## Why
`least_conn` mirror selection (`baseURLAttemptOrder`) scaled super-linearly. After rotating the pool by the round-robin cursor it `sort.SliceStable`d with a comparator that called `inflightCounter` on **every comparison** — and each call did a `remoteName+"\x00"+url` concat plus a `sync.Map` `LoadOrStore` with a speculative `new(atomic.Int64)`. So each selection cost O(n·log n) map lookups + allocations, all on the cache-miss/upstream path.
## How
Snapshot each mirror's in-flight count **once**, then sort the snapshot — O(n) map loads, zero comparator allocations.
- Add read-only `inflightCount(name, url) int64`: plain `sync.Map` `Load`, returns 0 when the gauge is absent (no `LoadOrStore`, no speculative allocation).
- `least_conn` branch builds a `{url, count}` snapshot via one `inflightCount` per rotated URL, `sort.SliceStable` by `count` ascending, then extracts the URLs.
- `beginAttempt`/`endAttempt` keep the create-on-write `inflightCounter` path — they legitimately need to create the gauge.
## Numbers (`BenchmarkBaseURLAttemptOrder_LeastConn`, Ryzen 7 4700U, best of 3)
| pool | before ns/op | after ns/op | before allocs | after allocs | before B/op | after B/op |
|------|-------------:|------------:|--------------:|-------------:|------------:|-----------:|
| 3 | ~2516 | ~1492 | 22 | 8 | 474 | 296 |
| 8 | ~14647 | ~3319 | 131–132 | 8 | 2688 | 568 |
Allocs are now **constant** regardless of pool size; pool-8 is ~4.8x faster with ~16x fewer allocations.
## Behavior
Unchanged: least-loaded first, RR rotation as the stable tie-break, `round_robin` and single-URL paths untouched. Pure internal optimization — no API/schema/DB change. Added a multi-mirror tie-break test asserting all-equal load yields the RR rotation; `make test` (`-race`) green, vet/fmt clean.
Reviewed-on: #124
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>
## Why
The mirrorlist (PR #121) always load-balances round-robin. Round-robin is oblivious to how busy each mirror is, so a slow or saturated mirror keeps getting its fair share of new requests. This adds an opt-in **least-connections** strategy that favors the mirror currently handling the fewest in-flight requests, steering new work toward idle mirrors. Round-robin stays the default, so existing remotes are unchanged.
## How
- **Model**: `models.Remote` gains `mirror_strategy` (`round_robin` default/empty, or `least_conn`). `ValidateMirrorStrategy` checks the enum and requires a non-empty mirrorlist for `least_conn`. Empty behaves as `round_robin` for back-compat.
- **DB**: additive `mirror_strategy TEXT NOT NULL DEFAULT 'round_robin'` column (CREATE TABLE + `ADD COLUMN IF NOT EXISTS`), wired through remoteCols/scanRemote/CreateRemote/UpdateRemote; empty normalized to `round_robin` on write.
- **Engine**: a per-remote, per-pool-URL atomic in-flight gauge is incremented around each upstream call (head/fetch/checkUpstream). For a `least_conn` remote the attempt order starts with the least-loaded pool URL (ties broken by the existing round-robin rotation). Round-robin path and failover order are unchanged; single-URL pools are a no-op.
- **Tests**: unit tests for least-loaded selection, round-robin default, gauge inc/dec, single-URL no-op, and validation; DB round-trip covers the new column; docker e2e adds a `least_conn` distribution test plus a real `dnf` install through a `least_conn` remote.
Back-compat: unset/empty `mirror_strategy` is `round_robin`, so all existing remotes keep their current behavior.
Reviewed-on: #122
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>
unkin-agent