plugin/health: remove ability to poll other plugins (#2547)

* plugin/health: remove ability to poll other plugins

This mechanism defeats the purpose any plugin (mostly) caching can still
be alive, we can probably forward queries still. Don't poll plugins,
just tell the world we're up and running.

It was only actually used in kubernetes; and there specifically would
mean any network hiccup would NACK the entire server health.

Fixes: #2534

Signed-off-by: Miek Gieben <miek@miek.nl>

* update docs based on feedback

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/health/health.go

@@ -5,7 +5,6 @@ import (
 	"io"
 	"net"
 	"net/http"
-	"sync"
 	"time"
 
 	clog "github.com/coredns/coredns/plugin/pkg/log"
@@ -22,18 +21,12 @@ type health struct {
 	nlSetup bool
 	mux     *http.ServeMux
 
-	// A slice of Healthers that the health plugin will poll every second for their health status.
-	h []Healther
-	sync.RWMutex
-	ok bool // ok is the global boolean indicating an all healthy plugin stack
-
-	stop     chan bool
-	pollstop chan bool
+	stop chan bool
 }
 
 // newHealth returns a new initialized health.
 func newHealth(addr string) *health {
-	return &health{Addr: addr, stop: make(chan bool), pollstop: make(chan bool)}
+	return &health{Addr: addr, stop: make(chan bool)}
 }
 
 func (h *health) OnStartup() error {
@@ -51,12 +44,10 @@ func (h *health) OnStartup() error {
 	h.nlSetup = true
 
 	h.mux.HandleFunc(path, func(w http.ResponseWriter, r *http.Request) {
-		if h.Ok() {
-			w.WriteHeader(http.StatusOK)
-			io.WriteString(w, ok)
-			return
-		}
-		w.WriteHeader(http.StatusServiceUnavailable)
+		// We're always healthy.
+		w.WriteHeader(http.StatusOK)
+		io.WriteString(w, ok)
+		return
 	})
 
 	go func() { http.Serve(h.ln, h.mux) }()
@@ -72,11 +63,6 @@ func (h *health) OnFinalShutdown() error {
 		return nil
 	}
 
-	// Stop polling plugins
-	h.pollstop <- true
-	// NACK health
-	h.SetOk(false)
-
 	if h.lameduck > 0 {
 		log.Infof("Going into lameduck mode for %s", h.lameduck)
 		time.Sleep(h.lameduck)
@@ -84,8 +70,8 @@ func (h *health) OnFinalShutdown() error {
 
 	h.ln.Close()
 
-	h.stop <- true
 	h.nlSetup = false
+	close(h.stop)
 	return nil
 }