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## Problem To implement split brain protection, we need tenants and timelines to be aware of their current generation, and use it when composing S3 keys. ## Summary of changes - A `Generation` type is introduced in the `utils` crate -- it is in this broadly-visible location because it will later be used from `control_plane/` as well as `pageserver/`. Generations can be a number, None, or Broken, to support legacy content (None), and Tenants in the broken state (Broken). - Tenant, Timeline, and RemoteTimelineClient all get a generation attribute - IndexPart's IndexLayerMetadata has a new `generation` attribute. Legacy layers' metadata will deserialize to Generation::none(). - Remote paths are composed with a trailing generation suffix. If a generation is equal to Generation::none() (as it currently always is), then this suffix is an empty string. - Functions for composing remote storage paths added in remote_timeline_client: these avoid the way that we currently always compose a local path and then strip the prefix, and avoid requiring a PageserverConf reference on functions that want to create remote paths (the conf is only needed for local paths). These are less DRY than the old functions, but remote storage paths are a very rarely changing thing, so it's better to write out our paths clearly in the functions than to compose timeline paths from tenant paths, etc. - Code paths that construct a Tenant take a `generation` argument in anticipation that we will soon load generations on startup before constructing Tenant. Until the whole feature is done, we don't want any generation-ful keys though: so initially we will carry this everywhere with the special Generation::none() value. Closes: https://github.com/neondatabase/neon/issues/5135 Co-authored-by: Christian Schwarz <christian@neon.tech>