tstime: add RandomDurationBetween helper

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstime/jitter.go

@@ -0,0 +1,44 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tstime
+
+import (
+	crand "crypto/rand"
+	"encoding/binary"
+	"math/rand"
+	"sync"
+	"time"
+)
+
+// crandSource is a rand.Source64 that gets its numbers from
+// crypto/rand.Reader.
+type crandSource struct{ sync.Mutex }
+
+var _ rand.Source64 = (*crandSource)(nil)
+
+func (s *crandSource) Int63() int64 { return int64(s.Uint64() >> 1) }
+
+func (s *crandSource) Uint64() uint64 {
+	s.Lock()
+	defer s.Unlock()
+	var buf [8]byte
+	crand.Read(buf[:])
+	return binary.BigEndian.Uint64(buf[:])
+}
+
+func (*crandSource) Seed(seed int64) {} // nope
+
+var durRand = rand.New(new(crandSource))
+
+// RandomDurationBetween returns a random duration in range [min,max).
+// If panics if max < min.
+func RandomDurationBetween(min, max time.Duration) time.Duration {
+	diff := max - min
+	if diff == 0 {
+		return min
+	}
+	ns := durRand.Int63n(int64(diff))
+	return min + time.Duration(ns)
+}

tstime/jitter_test.go

@@ -0,0 +1,23 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tstime
+
+import (
+	"testing"
+	"time"
+)
+
+func TestRandomDurationBetween(t *testing.T) {
+	if got := RandomDurationBetween(1, 1); got != 1 {
+		t.Errorf("between 1 and 1 = %v; want 1", int64(got))
+	}
+	const min = 1 * time.Second
+	const max = 10 * time.Second
+	for i := 0; i < 500; i++ {
+		if got := RandomDurationBetween(min, max); got < min || got >= max {
+			t.Fatalf("%v (%d) out of range", got, got)
+		}
+	}
+}