Reload hook (#1445)

* Add reload directive

* gofmt

* Fix default jitter and error message

* remove unneeded call to NextArg, add a couple negative setup tests

* Review feedback

plugin/reload/setup.go

@@ -0,0 +1,72 @@
+package reload
+
+import (
+	"math/rand"
+	"sync"
+	"time"
+
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("reload", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+var r *reload
+var once sync.Once
+
+func setup(c *caddy.Controller) error {
+	c.Next() // 'reload'
+	args := c.RemainingArgs()
+
+	if len(args) > 2 {
+		return plugin.Error("reload", c.ArgErr())
+	}
+
+	i := defaultInterval
+	if len(args) > 0 {
+		d, err := time.ParseDuration(args[0])
+		if err != nil {
+			return err
+		}
+		i = d
+	}
+
+	j := defaultJitter
+	if len(args) > 1 {
+		d, err := time.ParseDuration(args[1])
+		if err != nil {
+			return err
+		}
+		j = d
+	}
+
+	if j > i/2 {
+		j = i / 2
+	}
+
+	jitter := time.Duration(rand.Int63n(j.Nanoseconds()) - (j.Nanoseconds() / 2))
+	i = i + jitter
+
+	r = &reload{interval: i, quit: make(chan bool)}
+	once.Do(func() {
+		caddy.RegisterEventHook("reload", hook)
+	})
+
+	c.OnFinalShutdown(func() error {
+		r.quit <- true
+		return nil
+	})
+
+	return nil
+}
+
+const (
+	defaultInterval = 30 * time.Second
+	defaultJitter   = 15 * time.Second
+)