helper: guard against negative inputs into random stagger

.changelog/14497.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+helpers: Fixed a bug where random stagger func did not protect against negative inputs
+```

helper/cluster.go

@@ -1,4 +1,3 @@
-// These functions are coming from consul/lib/cluster.go
 package helper
 
 import (
@@ -13,11 +12,11 @@ const (
 )
 
 // RandomStagger returns an interval between 0 and the duration
-func RandomStagger(intv time.Duration) time.Duration {
-	if intv == 0 {
+func RandomStagger(interval time.Duration) time.Duration {
+	if interval <= 0 {
 		return 0
 	}
-	return time.Duration(uint64(rand.Int63()) % uint64(intv))
+	return time.Duration(uint64(rand.Int63()) % uint64(interval))
 }
 
 // RateScaledInterval is used to choose an interval to perform an action in

helper/cluster_test.go

@@ -0,0 +1,29 @@
+package helper
+
+import (
+	"testing"
+	"time"
+
+	"github.com/shoenig/test/must"
+)
+
+func TestCluster_RandomStagger(t *testing.T) {
+	cases := []struct {
+		name  string
+		input time.Duration
+	}{
+		{name: "positive", input: 1 * time.Second},
+		{name: "negative", input: -1 * time.Second},
+		{name: "zero", input: 0},
+	}
+
+	abs := func(d time.Duration) time.Duration {
+		return Max(d, -d)
+	}
+
+	for _, tc := range cases {
+		result := RandomStagger(tc.input)
+		must.GreaterEq(t, result, 0)
+		must.LessEq(t, result, abs(tc.input))
+	}
+}