mw/health: poll other middleware (#976)

This add the infrastructure to let other middleware report their health
status back to the health middleware. A health.Healther interface is
introduced and a middleware needs to implement that. A middleware
that supports healthchecks is statically configured.

Every second each supported middleware is queried and the global health
state is updated.

Actual tests have been disabled as no other middleware implements this
at the moment.

middleware/health/setup.go

@@ -1,6 +1,10 @@
 package health
 
 import (
+	"net"
+	"time"
+
+	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/middleware"
 
 	"github.com/mholt/caddy"
@@ -20,12 +24,32 @@ func setup(c *caddy.Controller) error {
 	}
 
 	h := &health{Addr: addr}
+
+	c.OnStartup(func() error {
+		for he := range healthers {
+			m := dnsserver.GetConfig(c).Handler(he)
+			if x, ok := m.(Healther); ok {
+				h.h = append(h.h, x)
+			}
+		}
+		return nil
+	})
+
+	c.OnStartup(func() error {
+		h.poll()
+		go func() {
+			for {
+				<-time.After(1 * time.Second)
+				h.poll()
+			}
+		}()
+		return nil
+	})
+
 	c.OnStartup(h.Startup)
 	c.OnShutdown(h.Shutdown)
 
-	// Don't do AddMiddleware, as health is not *really* a middleware just a separate
-	// webserver running.
-
+	// Don't do AddMiddleware, as health is not *really* a middleware just a separate webserver running.
 	return nil
 }
 
@@ -38,6 +62,9 @@ func healthParse(c *caddy.Controller) (string, error) {
 		case 0:
 		case 1:
 			addr = args[0]
+			if _, _, e := net.SplitHostPort(addr); e != nil {
+				return "", e
+			}
 		default:
 			return "", c.ArgErr()
 		}