fix(refresh): reset window to zero attempts and admit one half-open probe

The window and cooldown resets set attempts to 1. The gate then counted
the same call again, so a reset window allowed one call fewer than a
fresh one. With a limit of 1 the session locked out at once. Resets now
start at 0.

The half-open breaker admitted every caller. It now admits one probe,
claimed with a compare-and-swap. A stale probe is replaced after
OpenDuration, so a lost probe cannot wedge the breaker.

refresh_window_halfopen_test.go

@@ -0,0 +1,93 @@
+package traefikoidc
+
+import (
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// After a window reset the tracker must allow the full MaxRefreshAttempts,
+// the same as a fresh tracker.
+func TestRefreshWindowResetAllowsFullQuota(t *testing.T) {
+	cfg := DefaultRefreshCoordinatorConfig()
+	cfg.MaxRefreshAttempts = 3
+	cfg.RefreshAttemptWindow = 50 * time.Millisecond
+	cfg.RefreshCooldownPeriod = time.Hour
+	rc := NewRefreshCoordinator(cfg, GetSingletonNoOpLogger())
+	defer rc.Shutdown()
+
+	pass := func() int {
+		n := 0
+		for i := 0; i < 10; i++ {
+			if rc.isInCooldown("s") {
+				break
+			}
+			rc.recordRefreshAttempt("s")
+			n++
+		}
+		return n
+	}
+	if got := pass(); got != 3 {
+		t.Fatalf("fresh window passes=%d want 3", got)
+	}
+	// Window expiry alone cannot reset while in cooldown, so use a new
+	// session that never hits the cap: record 1, wait out the window.
+	rc.recordRefreshAttempt("w")
+	time.Sleep(80 * time.Millisecond)
+	n := 0
+	for i := 0; i < 10; i++ {
+		if rc.isInCooldown("w") {
+			break
+		}
+		rc.recordRefreshAttempt("w")
+		n++
+	}
+	if n != 3 {
+		t.Fatalf("post-reset window passes=%d want 3", n)
+	}
+}
+
+// Half-open admits a single probe until it reports.
+func TestRefreshBreakerHalfOpenSingleProbe(t *testing.T) {
+	cb := &RefreshCircuitBreaker{config: RefreshCircuitBreakerConfig{
+		MaxFailures: 1, OpenDuration: 20 * time.Millisecond, HalfOpenRequests: 1,
+	}}
+	cb.RecordFailure()
+	time.Sleep(40 * time.Millisecond)
+	var admitted int32
+	var wg sync.WaitGroup
+	for i := 0; i < 20; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if cb.AllowRequest() {
+				atomic.AddInt32(&admitted, 1)
+			}
+		}()
+	}
+	wg.Wait()
+	if admitted != 1 {
+		t.Fatalf("half-open admitted %d, want 1", admitted)
+	}
+	cb.RecordSuccess()
+	if !cb.AllowRequest() {
+		t.Fatal("breaker must close after probe success")
+	}
+}
+
+// A probe that never reports must not wedge the breaker.
+func TestRefreshBreakerHalfOpenProbeTimeout(t *testing.T) {
+	cb := &RefreshCircuitBreaker{config: RefreshCircuitBreakerConfig{
+		MaxFailures: 1, OpenDuration: 20 * time.Millisecond, HalfOpenRequests: 1,
+	}}
+	cb.RecordFailure()
+	time.Sleep(40 * time.Millisecond)
+	if !cb.AllowRequest() || cb.AllowRequest() {
+		t.Fatal("want exactly one probe")
+	}
+	time.Sleep(40 * time.Millisecond)
+	if !cb.AllowRequest() {
+		t.Fatal("stale probe must allow a new one")
+	}
+}