Do not deadlock when calling when calling GetLayerVisibility

... from multiple threads.

This is a partial solution (for most use cases, I believe).

Fix #53956

src/core/expression/qgsexpressioncontextutils.cpp

@@ -994,10 +994,39 @@ QVariant QgsExpressionContextUtils::GetLayerVisibility::func( const QVariantList
   }
 
   bool isVisible = false;
-  Q_NOWARN_DEPRECATED_PUSH
-  QgsMapLayer *layer = QgsExpressionUtils::getMapLayer( values.at( 0 ), context, parent );
-  Q_NOWARN_DEPRECATED_POP
-  if ( layer && mLayers.contains( layer ) )
+
+  // getMapLayer() will happily deadlock if multiple expression are evaluated at
+  // the same time from different threads
+  // see: https://github.com/qgis/QGIS/issues/53956
+  // Searching by layer name/id in mLayer is safe and fast, let's do it first
+
+  QgsMapLayer *layer = nullptr;
+
+  if ( values.at( 0 ).type() == QVariant::String )
+  {
+    const QString layerIdOrName { values.at( 0 ).toString() };
+
+    for ( const auto &layerPtr : std::as_const( mLayers ) )
+    {
+      if ( layerPtr->id() == layerIdOrName || layerPtr->name() == layerIdOrName )
+      {
+        layer = layerPtr;
+        break;
+      }
+    }
+  }
+  else
+  {
+    Q_NOWARN_DEPRECATED_PUSH
+    layer = QgsExpressionUtils::getMapLayer( values.at( 0 ), context, parent );
+    Q_NOWARN_DEPRECATED_POP
+    if ( ! mLayers.contains( layer ) )
+    {
+      layer = nullptr;
+    }
+  }
+
+  if ( layer )
   {
     isVisible = true;
     if ( mScaleBasedVisibilityDetails.contains( layer ) && !qgsDoubleNear( mScale, 0.0 ) )