Size-limited pool for GTFS cache #805
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Hard limit on total number of feeds open at once. get() blocks when full until a feed can be evicted (has no users). only one GTFSFeed can be open for each key at a time. Mostly untested draft, needs serious audit for concurrency issues. To simplify (while losing generality): reference counting logic could be pushed down into GTFSFeed itself. ReferenceCountingCache logic could be pulled up into GTFSCache.
considering LimitedPool as an option of LinkageCache
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Addresses #799. Related to #473. This is concurrent code managing its own locks and conditions, so should be subject to additional scrutiny and testing under load. That said, the locking has been kept relatively coarse to make it easier to follow.
This LimitedPool has been wired up to GTFSFeeds and spatial indexes, with the maximum size set very small for testing. It should also be applicable to things like linkages and distance tables and the AsyncPreloader, but those are not wired up yet.
For now the logging is set to be quite noisy on the LimitedPool class so we can observe how it behaves under load.