Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/file/notify.go

@@ -0,0 +1,82 @@
+package file
+
+import (
+	"fmt"
+	"log"
+	"net"
+
+	"github.com/coredns/coredns/plugin/pkg/rcode"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// isNotify checks if state is a notify message and if so, will *also* check if it
+// is from one of the configured masters. If not it will not be a valid notify
+// message. If the zone z is not a secondary zone the message will also be ignored.
+func (z *Zone) isNotify(state request.Request) bool {
+	if state.Req.Opcode != dns.OpcodeNotify {
+		return false
+	}
+	if len(z.TransferFrom) == 0 {
+		return false
+	}
+	// If remote IP matches we accept.
+	remote := state.IP()
+	for _, f := range z.TransferFrom {
+		from, _, err := net.SplitHostPort(f)
+		if err != nil {
+			continue
+		}
+		if from == remote {
+			return true
+		}
+	}
+	return false
+}
+
+// Notify will send notifies to all configured TransferTo IP addresses.
+func (z *Zone) Notify() {
+	go notify(z.origin, z.TransferTo)
+}
+
+// notify sends notifies to the configured remote servers. It will try up to three times
+// before giving up on a specific remote. We will sequentially loop through "to"
+// until they all have replied (or have 3 failed attempts).
+func notify(zone string, to []string) error {
+	m := new(dns.Msg)
+	m.SetNotify(zone)
+	c := new(dns.Client)
+
+	for _, t := range to {
+		if t == "*" {
+			continue
+		}
+		if err := notifyAddr(c, m, t); err != nil {
+			log.Printf("[ERROR] " + err.Error())
+		} else {
+			log.Printf("[INFO] Sent notify for zone %q to %q", zone, t)
+		}
+	}
+	return nil
+}
+
+func notifyAddr(c *dns.Client, m *dns.Msg, s string) error {
+	var err error
+
+	code := dns.RcodeServerFailure
+	for i := 0; i < 3; i++ {
+		ret, _, err := c.Exchange(m, s)
+		if err != nil {
+			continue
+		}
+		code = ret.Rcode
+		if code == dns.RcodeSuccess {
+			return nil
+		}
+	}
+	if err != nil {
+		return fmt.Errorf("notify for zone %q was not accepted by %q: %q", m.Question[0].Name, s, err)
+	}
+	return fmt.Errorf("notify for zone %q was not accepted by %q: rcode was %q", m.Question[0].Name, s, rcode.ToString(code))
+}