[MySQL Conformance] fix tree size bug

This returns the logic back to how it was before #281. This addresses part of #364, but I think only fixes a special case for full tiles. I think this needs more to address requesting partial sizes slightly larger than the current tree size.

storage/mysql/mysql.go

@@ -234,7 +234,7 @@ func (s *Storage) writeTreeState(ctx context.Context, tx *sql.Tx, size uint64, r
 	return nil
 }
 
-// ReadTile returns a full tile or a partial tile at the given level, index and width.
+// ReadTile returns a full tile or a partial tile at the given level, index and treeSize.
 // If the tile is not found, nil is returned with no error.
 //
 // TODO: Handle the following scenarios:
@@ -243,7 +243,7 @@ func (s *Storage) writeTreeState(ctx context.Context, tx *sql.Tx, size uint64, r
 // 3. Partial tile request with full/larger partial tile output: Return trimmed partial tile with correct tile width.
 // 4. Partial tile request with partial tile (same width) output: Return partial tile.
 // 5. Partial tile request with smaller partial tile output: Return error.
-func (s *Storage) ReadTile(ctx context.Context, level, index, width uint64) ([]byte, error) {
+func (s *Storage) ReadTile(ctx context.Context, level, index, treeSize uint64) ([]byte, error) {
 	row := s.db.QueryRowContext(ctx, selectSubtreeByLevelAndIndexSQL, level, index)
 	if err := row.Err(); err != nil {
 		return nil, err
@@ -258,8 +258,9 @@ func (s *Storage) ReadTile(ctx context.Context, level, index, width uint64) ([]b
 		return nil, fmt.Errorf("scan tile: %v", err)
 	}
 
+	width := treeSize % 256
 	// Return nil when returning a partial tile on a full tile request.
-	if width == 256 && uint64(len(tile)/32) != width {
+	if width == 0 && uint64(len(tile)/32) != width {
 		return nil, nil
 	}