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gmemorymonitorbase: Emit signals in the global default main context

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`GMemoryMonitor` is a singleton, which means we can’t use the usual
approach of emitting signals in the thread-default main context from the
time of construction of the object.

The next best thing is to emit them in the global default main context.
For many applications, this will be exactly what they are expecting. For
multi-threaded applications, they will need to implement their own
thread safety in the signal handler, but they would have to do that
anyway.

Currently, the signals are emitted in the GLib worker thread (for the
PSI and poll implementations of `GMemoryMonitor`) — this is the worst
option, because it means that third party signal handlers could block
the worker thread (which is precisely what the worker thread is meant to
avoid).

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
This commit is contained in:
Philip Withnall
2025-07-23 13:27:58 +01:00
parent ee5e750be3
commit 341cd08a42
2 changed files with 46 additions and 3 deletions
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8
gio/gmemorymonitor.c
View File

@@ -131,11 +131,17 @@ g_memory_monitor_default_init (GMemoryMonitorInterface *iface)
* @monitor: a #GMemoryMonitor
* @level: the #GMemoryMonitorWarningLevel warning level
*
* Emitted when the system is running low on free memory. The signal
* 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 the [iface@Gio.MemoryMonitor] is a singleton, this signal will be
* emitted in the [func@GLib.MainContext.default][global-default main
* context].
*
* Since: 2.64
*/
signals[LOW_MEMORY_WARNING] =

41
gio/gmemorymonitorbase.c
View File

@@ -103,6 +103,34 @@ g_memory_monitor_base_level_enum_to_byte (GMemoryMonitorLowMemoryLevel level)
return level_bytes[level];
}
typedef struct
{
GWeakRef monitor_weak;
GMemoryMonitorWarningLevel level;
} SendEventData;
static void
send_event_data_free (SendEventData *data)
{
g_weak_ref_clear (&data->monitor_weak);
g_free (data);
}
/* Invoked in the global default main context */
static gboolean
send_event_cb (void *user_data)
{
SendEventData *data = user_data;
GMemoryMonitor *monitor = g_weak_ref_get (&data->monitor_weak);
if (monitor != NULL)
g_signal_emit_by_name (monitor, "low-memory-warning", data->level);
g_clear_object (&monitor);
return G_SOURCE_REMOVE;
}
void
g_memory_monitor_base_send_event_to_user (GMemoryMonitorBase *monitor,
GMemoryMonitorLowMemoryLevel warning_level)
@@ -115,10 +143,19 @@ g_memory_monitor_base_send_event_to_user (GMemoryMonitorBase *monit
if (priv->last_trigger_us[warning_level] == 0 ||
(current_time - priv->last_trigger_us[warning_level]) > (RECOVERY_INTERVAL_SEC * G_USEC_PER_SEC))
{
SendEventData *data = NULL;
g_debug ("Send low memory signal with warning level %u", warning_level);
g_signal_emit_by_name (monitor, "low-memory-warning",
g_memory_monitor_base_level_enum_to_byte (warning_level));
/* The signal has to be emitted in the global default main context,
* because the `GMemoryMonitor` is a singleton which may have been created
* in an arbitrary thread, or which may be calling this function from the
* GLib worker thread. */
data = g_new0 (SendEventData, 1);
g_weak_ref_init (&data->monitor_weak, monitor);
data->level = g_memory_monitor_base_level_enum_to_byte (warning_level);
g_main_context_invoke_full (NULL, G_PRIORITY_DEFAULT, send_event_cb,
g_steal_pointer (&data), (GDestroyNotify) send_event_data_free);
priv->last_trigger_us[warning_level] = current_time;
}
}