// When the client confirms the validity of the tile data,
	// it will call SaveTiles to signal that they can be safely
	// written to persistent storage.
	// See also https://research.swtch.com/tlog#authenticating_tiles.
	ReadTiles(tiles []Tile) (data [][]byte, err error)

	// SaveTiles informs the TileReader that the tile data
	// returned by ReadTiles has been confirmed as valid

// markTileSaved records that tile is already present in the on-disk cache,
// so that a future SaveTiles for that tile can be ignored.
func (c *Client) markTileSaved(tile tlog.Tile) {
	c.tileSavedMu.Lock()
	c.tileSaved[tile] = true
	c.tileSavedMu.Unlock()
}

// SaveTiles saves the now validated tiles.
func (r *tileReader) SaveTiles(tiles []tlog.Tile, data [][]byte) {
	c := r.c

	// Determine which tiles need saving.