[relay] randomize the relay reconnect backoff

The relay client's reconnect backoff was built without a RandomizationFactor,
so it stayed at the zero value and every client that lost the same relay
server retried on the identical 2/4/8/16/32/60 second schedule, bunching up on
the server as soon as it came back.

Use backoff.DefaultRandomizationFactor, as the other backoffs in the client do.
The mean interval is unchanged; only the per-client offset differs. The
exponential backoff construction moves into newExponentialBackOff so the
configuration can be asserted directly: backoff.NewTicker fires its first tick
immediately, so the schedule is not observable through the ticker.

shared/relay/client/guard.go

@@ -154,14 +154,19 @@ func (g *Guard) notifyReconnected() {
 }
 
 func (g *Guard) exponentTicker(ctx context.Context) *backoff.Ticker {
-	bo := backoff.WithContext(&backoff.ExponentialBackOff{
-		InitialInterval: 2 * time.Second,
-		Multiplier:      2,
-		MaxInterval:     g.maxBackoffInterval,
-		Clock:           backoff.SystemClock,
-	}, ctx)
+	return backoff.NewTicker(backoff.WithContext(g.newExponentialBackOff(), ctx))
+}
 
-	return backoff.NewTicker(bo)
+func (g *Guard) newExponentialBackOff() *backoff.ExponentialBackOff {
+	return &backoff.ExponentialBackOff{
+		InitialInterval: 2 * time.Second,
+		// Spreads the reconnects of every client that lost the same relay server, so they do
+		// not hit it at the same instants.
+		RandomizationFactor: backoff.DefaultRandomizationFactor,
+		Multiplier:          2,
+		MaxInterval:         g.maxBackoffInterval,
+		Clock:               backoff.SystemClock,
+	}
 }
 
 func waiteBeforeRetry(ctx context.Context) bool {

shared/relay/client/guard_backoff_test.go

@@ -0,0 +1,40 @@
+package client
+
+import (
+	"testing"
+	"time"
+
+	"github.com/cenkalti/backoff/v4"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestExponentialBackOff_IsRandomized fails without the randomization factor, where every
+// client that lost the same relay server walked the identical 2/4/8/16... second schedule.
+func TestExponentialBackOff_IsRandomized(t *testing.T) {
+	g := NewGuard(&ServerPicker{}, 0)
+
+	const samples = 50
+	intervals := make(map[time.Duration]struct{}, samples)
+	for i := 0; i < samples; i++ {
+		bo := g.newExponentialBackOff()
+		bo.Reset()
+		intervals[bo.NextBackOff()] = struct{}{}
+	}
+
+	assert.Greater(t, len(intervals), samples/2, "backoff intervals are not randomized across clients")
+}
+
+func TestExponentialBackOff_RespectsMaxInterval(t *testing.T) {
+	const maxInterval = 10 * time.Second
+	g := NewGuard(&ServerPicker{}, maxInterval)
+
+	bo := g.newExponentialBackOff()
+	bo.Reset()
+
+	delta := time.Duration(backoff.DefaultRandomizationFactor * float64(maxInterval))
+	for i := 0; i < 20; i++ {
+		require.LessOrEqual(t, bo.NextBackOff(), maxInterval+delta,
+			"interval exceeded MaxInterval plus its randomization window")
+	}
+}