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

5 files changed, 22 insertions, 13 deletions

diff --git a/middleware/auto/setup.go b/middleware/auto/setup.go
index 98eefca4a..bd5949d7e 100644
--- a/middleware/auto/setup.go
+++ b/middleware/auto/setup.go
@@ -32,7 +32,9 @@ func setup(c *caddy.Controller) error {
 	}
 
 	// If we have enabled prometheus we should add newly discovered zones to it.
-	met := dnsserver.GetMiddleware(c, "prometheus")
+	// This does not have to happen in a on.Startup because monitoring is one of the first
+	// to be initialized.
+	met := dnsserver.GetConfig(c).GetHandler("prometheus")
 	if met != nil {
 		a.metrics = met.(*metrics.Metrics)
 	}
diff --git a/middleware/autopath/setup.go b/middleware/autopath/setup.go
index 804b61855..1855dd440 100644
--- a/middleware/autopath/setup.go
+++ b/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,
 }
diff --git a/middleware/kubernetes/setup.go b/middleware/kubernetes/setup.go
index 1c77c11ec..6b8cbba6e 100644
--- a/middleware/kubernetes/setup.go
+++ b/middleware/kubernetes/setup.go
@@ -51,6 +51,9 @@ func setup(c *caddy.Controller) error {
 		return kubernetes
 	})
 
+	// Also register kubernetes for use in autopath.
+	dnsserver.GetConfig(c).RegisterHandler(kubernetes)
+
 	return nil
 }
 
diff --git a/middleware/metrics/setup.go b/middleware/metrics/setup.go
index 1ed455ffc..b6b8e5291 100644
--- a/middleware/metrics/setup.go
+++ b/middleware/metrics/setup.go
@@ -38,6 +38,9 @@ func setup(c *caddy.Controller) error {
 	}
 	c.OnFinalShutdown(m.OnShutdown)
 
+	// Also register metrics for use in other middleware.
+	dnsserver.GetConfig(c).RegisterHandler(m)
+
 	return nil
 }
 
diff --git a/middleware/proxy/setup.go b/middleware/proxy/setup.go
index de979a5df..bd036d4cb 100644
--- a/middleware/proxy/setup.go
+++ b/middleware/proxy/setup.go
@@ -20,7 +20,7 @@ func setup(c *caddy.Controller) error {
 		return middleware.Error("proxy", err)
 	}
 
-	t := dnsserver.GetMiddleware(c, "trace")
+	t := dnsserver.GetConfig(c).GetHandler("trace")
 	P := &Proxy{Trace: t}
 	dnsserver.GetConfig(c).AddMiddleware(func(next middleware.Handler) middleware.Handler {
 		P.Next = next