From 3fef91d3428cf6d80b26dc64478ed1b1809c3275 Mon Sep 17 00:00:00 2001
From: binwiederhier <phil@heckel.io>
Date: Fri, 30 Oct 2026 21:21:59 +0000
Subject: [PATCH] Read the period end of a daily denial under the counter lock

---
 limit/service.go      |  3 ++-
 limit/service_test.go | 27 +++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/limit/service.go b/limit/service.go
index dde68cea..66b46738 100644
--- a/limit/service.go
+++ b/limit/service.go
@@ -125,11 +125,12 @@ func (s *Service) Allow(key string, rule Rule, n int64) Decision {
 	c := cs.counterNoLock(key, start)
 	total := c.base[i] + c.unflushed[i]
 	if rule.Daily > 0 && total+n > rule.Daily {
+		end := time.Unix(c.periodStart+daySeconds, 0) // Read under the lock: another call may roll the counter over
 		cs.mu.Unlock()
 		if rule.Burst > 0 {
 			s.refundTokens(key, rule.Kind, n)
 		}
-		return Decision{RetryAfter: time.Unix(c.periodStart+daySeconds, 0).Sub(dbNow)}
+		return Decision{RetryAfter: end.Sub(dbNow)}
 	}
 	c.unflushed[i] += n
 	if rule.Threshold == 0 || total+n >= rule.Threshold {
diff --git a/limit/service_test.go b/limit/service_test.go
index 904e75ec..0300ebae 100644
--- a/limit/service_test.go
+++ b/limit/service_test.go
@@ -841,3 +841,30 @@ func TestService_Persist_LeftoverAfterRollover(t *testing.T) {
 		}, usageRowOf(t, store, "b"))
 	})
 }
+
+func TestService_Allow_DenyRacesRollover(t *testing.T) {
+	// A daily denial computes its retry time from the counter it decided on, while other calls
+	// roll the counter over; run with -race
+	var ticks atomic.Int64
+	start := time.Date(2026, 10, 25, 12, 0, 0, 0, time.UTC)
+	s := newService(&Config{}, func() time.Time {
+		return start.Add(time.Duration(ticks.Add(1)) * time.Hour)
+	})
+	rule := Rule{
+		Kind:  KindMessages,
+		Daily: 1,
+	}
+	var wg sync.WaitGroup
+	for g := 0; g < 8; g++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for i := 0; i < 3000; i++ {
+				if d := s.Allow("k", rule, 1); !d.OK {
+					require.Greater(t, d.RetryAfter, time.Duration(0))
+				}
+			}
+		}()
+	}
+	wg.Wait()
+}