Optional Postgrex-backed range store using 1 MiB bytea chunks.

Pack and index chunks are immutable rows keyed by (pack_id, artifact, chunk_index). A singleton manifest row carries both the publisher generation column and the JSON manifest. publish/3 inserts all missing chunks and updates the manifest in one database transaction, so readers observe the previous generation or the complete new one.

Postgrex is an optional Gitility dependency. Applications that use this backend must include {:postgrex, "~> 0.22.4"}; calls return the explicit :postgrex_not_available error when it was omitted.

{:ok, supervisor} =
  Gitility.ODB.PackFetch.start_link(
    backend: {Gitility.ODB.RangeBackend.Postgres,
              [url: System.fetch_env!("DATABASE_URL")]},
    into: {:dir, "/var/cache/gitility"}
  )

init/1 accepts connection: pid or Postgrex connection options such as url:. A pid is treated as an existing supervised pool and is never stopped by this backend. URL/options make init/1 call Postgrex.start_link/1; that pool is owned and stopped by terminate/2, but because backend init runs in the PackFetch.start_link/1 caller it is linked to that caller rather than the provider tree. Production callers should therefore start Postgrex under their own supervisor and pass its name or pid (a pid today); URL/options are a convenience for scripts and tests. The optional prefix: (default "gitility_pack") is restricted to SQL identifier characters. publish/3 takes a repository, either a pid, URL, or Postgrex options, and backend options such as prefix:.