[Tile-serving] [openstreetmap/osm2pgsql] Lua relation members gets post-way transform member tags (#659)

[Sarah Hoffmann]

There is a comment in the code [here](https://github.com/openstreetmap/osm2pgsql/blob/master/output-pgsql.cpp#L423-L427) confirming that the filtering is intentional. It was introduced in 2565bf738fe69b18443971c40fe4a142163a3714 and the reasoning there is consistent with the old C implementation.

The easy way to work around your problem is to add a fixed tag to filtered ways and check for that, i.e. like that:

```
function wood_ways(tags, num_keys)
    if tags["natural"] == wood then
        return 0, { name = tags["name"] } , 1, 0
    end
    return 1, { notwood = "" }, 0, 0
end

function wood_rel_members (tags, member_tags, member_roles, membercount)
  if new-style MP then
    -- do stuff
  else
    local iswood = false
    for i = 1,membercount do
      if member_tags["notwood"] == nil then
        iswood = true
        break
      end
    end
    if iswood then
       -- do more stuff
    end
end
```
Sidenote: this hack exploits the fact that we always unpack four return arguments for the function, even when the element is filtered. It might be a good idea for efficiency to reconsider that and not return tags when the element is marked as filtered.

While I don't think that the current behaviour is a bug (it serves the limited use-case of old-style MP filtering as done in the C transform well enough), I would agree that it is far from ideal. I'd be tempted to move the filtering into the LUA functions/C transform, so that it is left to the user to decide if the second filter step is even necessary. However, I don't know how many people rely on the current behaviour.

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