plugin/ready: fix starts and restarts (#2814)

Add OnRestartFailed to the ready plugin and some various cleanups.

Document slightly better how things are supposed to work with multiple
`ready`'s in the multiple Server Blocks.

All manually tested with this Corefile:
~~~
. {
    log
    ready
}

example.org {
    log
    chaos
    ready
}
~~~
And then `kill -SIGUSR1` and curling the ready endpoint. This works
well, the FailedReload is triggered by adding a syntax error in the
Corefile.

See #2659

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

plugin/ready/setup.go

@@ -21,22 +21,25 @@ func setup(c *caddy.Controller) error {
 	if err != nil {
 		return plugin.Error("ready", err)
 	}
-
 	rd := &ready{Addr: addr}
 
 	uniqAddr.Set(addr, rd.onStartup)
+	c.OnStartup(func() error { uniqAddr.Set(addr, rd.onStartup); return nil })
+	c.OnRestartFailed(func() error { uniqAddr.Set(addr, rd.onStartup); return nil })
 
-	c.OncePerServerBlock(func() error {
-		c.OnStartup(func() error {
-			return uniqAddr.ForEach()
-		})
-		return nil
-	})
+	c.OnStartup(func() error { return uniqAddr.ForEach() })
+	c.OnRestartFailed(func() error { return uniqAddr.ForEach() })
 
 	c.OnStartup(func() error {
-		// Each plugin in this server block will (if they support it) report readiness.
-		plugs := dnsserver.GetConfig(c).Handlers()
-		for _, p := range plugs {
+		for _, p := range dnsserver.GetConfig(c).Handlers() {
+			if r, ok := p.(Readiness); ok {
+				plugins.Append(r, p.Name())
+			}
+		}
+		return nil
+	})
+	c.OnRestartFailed(func() error {
+		for _, p := range dnsserver.GetConfig(c).Handlers() {
 			if r, ok := p.(Readiness); ok {
 				plugins.Append(r, p.Name())
 			}
@@ -44,14 +47,14 @@ func setup(c *caddy.Controller) error {
 		return nil
 	})
 
-	c.OnRestart(rd.onRestart)
+	c.OnRestart(rd.onFinalShutdown)
 	c.OnFinalShutdown(rd.onFinalShutdown)
 
 	return nil
 }
 
 func parse(c *caddy.Controller) (string, error) {
-	addr := ""
+	addr := ":8181"
 	i := 0
 	for c.Next() {
 		if i > 0 {