Set handles both single-fetcher (one git repo) and multi-fetcher
(GeoLite2 City + ASN) cases uniformly. Any fetcher reporting an update
triggers a view reload. This replaces the per-caller FetcherFunc wrapper
that combined the two MaxMind cachers and the ad-hoc atomic.Pointer +
ticker goroutine in cmd/check-ip — geoip now rides on the same
Set/View/Load/Tick surface as the blocklists.
Stats the given paths and reports updated when any size/modtime
changes since the last call. First call always reports true so the
initial Load populates views.
check-ip uses it for --inbound/--outbound so edits to local lists
get picked up by Group.Tick without a restart.
Distilled from the previous net/dataset experiment and the inline
closure version in check-ip. Keeps what actually earned its keep:
- Group ties one Fetcher to N views; a single Load drives all swaps,
so shared sources (one git pull, one zip download) don't get
re-fetched per view.
- View[T].Value() is a lock-free atomic read; the atomic.Pointer is
hidden so consumers never see in-flight reloads.
- Tick runs Load on a ticker with stderr error logging.
Dropped from the v1 design: MultiSyncer (callers fan-out inline when
needed), Close (unused outside geoip), Name (callers wrap the logger),
standalone Dataset type (Group with one view covers it), Sync vs Init
asymmetry (Load handles first-call vs update internally).
check-ip rewires to use it — file/git/http modes all build a Group
with two views, uniform shape.
