  GMemoryMonitorDBus *dbus = user_data;
+  GDBusProxy *proxy;
+  GError *error = NULL;
+
+  proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
+                                         G_DBUS_PROXY_FLAGS_DO_NOT_AUTO_START,
+                                         NULL,
+                                         "org.freedesktop.LowMemoryMonitor",
+                                         "/org/freedesktop/LowMemoryMonitor",
+                                         "org.freedesktop.LowMemoryMonitor",
+                                         NULL,
+                                         &error);
+
+  if (!proxy)
+    {
+      g_debug ("Failed to create LowMemoryMonitor D-Bus proxy: %s",
+               error->message);
+      g_error_free (error);
+      return;
+    }
+
+  dbus->signal_id = g_signal_connect (G_OBJECT (proxy), "g-signal",
+                                      G_CALLBACK (proxy_signal_cb), dbus);
+  dbus->proxy = proxy;
+}
+
+static void
+lmm_vanished_cb (GDBusConnection *connection,
+                 const gchar     *name,
+                 gpointer         user_data)
+{
+  GMemoryMonitorDBus *dbus = user_data;
+
+  if (dbus->proxy != NULL)
+    g_clear_signal_handler (&dbus->signal_id, dbus->proxy);
+  g_clear_object (&dbus->proxy);
+}
+
+static gboolean
+g_memory_monitor_dbus_initable_init (GInitable     *initable,
+                                     GCancellable  *cancellable,
+                                     GError       **error)
+{
+  GMemoryMonitorDBus *dbus = G_MEMORY_MONITOR_DBUS (initable);
+
+  dbus->watch_id = g_bus_watch_name (G_BUS_TYPE_SYSTEM,
+                                     "org.freedesktop.LowMemoryMonitor",
+                                     G_BUS_NAME_WATCHER_FLAGS_AUTO_START,
+                                     lmm_appeared_cb,
+                                     lmm_vanished_cb,
+                                     dbus,
+                                     NULL);
+
+  return TRUE;
+}
+
+static void
+g_memory_monitor_dbus_finalize (GObject *object)
+{
+  GMemoryMonitorDBus *dbus = G_MEMORY_MONITOR_DBUS (object);
+
+  if (dbus->proxy != NULL)
+    g_clear_signal_handler (&dbus->signal_id, dbus->proxy);
+  g_clear_object (&dbus->proxy);
+  g_clear_handle_id (&dbus->watch_id, g_bus_unwatch_name);
+
+  G_OBJECT_CLASS (g_memory_monitor_dbus_parent_class)->finalize (object);
+}
+
+static void
+g_memory_monitor_dbus_class_init (GMemoryMonitorDBusClass *nl_class)
+{
+  GObjectClass *gobject_class = G_OBJECT_CLASS (nl_class);
+
+  gobject_class->finalize = g_memory_monitor_dbus_finalize;
+}
+
+static void
+g_memory_monitor_dbus_iface_init (GMemoryMonitorInterface *monitor_iface)
+{
+}
+
+static void
+g_memory_monitor_dbus_initable_iface_init (GInitableIface *iface)
+{
+  iface->init = g_memory_monitor_dbus_initable_init;
+}

gio/gmemorymonitordbus.h

@@ -0,0 +1,31 @@
+/* GIO - GLib Input, Output and Streaming Library
+ *
+ * Copyright 2019 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __G_MEMORY_MONITOR_DBUS_H__
+#define __G_MEMORY_MONITOR_DBUS_H__
+
+#include <glib-object.h>
+
+G_BEGIN_DECLS
+
+#define G_TYPE_MEMORY_MONITOR_DBUS         (g_memory_monitor_dbus_get_type ())
+G_DECLARE_FINAL_TYPE (GMemoryMonitorDBus, g_memory_monitor_dbus, G, MEMORY_MONITOR_DBUS, GObject)
+
+G_END_DECLS
+
+#endif /* __G_MEMORY_MONITOR_DBUS_H__ */

