monitor: Use current monitor mode to check whether active

For historical reasons meta_monitor_is_active() checked whether it is
active by checking whether the main output have a CRTC assigned and
whether that CRTC has a current mode. At a later point, the MetaMonitor
got its own mode abstraction (MetaMonitorMode), but
meta_monitor_is_active() was never updated to use this.

An issue with checking the main output's CRTC state is that, if there is
some CRTC mode combination that for some reason isn't properly detected
by the MetaMonitorMode abstraction (e.g. some tiling configuration not
yet handled), meta_monitor_is_active() would return TRUE, even though no
(abstracted) mode was set. This would cause confusion here and there,
leading to NULL pointer dereferences due to the assumption that if a
monitor is active, it has an active mode.

Instead, change meta_monitor_is_active() to directly check the current
monitor mode, and log a warning if the main output still happen to have
a CRTC with a mode assigned to it. This way, when an not undrestood CRTC
mode combination is encountered, instead of dereferencing NULL pointers,
simply assume the monitor is not active, which means that it will not be
managed or rendered by mutter at all.

https://gitlab.gnome.org/GNOME/mutter/issues/130

1 files changed, 16 insertions, 6 deletions

diff --git a/src/backends/meta-monitor.c b/src/backends/meta-monitor.c
index 92c61c037..60f36741a 100644
--- a/src/backends/meta-monitor.c
+++ b/src/backends/meta-monitor.c
@@ -203,13 +203,9 @@ meta_monitor_get_main_output (MetaMonitor *monitor)
 gboolean
 meta_monitor_is_active (MetaMonitor *monitor)
 {
-  MetaOutput *output;
-  MetaCrtc *crtc;
-
-  output = meta_monitor_get_main_output (monitor);
-  crtc = meta_output_get_assigned_crtc (output);
+  MetaMonitorPrivate *priv = meta_monitor_get_instance_private (monitor);
 
-  return crtc && crtc->current_mode;
+  return !!priv->current_mode;
 }
 
 gboolean
@@ -1411,6 +1407,18 @@ meta_monitor_get_current_mode (MetaMonitor *monitor)
   return priv->current_mode;
 }
 
+static gboolean
+is_current_mode_known (MetaMonitor *monitor)
+{
+  MetaOutput *output;
+  MetaCrtc *crtc;
+
+  output = meta_monitor_get_main_output (monitor);
+  crtc = meta_output_get_assigned_crtc (output);
+
+  return meta_monitor_is_active (monitor) == (crtc && crtc->current_mode);
+}
+
 void
 meta_monitor_derive_current_mode (MetaMonitor *monitor)
 {
@@ -1430,6 +1438,8 @@ meta_monitor_derive_current_mode (MetaMonitor *monitor)
     }
 
   priv->current_mode = current_mode;
+
+  g_warn_if_fail (is_current_mode_known (monitor));
 }
 
 void