eth/txtracker: track per-peer tx-request response latency

Adds NotifyRequestLatency(peer, latency) and a slow per-peer EMA
(alpha=0.01, ~70-sample half-life) that the dropper will use as a
new protection signal. The first sample seeds the EMA directly so
fresh peers don't ramp up from zero. RequestSamples is exposed
alongside the EMA so consumers can apply a minimum-samples bootstrap
guard before trusting the value.

Includes design notes for the broader peerdrop-latency feature.

eth/txtracker/tracker.go

@@ -28,6 +28,7 @@ package txtracker
 
 import (
 	"sync"
+	"time"
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core"
@@ -46,12 +47,22 @@ const (
 	// sustained contribution over long windows, not recent bursts.
 	// Half-life ≈ 6930 chain heads (~23 hours on 12s blocks).
 	finalizedEMAAlpha = 0.0001
+	// EMA smoothing factor for per-request latency average. Slow on purpose:
+	// short bursts shouldn't shift the score, sustained behavior should.
+	// Half-life ≈ ln(0.5)/ln(0.99) ≈ 69 samples.
+	latencyEMAAlpha = 0.01
+	// MinLatencySamples is the number of latency samples a peer must accumulate
+	// before its RequestLatencyEMA is considered meaningful for protection.
+	// Prevents a single lucky-fast reply from displacing established peers.
+	MinLatencySamples = 10
 )
 
-// PeerStats holds the per-peer inclusion data.
+// PeerStats holds the per-peer inclusion and responsiveness data.
 type PeerStats struct {
-	RecentFinalized float64 // EMA of per-block finalization credits (slow)
-	RecentIncluded  float64 // EMA of per-block inclusions (fast)
+	RecentFinalized   float64       // EMA of per-block finalization credits (slow)
+	RecentIncluded    float64       // EMA of per-block inclusions (fast)
+	RequestLatencyEMA time.Duration // Slow EMA of tx-request response latency (timeouts count as the timeout value)
+	RequestSamples    int64         // Number of latency samples seen for this peer
 }
 
 // Chain is the blockchain interface needed by the tracker.
@@ -63,8 +74,10 @@ type Chain interface {
 }
 
 type peerStats struct {
-	recentFinalized float64
-	recentIncluded  float64
+	recentFinalized   float64
+	recentIncluded    float64
+	requestLatencyEMA time.Duration
+	requestSamples    int64
 }
 
 // Tracker records which peer delivered each transaction and credits peers
@@ -155,6 +168,33 @@ func (t *Tracker) NotifyAccepted(peer string, hashes []common.Hash) {
 	}
 }
 
+// NotifyRequestLatency records a tx-request response latency sample for the
+// given peer. Timeouts should be reported as the timeout value (so they count
+// against the EMA rather than being silently omitted). The EMA uses a slow
+// alpha so isolated bursts don't shift the score appreciably.
+// Safe to call from any goroutine.
+func (t *Tracker) NotifyRequestLatency(peer string, latency time.Duration) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	ps := t.peers[peer]
+	if ps == nil {
+		ps = &peerStats{}
+		t.peers[peer] = ps
+	}
+	if ps.requestSamples == 0 {
+		// Bootstrap the EMA with the first sample so it doesn't drift up
+		// from zero over many samples before reaching realistic values.
+		ps.requestLatencyEMA = latency
+	} else {
+		ps.requestLatencyEMA = time.Duration(
+			float64(ps.requestLatencyEMA)*(1-latencyEMAAlpha) +
+				float64(latency)*latencyEMAAlpha,
+		)
+	}
+	ps.requestSamples++
+}
+
 // GetAllPeerStats returns a snapshot of per-peer inclusion statistics.
 // Safe to call from any goroutine.
 func (t *Tracker) GetAllPeerStats() map[string]PeerStats {
@@ -164,8 +204,10 @@ func (t *Tracker) GetAllPeerStats() map[string]PeerStats {
 	result := make(map[string]PeerStats, len(t.peers))
 	for id, ps := range t.peers {
 		result[id] = PeerStats{
-			RecentFinalized: ps.recentFinalized,
-			RecentIncluded:  ps.recentIncluded,
+			RecentFinalized:   ps.recentFinalized,
+			RecentIncluded:    ps.recentIncluded,
+			RequestLatencyEMA: ps.requestLatencyEMA,
+			RequestSamples:    ps.requestSamples,
 		}
 	}
 	return result

