2020-11-09 Lua module for highway shields committed

Today, I tidied up, tested and committed a Lua module for processing highway shields in osm2pgsql.

I've tried to keep the design as non-intrusive as possible to people's existing workflow, assuming they already use the flex backend. There are just four changes needed to the Lua style file.

In initialization, add the lines:

      local shieldtables = require("shieldtables")
      local shieldt = shieldtables.new(prefix)
      

where prefix is the prefix (e.g., planet_osm) being used in the database for the OSM tables.

In the osm2pgsql.process_way function, add the line:

      shieldt:process_way(object);
      

In the osm2pgsql.process_relation function, add the line:

      shieldt:process_way(object);
      

Create a osm2pgsql.select_relation_members function if you don't already have one. There's a call:

      shieldt:select_relation_members(object)
      

with the same API that returns the necessary selection. For all the style sheets that I was using, I was able just to use:

      function osm2pgsql.select_relation_members(object)
	     return shieldt:select_relation_members(object)
      end
      

And that's pretty much it, for how to use it. The database tables prefix_shieldroute and prefix_shieldway are managed entirely by those procedures.

I still need to test to make sure that incremental update works with the new setup. If it does, that's half the job of making minutely updates work, I think.