[Tile-serving] [osm2pgsql-dev/osm2pgsql] Allow a limit to be set on expiry of line segments (PR #2185)

[Sarah Hoffmann]

A new release of osm2pgsql is imminent, so we need to decide what happens to the this PR. As said above, I would be okay to merge if this is essential to the operations of the OSMF servers. However, I do share some of @joto's concerns that this isn't really addressing the root cause and thus only working around one very specific form of vandalism, which may never occur again in this form.

@tomhughes what is the status here? I understand that we have switched the servers to expire on metatile level. This should address the memory issue (#2190) and the issue of being stuck for hours writing out the expired tiles. Would that be sufficient for now?

The other possible advantage I can see is that the render servers wouldn't end up with basically all tiles marked as outdated. Was that a concern? Was the additional load on rerendering causing issues?

We do know that the chance will not prevent vandalised tiles from appearing. It just will be on less tiles. And after the change, it will be necessary to manually expire all affected tiles. Probably not much of a change in the case of global vandalism. However I can see where this might become a major pain when we run into the case where a user accidentally draws a single line over half the globe and then globally all tiles rendered in a certain timespan need to be invalidated because finding just the ones that touch the single line is difficult.

Any other pros and cons?

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