Fix #9459 (layer fails to render when rule-based symbology make use of $atlasfeatureid)

Author: wonder-sk

python/core/qgsexpression.sip

@@ -57,6 +57,9 @@ class QgsExpression
     static void unsetSpecialColumn( const QString& name );
     //! Return the value of the given special column or a null QVariant if undefined
     static QVariant specialColumn( const QString& name );
+    //! Check whether a special column exists
+    //! @note added in 2.2
+    static bool hasSpecialColumn( const QString& name );
 
     void setScale( double scale );
 

src/core/qgsexpression.cpp

@@ -1583,6 +1583,28 @@ QVariant QgsExpression::specialColumn( const QString& name )
   return it.value();
 }
 
+bool QgsExpression::hasSpecialColumn( const QString& name )
+{
+  static bool initialized = false;
+  if ( !initialized )
+  {
+    // Pre-register special columns that will exist within QGIS so that expressions that may use them are parsed correctly.
+    // This is really sub-optimal, we should get rid of the special columns and instead have contexts in which some values
+    // are defined and some are not ($rownum makes sense only in field calculator, $scale only when rendering, $page only for composer etc.)
+
+    QStringList lst;
+    lst << "$page" << "$feature" << "$numpages" << "$numfeatures" << "$atlasfeatureid" << "$atlasgeometry" << "$map";
+    foreach ( QString c, lst )
+      setSpecialColumn( c, QVariant() );
+
+    initialized = true;
+  }
+
+  if ( functionIndex( name ) != -1 )
+    return false;
+  return gmSpecialColumns.contains( name );
+}
+
 QList<QgsExpression::Function*> QgsExpression::specialColumns()
 {
   QList<Function*> defs;

src/core/qgsexpression.h

@@ -147,6 +147,9 @@ class CORE_EXPORT QgsExpression
     static void unsetSpecialColumn( const QString& name );
     //! Return the value of the given special column or a null QVariant if undefined
     static QVariant specialColumn( const QString& name );
+    //! Check whether a special column exists
+    //! @note added in 2.2
+    static bool hasSpecialColumn( const QString& name );
 
     void setScale( double scale ) { mScale = scale; }
 

src/core/qgsexpressionparser.yy

@@ -193,8 +193,7 @@ expression:
           int fnIndex = QgsExpression::functionIndex(*$1);
           if (fnIndex == -1)
           {
-	    QVariant userVar = QgsExpression::specialColumn( *$1 );
-	    if ( userVar.isNull() )
+      if ( !QgsExpression::hasSpecialColumn( *$1 ) )
 	    {
 	      exp_error("Special column is not known");
 	      YYERROR;