Fix OGR expression compiler with GDAL >= 2.0

Some extreme corner cases (eg "NULL OR TRUE") tested by the provider
tests were failing with GDAL 2.0. In GDAL<2.0 these expressions would
be rejected by OGR, but it seems GDAL>=2.0 has partial support for
these so the expression compiler needs to handle them before handing
off to GDAL.

src/core/qgssqlexpressioncompiler.cpp

@@ -162,10 +162,22 @@ QgsSqlExpressionCompiler::Result QgsSqlExpressionCompiler::compileNode( const Qg
           break;
 
         case QgsExpression::boOr:
+          if ( mFlags.testFlag( NoNullInBooleanLogic ) )
+          {
+            if ( nodeIsNullLiteral( n->opLeft() ) || nodeIsNullLiteral( n->opRight() ) )
+              return Fail;
+          }
+
           op = "OR";
           break;
 
         case QgsExpression::boAnd:
+          if ( mFlags.testFlag( NoNullInBooleanLogic ) )
+          {
+            if ( nodeIsNullLiteral( n->opLeft() ) || nodeIsNullLiteral( n->opRight() ) )
+              return Fail;
+          }
+
           op = "AND";
           break;
 
@@ -297,3 +309,12 @@ QgsSqlExpressionCompiler::Result QgsSqlExpressionCompiler::compileNode( const Qg
 
   return Fail;
 }
+
+bool QgsSqlExpressionCompiler::nodeIsNullLiteral( const QgsExpression::Node* node ) const
+{
+  if ( node->nodeType() != QgsExpression::ntLiteral )
+    return false;
+
+  const QgsExpression::NodeLiteral* nLit = static_cast<const QgsExpression::NodeLiteral*>( node );
+  return nLit->value().isNull();
+}

src/core/qgssqlexpressioncompiler.h

@@ -49,6 +49,7 @@ class CORE_EXPORT QgsSqlExpressionCompiler
     {
       CaseInsensitiveStringMatch = 0x01,  //!< Provider performs case-insensitive string matching for all strings
       LikeIsCaseInsensitive = 0x02, //!< Provider treats LIKE as case-insensitive
+      NoNullInBooleanLogic = 0x04, //!< Provider does not support using NULL with boolean logic, eg "(...) OR NULL"
     };
     Q_DECLARE_FLAGS( Flags, Flag )
 
@@ -98,6 +99,8 @@ class CORE_EXPORT QgsSqlExpressionCompiler
 
     Flags mFlags;
 
+    bool nodeIsNullLiteral( const QgsExpression::Node* node ) const;
+
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS( QgsSqlExpressionCompiler::Flags )

src/providers/ogr/qgsogrexpressioncompiler.cpp

@@ -17,7 +17,7 @@
 #include "qgsogrprovider.h"
 
 QgsOgrExpressionCompiler::QgsOgrExpressionCompiler( QgsOgrFeatureSource* source )
-    : QgsSqlExpressionCompiler( source->mFields, QgsSqlExpressionCompiler::CaseInsensitiveStringMatch )
+    : QgsSqlExpressionCompiler( source->mFields, QgsSqlExpressionCompiler::CaseInsensitiveStringMatch | QgsSqlExpressionCompiler::NoNullInBooleanLogic )
     , mSource( source )
 {
 }
@@ -95,5 +95,12 @@ QString QgsOgrExpressionCompiler::quotedIdentifier( const QString& identifier )
 QString QgsOgrExpressionCompiler::quotedValue( const QVariant& value, bool& ok )
 {
   ok = true;
+
+  if ( value.type() == QVariant::Bool )
+  {
+    // No support for boolean literals, so fake them
+    return value.toBool() ? "(1=1)" : "(1=0)";
+  }
+
   return QgsOgrProviderUtils::quotedValue( value );
 }