plugin/health: add lameduck mode (#1379)

* plugin/health: add lameduck mode

Add a way to configure lameduck more, i.e. set health to false, stop
polling plugins. Then wait for a duration before shutting down. As the
health middleware is configured early on in the plugin list, it will
hold up all other shutdown, meaning we still answer queries.

* Add New

* More tests

* golint

* remove confusing text

plugin/health/health_test.go

@@ -5,12 +5,13 @@ import (
 	"io/ioutil"
 	"net/http"
 	"testing"
+	"time"
 
 	"github.com/coredns/coredns/plugin/erratic"
 )
 
 func TestHealth(t *testing.T) {
-	h := health{Addr: ":0"}
+	h := newHealth(":0")
 	h.h = append(h.h, &erratic.Erratic{})
 
 	if err := h.OnStartup(); err != nil {
@@ -18,6 +19,11 @@ func TestHealth(t *testing.T) {
 	}
 	defer h.OnShutdown()
 
+	go func() {
+		<-h.pollstop
+		return
+	}()
+
 	// Reconstruct the http address based on the port allocated by operating system.
 	address := fmt.Sprintf("http://%s%s", h.ln.Addr().String(), path)
 
@@ -50,3 +56,22 @@ func TestHealth(t *testing.T) {
 		t.Errorf("Invalid response body: expecting 'OK', got '%s'", string(content))
 	}
 }
+
+func TestHealthLameduck(t *testing.T) {
+	h := newHealth(":0")
+	h.lameduck = 250 * time.Millisecond
+	h.h = append(h.h, &erratic.Erratic{})
+
+	if err := h.OnStartup(); err != nil {
+		t.Fatalf("Unable to startup the health server: %v", err)
+	}
+
+	// Both these things are behind a sync.Once, fake reading from the channels.
+	go func() {
+		<-h.pollstop
+		<-h.stop
+		return
+	}()
+
+	h.OnShutdown()
+}