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/xfr.go

@@ -0,0 +1,62 @@
+package file
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// Xfr serves up an AXFR.
+type Xfr struct {
+	*Zone
+}
+
+// ServeDNS implements the plugin.Handler interface.
+func (x Xfr) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := request.Request{W: w, Req: r}
+	if !x.TransferAllowed(state) {
+		return dns.RcodeServerFailure, nil
+	}
+	if state.QType() != dns.TypeAXFR && state.QType() != dns.TypeIXFR {
+		return 0, plugin.Error(x.Name(), fmt.Errorf("xfr called with non transfer type: %d", state.QType()))
+	}
+
+	records := x.All()
+	if len(records) == 0 {
+		return dns.RcodeServerFailure, nil
+	}
+
+	ch := make(chan *dns.Envelope)
+	defer close(ch)
+	tr := new(dns.Transfer)
+	go tr.Out(w, r, ch)
+
+	j, l := 0, 0
+	records = append(records, records[0]) // add closing SOA to the end
+	log.Printf("[INFO] Outgoing transfer of %d records of zone %s to %s started", len(records), x.origin, state.IP())
+	for i, r := range records {
+		l += dns.Len(r)
+		if l > transferLength {
+			ch <- &dns.Envelope{RR: records[j:i]}
+			l = 0
+			j = i
+		}
+	}
+	if j < len(records) {
+		ch <- &dns.Envelope{RR: records[j:]}
+	}
+
+	w.Hijack()
+	// w.Close() // Client closes connection
+	return dns.RcodeSuccess, nil
+}
+
+// Name implements the plugin.Hander interface.
+func (x Xfr) Name() string { return "xfr" }
+
+const transferLength = 1000 // Start a new envelop after message reaches this size in bytes. Intentionally small to test multi envelope parsing.