gio/gmemorymonitorportal.c

@@ -0,0 +1,152 @@
+/* GIO - GLib Input, Output and Streaming Library
+ *
+ * Copyright 2019 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "config.h"
+
+#include "gmemorymonitor.h"
+#include "gmemorymonitorportal.h"
+#include "ginitable.h"
+#include "giomodule-priv.h"
+#include "xdp-dbus.h"
+#include "gportalsupport.h"
+
+#define G_MEMORY_MONITOR_PORTAL_GET_INITABLE_IFACE(o) (G_TYPE_INSTANCE_GET_INTERFACE ((o), G_TYPE_INITABLE, GInitable))
+
+static void g_memory_monitor_portal_iface_init (GMemoryMonitorInterface *iface);
+static void g_memory_monitor_portal_initable_iface_init (GInitableIface *iface);
+
+struct _GMemoryMonitorPortal
+{
+  GObject parent_instance;
+
+  GDBusProxy *proxy;
+  gulong signal_id;
+};
+
+G_DEFINE_TYPE_WITH_CODE (GMemoryMonitorPortal, g_memory_monitor_portal, G_TYPE_OBJECT,
+                         G_IMPLEMENT_INTERFACE (G_TYPE_INITABLE,
+                                                g_memory_monitor_portal_initable_iface_init)
+                         G_IMPLEMENT_INTERFACE (G_TYPE_MEMORY_MONITOR,
+                                                g_memory_monitor_portal_iface_init)
+                         _g_io_modules_ensure_extension_points_registered ();
+                         g_io_extension_point_implement (G_MEMORY_MONITOR_EXTENSION_POINT_NAME,
+                                                         g_define_type_id,
+                                                         "portal",
+                                                         40))
+
+static void
+g_memory_monitor_portal_init (GMemoryMonitorPortal *portal)
+{
+}
+
+static void
+proxy_signal (GDBusProxy            *proxy,
+              const char            *sender,
+              const char            *signal,
+              GVariant              *parameters,
+              GMemoryMonitorPortal *portal)
+{
+  guint8 level;
+
+  if (strcmp (signal, "LowMemoryWarning") != 0)
+    return;
+  if (!parameters)
+    return;
+
+  g_variant_get (parameters, "(y)", &level);
+  g_signal_emit_by_name (portal, "low-memory-warning", level);
+}
+
+static gboolean
+g_memory_monitor_portal_initable_init (GInitable     *initable,
+                                        GCancellable  *cancellable,
+                                        GError       **error)
+{
+  GMemoryMonitorPortal *portal = G_MEMORY_MONITOR_PORTAL (initable);
+  GDBusProxy *proxy;
+  gchar *name_owner = NULL;
+
+  if (!glib_should_use_portal ())
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED, "Not using portals");
+      return FALSE;
+    }
+
+  proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SESSION,
+                                         G_DBUS_PROXY_FLAGS_DO_NOT_LOAD_PROPERTIES,
+                                         NULL,
+                                         "org.freedesktop.portal.Desktop",
+                                         "/org/freedesktop/portal/desktop",
+                                         "org.freedesktop.portal.MemoryMonitor",
+                                         cancellable,
+                                         error);
+  if (!proxy)
+    return FALSE;
+
+  name_owner = g_dbus_proxy_get_name_owner (proxy);
+
+  if (name_owner == NULL)
+    {
+      g_object_unref (proxy);
+      g_set_error (error,
+                   G_DBUS_ERROR,
+                   G_DBUS_ERROR_NAME_HAS_NO_OWNER,
+                   "Desktop portal not found");
+      return FALSE;
+    }
+
+  g_free (name_owner);
+
+  portal->signal_id = g_signal_connect (proxy, "g-signal",
+                                        G_CALLBACK (proxy_signal), portal);
+
+  portal->proxy = proxy;
+
+  return TRUE;
+}
+
+static void
+g_memory_monitor_portal_finalize (GObject *object)
+{
+  GMemoryMonitorPortal *portal = G_MEMORY_MONITOR_PORTAL (object);
+
+  if (portal->proxy != NULL)
+    g_clear_signal_handler (&portal->signal_id, portal->proxy);
+  g_clear_object (&portal->proxy);
+
+  G_OBJECT_CLASS (g_memory_monitor_portal_parent_class)->finalize (object);
+}
+
+static void
+g_memory_monitor_portal_class_init (GMemoryMonitorPortalClass *nl_class)
+{
+  GObjectClass *gobject_class = G_OBJECT_CLASS (nl_class);
+
+  gobject_class->finalize  = g_memory_monitor_portal_finalize;
+}
+
+static void
+g_memory_monitor_portal_iface_init (GMemoryMonitorInterface *monitor_iface)
+{
+}
+
+static void
+g_memory_monitor_portal_initable_iface_init (GInitableIface *iface)
+{
+  iface->init = g_memory_monitor_portal_initable_init;
+}

gio/gmemorymonitorportal.h

@@ -0,0 +1,31 @@
+/* GIO - GLib Input, Output and Streaming Library
+ *
+ * Copyright 2019 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __G_MEMORY_MONITOR_PORTAL_H__
+#define __G_MEMORY_MONITOR_PORTAL_H__
+
+#include <glib-object.h>
+
+G_BEGIN_DECLS
+
+#define G_TYPE_MEMORY_MONITOR_PORTAL         (g_memory_monitor_portal_get_type ())
+G_DECLARE_FINAL_TYPE (GMemoryMonitorPortal, g_memory_monitor_portal, G, MEMORY_MONITOR_PORTAL, GObject)
+
+G_END_DECLS
+
+#endif /* __G_MEMORY_MONITOR_PORTAL_H__ */

gio/meson.build

@@ -393,6 +393,7 @@ if host_system != 'windows'
   portal_sources = [files(
     'gdocumentportal.c',
     'gopenuriportal.c',
+    'gmemorymonitorportal.c',
     'gnetworkmonitorportal.c',
     'gproxyresolverportal.c',
     'gtrashportal.c',
@@ -528,6 +529,8 @@ gio_sources = files(
   'gloadableicon.c',
   'gmarshal-internal.c',
   'gmount.c',
+  'gmemorymonitor.c',
+  'gmemorymonitordbus.c',
   'gmemoryinputstream.c',
   'gmemoryoutputstream.c',
   'gmountoperation.c',
@@ -670,6 +673,7 @@ gio_headers = files(
   'gloadableicon.h',
   'gmount.h',
   'gmemoryinputstream.h',
+  'gmemorymonitor.h',
   'gmemoryoutputstream.h',
   'gmountoperation.h',
   'gnativesocketaddress.h',