Save both forward and reverse datasources.

[danpat]

This PR addresses #4346 where we were not correctly saving or retrieving both forward and reverse datasource attributes for road segments (roads can have data from different sources in different directions).

The basic summary is that we did not have separate storage for forward and reverse data - so we were randomly overwriting with whichever datasource was applied to the graph last. There was also an off-by-one bug that meant we were often seeing datasource values from other roads on the first/last segment of the road in question.

In the following two images, colours represent different sources of data:

Before this change, there was a lot of this going on:

After this change, the map is a lot more consistent, as expected:

NOTE: this doubles the amount of disk/RAM needed to store datasource IDs. The good news is that we only use std::uint8_t for each segment.

I tested with mexico-latest.osm.pbf just to confirm, and the .geometry file increased by the expected amount:

before: 211,090,822
after: 223,418,490
% increase: ~5% (note this is just for this file, it's closer to 2% for the total dataset)

I would expect this to scale for the planet file as well.

NOTE 2: This PR changes the file format (we store more data now) - there's no way to backport this compatibly to 5.9.

Tasklist

• add regression / cucumber cases (see docs/testing.md)
• review
• adjust for comments

[TheMarex]

Looks good to me. 👍