[Tile-serving] [openstreetmap/osm2pgsql] Add whitelist functionality to osm2pgsql.make_clean_tags_func (Discussion #1955)

[Tilman M. Jaeschke]

I use [PostGIS](https://postgis.net) and it's built-in function [ST_AsMVT](https://postgis.net/docs/ST_AsMVT.html) to provide my application client with tile data. My app is not a general purpose map and it uses only a specific set of tags. By whitelisting tag keys instead of blacklisting them i can be sure that nothing i do not need unnecessarily wastes any resources and i do not need to know every key there is and maintain a complete list.

Add this function to your flex config and call it with whitelist set to `true` to filter out tags with keys not present in the keys list. With whitelist set to `false` or not set at all it behaves exactly like the built-in function.

```lua
function osm2pgsql.make_clean_tags_func(keys, whitelist)
  whitelist = whitelist or false;

  local keys_to_consider = {}
  local prefixes_to_consider = {}
  local suffixes_to_consider = {}

  for _, k in ipairs(keys) do
    if k:sub(-1) == '*' then
      prefixes_to_consider[#prefixes_to_consider + 1] = k:sub(1, -2)
    elseif k:sub(1, 1) == '*' then
      suffixes_to_consider[#suffixes_to_consider + 1] = k:sub(2)
    else
      keys_to_consider[k] = true
    end
  end

  return function(tags)
    local delete_tags = {}
    local tag_matched

    for key, _ in pairs(tags) do
      tag_matched = keys_to_consider[key] or false

      if not tag_matched then
        for _, k in ipairs(prefixes_to_consider) do
          if osm2pgsql.has_prefix(key, k) then
            tag_matched = true
            break
          end
        end
      end

      if not tag_matched then
        for _, k in ipairs(suffixes_to_consider) do
          if osm2pgsql.has_suffix(key, k) then
            tag_matched = true
            break
          end
        end
      end

      if tag_matched ~= whitelist then
        delete_tags[#delete_tags + 1] = key
      end
    end

    for _, key in ipairs(delete_tags) do
      tags[key] = nil
    end

    return next(tags) == nil
  end
end
```

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