Handle VTPK tile data with zero sizes by falling back to lower

zoom levels for the tile's data

It seems that every VTPK behaves a bit like an indexed VTPK
where higher zoom level tiles may be replaced with lower
zoom level tiles. This new special handling is required
for non-indexed VTPKs where the high zoom level tile data
is empty, but there's not explicit tilemap present to
advise of us this situation in advance.

Fixes #52872

python/core/auto_generated/vectortile/qgsvtpktiles.sip.in

@@ -94,6 +94,11 @@ Returns bounding box from metadata, given in the tiles :py:func:`~QgsVtpkTiles.c
     QByteArray tileData( int z, int x, int y );
 %Docstring
 Returns the raw tile data for the matching tile.
+
+Returns a null byte array if the requested tile does not exist.
+
+Will return an empty byte array (as opposed to a null byte array) if the tile
+exists but has a zero size.
 %End
 
   private:

src/core/vectortile/qgsvtpktiles.cpp

@@ -473,7 +473,12 @@ QByteArray QgsVtpkTiles::tileData( int z, int x, int y )
       const std::size_t tileOffset = indexValue % ( 2ULL << 39 );
       const std::size_t tileSize = static_cast< std::size_t>( std::floor( indexValue / ( 2ULL << 39 ) ) );
       // bundle is a gzip file;
-      if ( tileSize > 0 && !QgsZipUtils::decodeGzip( buf.get() + tileOffset, tileSize, res ) )
+      if ( tileSize == 0 )
+      {
+        // construct a non-null bytearray
+        res = QByteArray( "" );
+      }
+      else if ( !QgsZipUtils::decodeGzip( buf.get() + tileOffset, tileSize, res ) )
       {
         QgsMessageLog::logMessage( QObject::tr( "Error extracting bundle contents as gzip: %1" ).arg( fileName ) );
       }

src/core/vectortile/qgsvtpktiles.h

@@ -107,6 +107,11 @@ class CORE_EXPORT QgsVtpkTiles
 
     /**
      * Returns the raw tile data for the matching tile.
+     *
+     * Returns a null byte array if the requested tile does not exist.
+     *
+     * Will return an empty byte array (as opposed to a null byte array) if the tile
+     * exists but has a zero size.
      */
     QByteArray tileData( int z, int x, int y );
 

src/core/vectortile/qgsvtpkvectortiledataprovider.cpp

@@ -198,7 +198,9 @@ QList<QgsVectorTileRawData> QgsVtpkVectorTileDataProvider::readTiles( const QgsT
   reader.open();
 
   QList<QgsVectorTileRawData> rawTiles;
+  QSet< QgsTileXYZ > fetchedTiles;
   rawTiles.reserve( tiles.size() );
+  fetchedTiles.reserve( tiles.size() );
   for ( QgsTileXYZ id : std::as_const( tiles ) )
   {
     if ( feedback && feedback->isCanceled() )
@@ -208,6 +210,7 @@ QList<QgsVectorTileRawData> QgsVtpkVectorTileDataProvider::readTiles( const QgsT
     if ( !rawData.data.isEmpty() )
     {
       rawTiles.append( rawData );
+      fetchedTiles.insert( rawData.id );
     }
   }
   return rawTiles;
@@ -232,20 +235,22 @@ QString QgsVtpkVectorTileDataProvider::htmlMetadata() const
 
 QgsVectorTileRawData QgsVtpkVectorTileDataProvider::loadFromVtpk( QgsVtpkTiles &vtpkTileReader, const QgsTileXYZ &id, QgsFeedback * )
 {
-  const QByteArray tileData = vtpkTileReader.tileData( id.zoomLevel(), id.column(), id.row() );
-  if ( tileData.isEmpty() )
+  QgsTileXYZ requestedTile = id;
+  QByteArray tileData = vtpkTileReader.tileData( requestedTile.zoomLevel(), requestedTile.column(), requestedTile.row() );
+  // I **think** here ESRI software will detect a zero size tile and automatically fallback to lower zoom level tiles
+  // I.e. they treat EVERY vtpk a bit like an indexed VTPK, but without the up-front tilemap information.
+  // See https://github.com/qgis/QGIS/issues/52872
+  while ( !tileData.isNull() && tileData.size() == 0 && requestedTile.zoomLevel() > vtpkTileReader.matrixSet().minimumZoom() )
   {
-    // TODO -- I think here ESRI software will detect a zero size tile and automatically fallback to lower zoom level tiles
-    // I.e. they treat EVERY vtpk a bit like an indexed VTPK, but without the up-front tilemap information.
-    // See https://github.com/qgis/QGIS/issues/52872
-
-    return QByteArray();
+    requestedTile = QgsTileXYZ( requestedTile.column() / 2, requestedTile.row() / 2, requestedTile.zoomLevel() - 1 );
+    tileData = vtpkTileReader.tileData( requestedTile.zoomLevel(), requestedTile.column(), requestedTile.row() );
   }
 
-  if ( tileData.isEmpty() )
+  if ( tileData.isNull() )
     return QgsVectorTileRawData();
 
   QgsVectorTileRawData res( id, tileData );
+  res.tileGeometryId = requestedTile;
   return res;
 }