render missing SVG symbol (?) if SVG file does not exist

src/core/symbology-ng/qgssvgcache.cpp

@@ -93,6 +93,7 @@ QgsSvgCache::QgsSvgCache( QObject *parent )
     , mLeastRecentEntry( 0 )
     , mMostRecentEntry( 0 )
 {
+  mMissingSvg = QString( "<svg width='10' height='10'><text x='5' y='10' font-size='10' text-anchor='middle'>?</text></svg>" ).toAscii();
 }
 
 QgsSvgCache::~QgsSvgCache()
@@ -246,20 +247,20 @@ QByteArray QgsSvgCache::getImageData( const QString &path ) const
     }
     else
     {
-      return QByteArray();
+      return mMissingSvg;
     }
   }
 
   // maybe it's a url...
   if ( !path.contains( "://" ) ) // otherwise short, relative SVG paths might be considered URLs
   {
-    return QByteArray();
+    return mMissingSvg;
   }
 
   QUrl svgUrl( path );
   if ( !svgUrl.isValid() )
   {
-    return QByteArray();
+    return mMissingSvg;
   }
 
   // check whether it's a url pointing to a local file
@@ -275,7 +276,7 @@ QByteArray QgsSvgCache::getImageData( const QString &path ) const
     }
 
     // not found...
-    return QByteArray();
+    return mMissingSvg;
   }
 
   // the url points to a remote resource, download it!
@@ -331,15 +332,15 @@ QByteArray QgsSvgCache::getImageData( const QString &path ) const
     QgsMessageLog::logMessage( tr( "SVG request error [status: %1 - reason phrase: %2]" ).arg( status.toInt() ).arg( phrase.toString() ), tr( "SVG" ) );
 
     reply->deleteLater();
-    return QByteArray();
+    return mMissingSvg;
   }
 
   QString contentType = reply->header( QNetworkRequest::ContentTypeHeader ).toString();
   QgsDebugMsg( "contentType: " + contentType );
   if ( !contentType.startsWith( "image/svg+xml", Qt::CaseInsensitive ) )
   {
     reply->deleteLater();
-    return QByteArray();
+    return mMissingSvg;
   }
 
   // read the image data

src/core/symbology-ng/qgssvgcache.h

@@ -147,6 +147,9 @@ class CORE_EXPORT QgsSvgCache : public QObject
 
     /**For debugging*/
     void printEntryList();
+
+    /** SVG content to be rendered if SVG file was not found. */
+    QByteArray mMissingSvg;
 };
 
 #endif // QGSSVGCACHE_H