eth/txtracker/tracker_test.go

@@ -454,3 +454,105 @@ func TestRecentFinalizedDecays(t *testing.T) {
 		t.Fatalf("expected RecentFinalized to decay, got %f >= peak %f", after, peak)
 	}
 }
+
+// TestRequestLatencyFirstSampleBootstrap asserts that the first latency
+// sample seeds the EMA directly (no slow ramp-up from zero), and that the
+// sample counter starts at 1.
+func TestRequestLatencyFirstSampleBootstrap(t *testing.T) {
+	tr := New()
+	tr.NotifyRequestLatency("peerA", 200*time.Millisecond)
+
+	stats := tr.GetAllPeerStats()
+	ps := stats["peerA"]
+	if ps.RequestLatencyEMA != 200*time.Millisecond {
+		t.Fatalf("expected first sample to seed EMA at 200ms, got %v", ps.RequestLatencyEMA)
+	}
+	if ps.RequestSamples != 1 {
+		t.Fatalf("expected RequestSamples=1, got %d", ps.RequestSamples)
+	}
+}
+
+// TestRequestLatencyEMAUpdate verifies the EMA formula (1-α)·old + α·new.
+func TestRequestLatencyEMAUpdate(t *testing.T) {
+	tr := New()
+	tr.NotifyRequestLatency("peerA", 100*time.Millisecond)
+	tr.NotifyRequestLatency("peerA", 1000*time.Millisecond)
+
+	// Expected: 0.99*100ms + 0.01*1000ms = 109ms
+	got := tr.GetAllPeerStats()["peerA"].RequestLatencyEMA
+	want := 109 * time.Millisecond
+	delta := got - want
+	if delta < 0 {
+		delta = -delta
+	}
+	if delta > 1*time.Microsecond {
+		t.Fatalf("EMA mismatch: got %v, want %v (delta %v)", got, want, delta)
+	}
+	if samples := tr.GetAllPeerStats()["peerA"].RequestSamples; samples != 2 {
+		t.Fatalf("expected RequestSamples=2, got %d", samples)
+	}
+}
+
+// TestRequestLatencySlowEMAConvergence verifies that the slow alpha
+// requires many samples to noticeably shift the EMA. Starting at 100ms
+// and feeding 5s (timeout) samples, the EMA should still be well below
+// 1s after 50 samples.
+func TestRequestLatencySlowEMAConvergence(t *testing.T) {
+	tr := New()
+	tr.NotifyRequestLatency("peerA", 100*time.Millisecond)
+	for i := 0; i < 50; i++ {
+		tr.NotifyRequestLatency("peerA", 5*time.Second)
+	}
+	got := tr.GetAllPeerStats()["peerA"].RequestLatencyEMA
+	if got < 1*time.Second {
+		// Expected ≈ (0.99)^50 * 100ms + (1-(0.99)^50) * 5s ≈ 1.99s
+		// The lower bound proves a meaningful shift; the upper bound (below)
+		// proves the slow alpha damped the convergence.
+		t.Fatalf("EMA did not move enough under sustained timeouts, got %v", got)
+	}
+	if got > 3*time.Second {
+		t.Fatalf("EMA converged too fast for slow alpha=0.01, got %v", got)
+	}
+}
+
+// TestRequestLatencyMultiplePeersIsolated verifies per-peer isolation: a
+// sample for peerA does not affect peerB's stats.
+func TestRequestLatencyMultiplePeersIsolated(t *testing.T) {
+	tr := New()
+	tr.NotifyRequestLatency("peerA", 100*time.Millisecond)
+	tr.NotifyRequestLatency("peerB", 5*time.Second)
+
+	stats := tr.GetAllPeerStats()
+	if stats["peerA"].RequestLatencyEMA != 100*time.Millisecond {
+		t.Errorf("peerA EMA: got %v, want 100ms", stats["peerA"].RequestLatencyEMA)
+	}
+	if stats["peerB"].RequestLatencyEMA != 5*time.Second {
+		t.Errorf("peerB EMA: got %v, want 5s", stats["peerB"].RequestLatencyEMA)
+	}
+	if stats["peerA"].RequestSamples != 1 || stats["peerB"].RequestSamples != 1 {
+		t.Errorf("expected RequestSamples=1 for each peer, got A=%d B=%d",
+			stats["peerA"].RequestSamples, stats["peerB"].RequestSamples)
+	}
+}
+
+// TestRequestLatencyPeerDropResetsStats verifies that NotifyPeerDrop
+// removes the peer's latency history along with its other stats.
+func TestRequestLatencyPeerDropResetsStats(t *testing.T) {
+	tr := New()
+	tr.NotifyRequestLatency("peerA", 200*time.Millisecond)
+	tr.NotifyPeerDrop("peerA")
+
+	if _, ok := tr.GetAllPeerStats()["peerA"]; ok {
+		t.Fatal("peerA stats should be removed after NotifyPeerDrop")
+	}
+
+	// A subsequent latency sample re-creates the entry as a fresh peer.
+	tr.NotifyRequestLatency("peerA", 50*time.Millisecond)
+	ps := tr.GetAllPeerStats()["peerA"]
+	if ps.RequestSamples != 1 {
+		t.Fatalf("expected RequestSamples=1 after re-add, got %d", ps.RequestSamples)
+	}
+	if ps.RequestLatencyEMA != 50*time.Millisecond {
+		t.Fatalf("expected fresh EMA bootstrap, got %v", ps.RequestLatencyEMA)
+	}
+}