core: replace GetMiddleware (#885)

* core: replace GetMiddleware

See the discussion in #881. GetMiddleware would add a `nil` middleware
to the callstack thereby breaking functionality.

This PR drops it in favor of RegisterHandler which is a completely
standalone registry for middleware that want to let it self know to
other middleware.

Currenly *autopath* uses this to call *kubernetes*'s AutoPath method
for dynamic autopathing.

* Drop GetMiddleware

* Register metrics

* drop the panic

middleware/autopath/setup.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/middleware"
+	"github.com/coredns/coredns/middleware/kubernetes"
 
 	"github.com/mholt/caddy"
 	"github.com/miekg/dns"
@@ -19,23 +20,21 @@ func init() {
 }
 
 func setup(c *caddy.Controller) error {
-	ap, _, err := autoPathParse(c)
+	ap, mw, err := autoPathParse(c)
 	if err != nil {
 		return middleware.Error("autopath", err)
 	}
 
+	// Do this in OnStartup, so all middleware has been initialized.
 	c.OnStartup(func() error {
-		// Do this in OnStartup, so all middleware has been initialized.
 		// TODO(miek): fabricate test to proof this is not thread safe.
-		// TODO(miek): disable this for now: See https://github.com/coredns/coredns/issues/881
-		/*
-			switch mw {
-			case "kubernetes":
-				if k, ok := m.(kubernetes.Kubernetes); ok {
-					ap.searchFunc = k.AutoPath
-				}
-			}
-		*/
+		m := dnsserver.GetConfig(c).GetHandler(mw)
+		if m == nil {
+			return nil
+		}
+		if k, ok := m.(kubernetes.Kubernetes); ok {
+			ap.searchFunc = k.AutoPath
+		}
 		return nil
 	})
 
@@ -47,6 +46,8 @@ func setup(c *caddy.Controller) error {
 	return nil
 }
 
+// allowedMiddleware has a list of middleware that can be used by autopath. For this to work, they
+// need to register themselves with dnsserver.RegisterHandler.
 var allowedMiddleware = map[string]bool{
 	"@kubernetes": true,
 }