This change set introduces per-user Bitbucket ACLs caching that respects a user configured TTL. It leverages Postgres row locking for concurrency control of cache fill events, so that many concurrent requests during an expiration don't overload the Bitbucket Server API.
It features a second in-memory read-through cache to avoid the marshaling and network round trip costs during steady state.
We cache the complete set of Sourcegraph repository ids a user has access to. This set is represented as a highly compressed bitmap to save on storage, memory and compute resources. For a hypothetical upper bound of 100k repositories, and 10k users, the aggregate memory usage would be rounding 329,36 MB per frontend API instance. For 30k repositories and 1k users, that number would be 16,472MB. This is computed with https://godoc.org/github.com/RoaringBitmap/roaring#BoundSerializedSizeInBytes
