helper: guard against negative inputs into random stagger

This PR modifies RandomStagger to protect against negative input
values. If the given interval is negative, the value returned will
be somewhere in the stratosphere. Instead, treat negative inputs
like zero, returning zero.

.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))
+	}
+}