Mapnik.VectorTile.painted() doesn't do what we want it to do

TLDR

mapnik.VectorTile.painted() doesn't do exactly what we want any longer, which means our check here isn't as strong as expected.

Problem

When generating tiles and copying them (via tilelive), we have a check in the pyramid tiling scheme to ensure we should continue rendering and copying child tiles. This check is here.

There are some situations where we could have a datasource (geojson for this example) that has data in the z0 tile, but has no data in the z1 tile. This means tilelive-bridge checks vtile.painted() and sets the x-tilelive-contains-data header to false. If this is set to false, the rest of the tiling scheme is skipped, resulting in no tiles.

Node Mapnik essentially uses empty() to check for painted tiles via mapnik-vector-tile, which is exactly what we don't want. Right now this z1 tile returns false for empty(), which tells tilelive to stop the copy process and skip the rest of the tiles. We need to know if the source data is there (even if tile features are not) so we can continue to tile the children.

Why

Currently, I'm unable to figure out why a datasource in Node Mapnik would add data at z0 but not at z1.

Solution

A couple solutions come to mind:

• [time intensive] improve the painted(), solid(), and empty() architecture in Mapnik. cc @flippmoke
• [quick fix, potentially at CPU expense] don't check for painted/empty tiles since we already have a relatively solid pyramid built via mapnik-omnivore and spherical mercator. This either means we always set x-tilelive-contains-data to true or remove the if statement in tilelive.

cc @rclark @GretaCB @springmeyer @yhahn @flippmoke @captainbarbosa

Start with the check in tilelive-bridge/index.js and the child-tile decision in tilelive/lib/stream-pyramid.js, then compare their behavior with empty() in mapnik-vector-tile/src/vector_tile_tile.hpp. Determine how source data should be detected when a tile has no features, and choose between fixing the painted/solid/empty architecture or removing the check. Done means child tiling continues for the described z0/z1 case without incorrectly reporting the tile as empty.