config: add jitter interval for watcher to avoid dos

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

util/jitter/random.go

@@ -7,8 +7,8 @@ import (
 	"github.com/unistack-org/micro/v3/util/rand"
 )
 
-// Do returns a random time to jitter with max cap specified
-func Do(d time.Duration) time.Duration {
+// Random returns a random time to jitter with max cap specified
+func Random(d time.Duration) time.Duration {
 	var rng rand.Rand
 	v := rng.Float64() * float64(d.Nanoseconds())
 	return time.Duration(v)

util/jitter/ticker.go

@@ -0,0 +1,65 @@
+package jitter
+
+import (
+	"time"
+
+	"github.com/unistack-org/micro/v3/util/rand"
+)
+
+// Ticker is similar to time.Ticker but ticks at random intervals between
+// the min and max duration values (stored internally as int64 nanosecond
+// counts).
+type Ticker struct {
+	C    chan time.Time
+	done chan chan struct{}
+	min  int64
+	max  int64
+	rng  rand.Rand
+}
+
+// NewTicker returns a pointer to an initialized instance of the Ticker.
+// Min and max are durations of the shortest and longest allowed
+// ticks. Ticker will run in a goroutine until explicitly stopped.
+func NewTicker(min, max time.Duration) *Ticker {
+	ticker := &Ticker{
+		C:    make(chan time.Time),
+		done: make(chan chan struct{}),
+		min:  min.Nanoseconds(),
+		max:  max.Nanoseconds(),
+	}
+	go ticker.run()
+	return ticker
+}
+
+// Stop terminates the ticker goroutine and closes the C channel.
+func (ticker *Ticker) Stop() {
+	c := make(chan struct{})
+	ticker.done <- c
+	<-c
+}
+
+func (ticker *Ticker) run() {
+	defer close(ticker.C)
+	t := time.NewTimer(ticker.nextInterval())
+	for {
+		// either a stop signal or a timeout
+		select {
+		case c := <-ticker.done:
+			t.Stop()
+			close(c)
+			return
+		case <-t.C:
+			select {
+			case ticker.C <- time.Now():
+				t.Stop()
+				t = time.NewTimer(ticker.nextInterval())
+			default:
+				// there could be noone receiving...
+			}
+		}
+	}
+}
+
+func (ticker *Ticker) nextInterval() time.Duration {
+	return time.Duration(ticker.rng.Int63n(ticker.max-ticker.min)+ticker.min) * time.Nanosecond
+}