plugin/k8s_extenral: Supports fallthrough option (#5959)

* Add fallthrough option to k8s_external plugin to allow transitioning control to the next plugin if the domain is not found
* Exit on start up if required plugin is not present.

Signed-off-by: vanceli <vanceli@tencent.com>

---------

Signed-off-by: vanceli <vanceli@tencent.com>
Co-authored-by: vanceli <vanceli@tencent.com>

1 files changed, 16 insertions, 7 deletions

diff --git a/plugin/k8s_external/setup.go b/plugin/k8s_external/setup.go
index dbb1372e5..f42f7de23 100644
--- a/plugin/k8s_external/setup.go
+++ b/plugin/k8s_external/setup.go
@@ -1,6 +1,7 @@
 package external
 
 import (
+	"errors"
 	"strconv"
 
 	"github.com/coredns/caddy"
@@ -9,7 +10,9 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/upstream"
 )
 
-func init() { plugin.Register("k8s_external", setup) }
+const pluginName = "k8s_external"
+
+func init() { plugin.Register(pluginName, setup) }
 
 func setup(c *caddy.Controller) error {
 	e, err := parse(c)
@@ -21,14 +24,18 @@ func setup(c *caddy.Controller) error {
 	c.OnStartup(func() error {
 		m := dnsserver.GetConfig(c).Handler("kubernetes")
 		if m == nil {
-			return nil
+			return plugin.Error(pluginName, errors.New("kubernetes plugin not loaded"))
 		}
-		if x, ok := m.(Externaler); ok {
-			e.externalFunc = x.External
-			e.externalAddrFunc = x.ExternalAddress
-			e.externalServicesFunc = x.ExternalServices
-			e.externalSerialFunc = x.ExternalSerial
+
+		x, ok := m.(Externaler)
+		if !ok {
+			return plugin.Error(pluginName, errors.New("kubernetes plugin does not implement the Externaler interface"))
 		}
+
+		e.externalFunc = x.External
+		e.externalAddrFunc = x.ExternalAddress
+		e.externalServicesFunc = x.ExternalServices
+		e.externalSerialFunc = x.ExternalSerial
 		return nil
 	})
 
@@ -70,6 +77,8 @@ func parse(c *caddy.Controller) (*External, error) {
 				e.apex = args[0]
 			case "headless":
 				e.headless = true
+			case "fallthrough":
+				e.Fall.SetZonesFromArgs(c.RemainingArgs())
 			default:
 				return nil, c.Errf("unknown property '%s'", c.Val())
 			}