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/loadbalance/loadbalance.go

@@ -0,0 +1,87 @@
+// Package loadbalance shuffles A and AAAA records.
+package loadbalance
+
+import (
+	"log"
+
+	"github.com/miekg/dns"
+)
+
+// RoundRobinResponseWriter is a response writer that shuffles A and AAAA records.
+type RoundRobinResponseWriter struct {
+	dns.ResponseWriter
+}
+
+// WriteMsg implements the dns.ResponseWriter interface.
+func (r *RoundRobinResponseWriter) WriteMsg(res *dns.Msg) error {
+	if res.Rcode != dns.RcodeSuccess {
+		return r.ResponseWriter.WriteMsg(res)
+	}
+
+	res.Answer = roundRobin(res.Answer)
+	res.Ns = roundRobin(res.Ns)
+	res.Extra = roundRobin(res.Extra)
+
+	return r.ResponseWriter.WriteMsg(res)
+}
+
+func roundRobin(in []dns.RR) []dns.RR {
+	cname := []dns.RR{}
+	address := []dns.RR{}
+	mx := []dns.RR{}
+	rest := []dns.RR{}
+	for _, r := range in {
+		switch r.Header().Rrtype {
+		case dns.TypeCNAME:
+			cname = append(cname, r)
+		case dns.TypeA, dns.TypeAAAA:
+			address = append(address, r)
+		case dns.TypeMX:
+			mx = append(mx, r)
+		default:
+			rest = append(rest, r)
+		}
+	}
+
+	roundRobinShuffle(address)
+	roundRobinShuffle(mx)
+
+	out := append(cname, rest...)
+	out = append(out, address...)
+	out = append(out, mx...)
+	return out
+}
+
+func roundRobinShuffle(records []dns.RR) {
+	switch l := len(records); l {
+	case 0, 1:
+		break
+	case 2:
+		if dns.Id()%2 == 0 {
+			records[0], records[1] = records[1], records[0]
+		}
+	default:
+		for j := 0; j < l*(int(dns.Id())%4+1); j++ {
+			q := int(dns.Id()) % l
+			p := int(dns.Id()) % l
+			if q == p {
+				p = (p + 1) % l
+			}
+			records[q], records[p] = records[p], records[q]
+		}
+	}
+}
+
+// Write implements the dns.ResponseWriter interface.
+func (r *RoundRobinResponseWriter) Write(buf []byte) (int, error) {
+	// Should we pack and unpack here to fiddle with the packet... Not likely.
+	log.Printf("[WARNING] RoundRobin called with Write: no shuffling records")
+	n, err := r.ResponseWriter.Write(buf)
+	return n, err
+}
+
+// Hijack implements the dns.ResponseWriter interface.
+func (r *RoundRobinResponseWriter) Hijack() {
+	r.ResponseWriter.Hijack()
+	return
+}