fix(rps): stop dropping requests recorded during a sample

The sampler read the counter and then reset it in two steps. Requests recorded between the steps were lost. The sampler now swaps the counter to zero in one atomic step.

rps_tracker.go

@@ -57,17 +57,23 @@ func (r *RPSTracker) Shutdown() {
 
 // sample calculates RPS since last sample
 func (r *RPSTracker) sample() {
-	now := time.Now()
-	nowNano := now.UnixNano()
+	r.sampleCount()
+}
+
+// sampleCount performs one sample and returns the requests it consumed.
+func (r *RPSTracker) sampleCount() int64 {
+	nowNano := time.Now().UnixNano()
 
-	currentCount := r.lastCount.Load()
 	lastSampleNano := r.lastSampleTime.Load()
 
 	if lastSampleNano == 0 {
 		r.lastSampleTime.Store(nowNano)
-		return
+		return 0
 	}
 
+	// Swap, not Load+Store(0): requests recorded between the two would be lost.
+	currentCount := r.lastCount.Swap(0)
+
 	elapsed := float64(nowNano-lastSampleNano) / float64(time.Second)
 	if elapsed > 0 {
 		rps := float64(currentCount) / elapsed
@@ -75,9 +81,8 @@ func (r *RPSTracker) sample() {
 		r.currentRPS.Store(uint64(rps * 100))
 	}
 
-	// Reset for next sample
-	r.lastCount.Store(0)
 	r.lastSampleTime.Store(nowNano)
+	return currentCount
 }
 
 // GetCurrentRPS returns the current requests per second

rps_tracker_sample_test.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"context"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// Pins theorems/Theorems/RpsSample.lean: no recorded request is dropped by sample().
+func TestRPSTracker_SampleKeepsConcurrentRequests(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	tr := NewRPSTracker(ctx)
+	tr.Shutdown() // stop the background sampler; sample() is driven by the test
+
+	const writers, perWriter = 8, 20000
+	var wg sync.WaitGroup
+	for i := 0; i < writers; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for j := 0; j < perWriter; j++ {
+				tr.RecordRequest()
+			}
+		}()
+	}
+	var sampled int64
+	done := make(chan struct{})
+	go func() { wg.Wait(); close(done) }()
+loop:
+	for {
+		select {
+		case <-done:
+			break loop
+		default:
+			sampled += tr.sampleCount()
+		}
+	}
+	sampled += tr.sampleCount()
+	assert.Equal(t, int64(writers*perWriter), sampled)
+}