Add method to QgsRasterBlockFeedback to collect error messages

And append raster errors to rendering errors whenever encountered

python/core/auto_generated/raster/qgsrasterinterface.sip.in

@@ -61,6 +61,25 @@ Whether our painter is drawing to a temporary image used just by this layer
 Set whether our painter is drawing to a temporary image used just by this layer
 
 .. seealso:: :py:func:`renderPartialOutput`
+%End
+
+    void appendError( const QString &error );
+%Docstring
+Appends an error message to the stored list of errors. Should be called
+whenever an error is encountered while retrieving a raster block.
+
+.. seealso:: :py:func:`errors`
+
+.. versionadded:: 3.8.0
+%End
+
+    QStringList errors() const;
+%Docstring
+Returns a list of any errors encountered while retrieving the raster block.
+
+.. seealso:: :py:func:`appendError`
+
+.. versionadded:: 3.8.0
 %End
 
 };

src/core/raster/qgsrasterinterface.h

@@ -76,6 +76,23 @@ class CORE_EXPORT QgsRasterBlockFeedback : public QgsFeedback
      */
     void setRenderPartialOutput( bool enable ) { mRenderPartialOutput = enable; }
 
+    /**
+     * Appends an error message to the stored list of errors. Should be called
+     * whenever an error is encountered while retrieving a raster block.
+     *
+     * \see errors()
+     * \since QGIS 3.8.0
+     */
+    void appendError( const QString &error ) { mErrors.append( error ); }
+
+    /**
+     * Returns a list of any errors encountered while retrieving the raster block.
+     *
+     * \see appendError()
+     * \since QGIS 3.8.0
+     */
+    QStringList errors() const { return mErrors; }
+
   private:
 
     /**
@@ -86,6 +103,9 @@ class CORE_EXPORT QgsRasterBlockFeedback : public QgsFeedback
 
     //! Whether our painter is drawing to a temporary image used just by this layer
     bool mRenderPartialOutput = false;
+
+    //! List of errors encountered while retrieving block
+    QStringList mErrors;
 };
 
 

src/core/raster/qgsrasterlayerrenderer.cpp

@@ -272,6 +272,12 @@ bool QgsRasterLayerRenderer::render()
   QgsRasterDrawer drawer( &iterator );
   drawer.draw( mPainter, mRasterViewPort, mMapToPixel, mFeedback );
 
+  const QStringList errors = mFeedback->errors();
+  for ( const QString &error : errors )
+  {
+    mErrors.append( error );
+  }
+
   QgsDebugMsgLevel( QStringLiteral( "total raster draw time (ms):     %1" ).arg( time.elapsed(), 5 ), 4 );
 
   return true;

src/providers/arcgisrest/qgsamsprovider.cpp

@@ -463,6 +463,9 @@ bool QgsAmsProvider::readBlock( int /*bandNo*/, const QgsRectangle &viewExtent,
   draw( viewExtent, width, height );
   if ( mCachedImage.width() != width || mCachedImage.height() != height )
   {
+    if ( feedback )
+      feedback->appendError( tr( "Unexpected image size for block" ) );
+
     QgsDebugMsg( QStringLiteral( "Unexpected image size for block" ) );
     return false;
   }

src/providers/gdal/qgsgdalprovider.cpp

@@ -904,7 +904,11 @@ bool QgsGdalProvider::readBlock( int bandNo, QgsRectangle  const &extent, int pi
 
   if ( err != CPLE_None )
   {
-    QgsLogger::warning( "RasterIO error: " + QString::fromUtf8( CPLGetLastErrorMsg() ) );
+    const QString lastError = QString::fromUtf8( CPLGetLastErrorMsg() ) ;
+    if ( feedback )
+      feedback->appendError( lastError );
+
+    QgsLogger::warning( "RasterIO error: " + lastError );
     qgsFree( tmpBlock );
     return false;
   }