remotes: add least-connections mirror strategy (round-robin remains default)

Add a per-remote mirror_strategy selector for the mirrorlist, supporting
round_robin (default, unchanged) and least_conn.

- models.Remote gains MirrorStrategy string + MirrorStrategy{RoundRobin,LeastConn}
  constants and ValidateMirrorStrategy (enum check; least_conn requires a
  non-empty mirrorlist). 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: least_conn starts each attempt with the pool URL holding the fewest
  in-flight requests via a per-remote/per-URL atomic gauge (incremented around
  each upstream call in head/fetch/checkUpstream), ties broken by the existing
  round-robin rotation. Round-robin path and failover order 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 strategy validation; DB round-trip covers the
  new column; docker e2e adds a least_conn distribution test and a real dnf
  install through a least_conn remote.

internal/proxy/leastconn_test.go

@@ -0,0 +1,103 @@
+package proxy
+
+import (
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+// baseURLAttemptOrder and the in-flight gauge only touch the engine's sync.Map
+// fields, so these tests run against a zero-value Engine without a DB/S3/redis
+// stack and can drive the gauge deterministically.
+
+// TestLeastConnPicksLeastLoaded pre-loads one mirror's in-flight gauge and
+// asserts a least_conn remote starts its attempt order with the idle mirror.
+func TestLeastConnPicksLeastLoaded(t *testing.T) {
+	e := &Engine{}
+	r := models.Remote{
+		Name:           "lc",
+		BaseURL:        "https://a.example",
+		Mirrorlist:     []string{"https://b.example"},
+		MirrorStrategy: models.MirrorStrategyLeastConn,
+	}
+
+	// Make A appear busy: least_conn must prefer B regardless of RR rotation.
+	e.inflightCounter(r.Name, "https://a.example").Add(3)
+	for i := 0; i < 5; i++ {
+		order := e.baseURLAttemptOrder(r)
+		if len(order) != 2 {
+			t.Fatalf("attempt %d: order len = %d, want 2", i, len(order))
+		}
+		if order[0] != "https://b.example" {
+			t.Fatalf("attempt %d: least_conn started with %q, want idle mirror https://b.example", i, order[0])
+		}
+	}
+
+	// Once B is the busier mirror, the starting pick flips to A.
+	e.inflightCounter(r.Name, "https://b.example").Add(10)
+	if order := e.baseURLAttemptOrder(r); order[0] != "https://a.example" {
+		t.Fatalf("after loading B, least_conn started with %q, want https://a.example", order[0])
+	}
+}
+
+// TestRoundRobinDefaultUnchanged asserts an unset strategy still rotates the
+// starting mirror across the pool and ignores the in-flight gauge.
+func TestRoundRobinDefaultUnchanged(t *testing.T) {
+	e := &Engine{}
+	r := models.Remote{
+		Name:       "rr",
+		BaseURL:    "https://a.example",
+		Mirrorlist: []string{"https://b.example"},
+	}
+
+	// Even with A heavily loaded, round-robin must still rotate (not avoid A).
+	e.inflightCounter(r.Name, "https://a.example").Add(100)
+	starts := map[string]int{}
+	for i := 0; i < 4; i++ {
+		starts[e.baseURLAttemptOrder(r)[0]]++
+	}
+	if starts["https://a.example"] == 0 || starts["https://b.example"] == 0 {
+		t.Fatalf("round-robin did not rotate starting mirror: %v", starts)
+	}
+}
+
+// TestLeastConnSingleURLNoOp asserts a single-URL pool yields exactly [base_url]
+// and beginAttempt is a no-op there and for round-robin remotes.
+func TestLeastConnSingleURLNoOp(t *testing.T) {
+	e := &Engine{}
+	solo := models.Remote{Name: "solo", BaseURL: "https://a.example", MirrorStrategy: models.MirrorStrategyLeastConn}
+	if order := e.baseURLAttemptOrder(solo); len(order) != 1 || order[0] != "https://a.example" {
+		t.Fatalf("single-url order = %v, want [base_url]", order)
+	}
+	if ctr := e.beginAttempt(solo, "https://a.example"); ctr != nil {
+		t.Fatal("beginAttempt on single-url pool should be a no-op (nil)")
+	}
+
+	rr := models.Remote{Name: "rr2", BaseURL: "https://a.example", Mirrorlist: []string{"https://b.example"}}
+	if ctr := e.beginAttempt(rr, "https://a.example"); ctr != nil {
+		t.Fatal("beginAttempt on round-robin remote should be a no-op (nil)")
+	}
+}
+
+// TestBeginEndAttemptGauge asserts the gauge increments on begin and returns to
+// zero after endAttempt, so it tracks live in-flight requests.
+func TestBeginEndAttemptGauge(t *testing.T) {
+	e := &Engine{}
+	r := models.Remote{
+		Name:           "g",
+		BaseURL:        "https://a.example",
+		Mirrorlist:     []string{"https://b.example"},
+		MirrorStrategy: models.MirrorStrategyLeastConn,
+	}
+	c1 := e.beginAttempt(r, "https://a.example")
+	c2 := e.beginAttempt(r, "https://a.example")
+	if got := e.inflightCounter(r.Name, "https://a.example").Load(); got != 2 {
+		t.Fatalf("gauge after two begins = %d, want 2", got)
+	}
+	endAttempt(c1)
+	endAttempt(c2)
+	if got := e.inflightCounter(r.Name, "https://a.example").Load(); got != 0 {
+		t.Fatalf("gauge after matching ends = %d, want 0", got)
+	}
+	endAttempt(nil) // tolerated
+}