Add a helper function DurationMinusBuffer() to calculate an interval

used to schedule a TTL check.  e.g.

d := lib.DurationMinusBuffer(60 * time.Duration, 10 * time.Second, 16)

will return a duration between 46.875s and 50s.

lib/cluster.go

@@ -5,6 +5,15 @@ import (
 	"time"
 )
 
+// DurationMinusBuffer returns a duration, minus a buffer and jitter
+// subtracted from the duration.  This function is used primarily for
+// servicing Consul TTL Checks in advance of the TTL.
+func DurationMinusBuffer(intv time.Duration, buffer time.Duration, jitter int64) time.Duration {
+	d := intv - buffer
+	d -= RandomStagger(time.Duration(int64(d) / jitter))
+	return d
+}
+
 // Returns a random stagger interval between 0 and the duration
 func RandomStagger(intv time.Duration) time.Duration {
 	return time.Duration(uint64(rand.Int63()) % uint64(intv))

lib/cluster_test.go

@@ -5,6 +5,22 @@ import (
 	"time"
 )
 
+func TestDurationMinusBuffer(t *testing.T) {
+	const (
+		buffer = 10 * time.Second
+		jitter = 16
+	)
+	intv := 1 * time.Minute
+	minValue := (intv - buffer) - ((intv - buffer) / jitter)
+	maxValue := intv - buffer
+	for i := 0; i < 10; i++ {
+		d := DurationMinusBuffer(intv, buffer, jitter)
+		if d < minValue || d > maxValue {
+			t.Fatalf("Bad: %v", d)
+		}
+	}
+}
+
 func TestRandomStagger(t *testing.T) {
 	intv := time.Minute
 	for i := 0; i < 10; i++ {