Add *ready* plugin (#2616)

Add a ready plugin that allows plugin to signal when they are ready.
Once a plugin is ready it is not queried again.

This uses same mechanism as the health plugin: each plugin needs to
implement an interface.

Implement readines for the *erratic* plugin to aid in testing.

Add README.md and tests moduled after the health plugin; which will be
relegated to just providing process health. In similar vein to health
this is a process wide setting.

With this Corefile:
~~~
. {
    erratic
    whoami
    ready
}

bla {
    erratic
    whoami
}
~~~

ready will lead to:

~~~ sh
% curl localhost:8181/ready
% dig @localhost -p 1053 mx example.org
% curl localhost:8181/ready
OK%
~~~

Meanwhile CoreDNS logs:

~~~
.:1053
bla.:1053
2019-02-26T20:59:07.137Z [INFO] CoreDNS-1.3.1
2019-02-26T20:59:07.137Z [INFO] linux/amd64, go1.11.4,
CoreDNS-1.3.1
linux/amd64, go1.11.4,
2019-02-26T20:59:11.415Z [INFO] plugin/ready: Still waiting on: "erratic"
2019-02-26T20:59:13.510Z [INFO] plugin/ready: Still waiting on: "erratic"
~~~

*ready* can be used in multiple server blocks and will do the right
thing; query all those plugins from all server blocks for readiness.
This does a similar thing to the prometheus plugin.

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

plugin/ready/setup.go

@@ -0,0 +1,75 @@
+package ready
+
+import (
+	"net"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("ready", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	addr, err := parse(c)
+	if err != nil {
+		return plugin.Error("ready", err)
+	}
+
+	rd := &ready{Addr: addr}
+
+	uniqAddr.Set(addr, rd.onStartup, rd)
+
+	c.OncePerServerBlock(func() error {
+		c.OnStartup(func() error {
+			return uniqAddr.ForEach()
+		})
+		return nil
+	})
+
+	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 {
+			if r, ok := p.(Readiness); ok {
+				plugins.Append(r, p.Name())
+			}
+		}
+		return nil
+	})
+
+	c.OnRestart(rd.onRestart)
+	c.OnFinalShutdown(rd.onFinalShutdown)
+
+	return nil
+}
+
+func parse(c *caddy.Controller) (string, error) {
+	addr := ""
+	i := 0
+	for c.Next() {
+		if i > 0 {
+			return "", plugin.ErrOnce
+		}
+		i++
+		args := c.RemainingArgs()
+
+		switch len(args) {
+		case 0:
+		case 1:
+			addr = args[0]
+			if _, _, e := net.SplitHostPort(addr); e != nil {
+				return "", e
+			}
+		default:
+			return "", c.ArgErr()
+		}
+	}
+	return addr, nil
+}