support for zone transfer for kubernetes (#1259)

* plugin/kubernetes: axfr

Hook up dynamic SOA serial by setting ResourceEventHandlerFuncs in
dnsController.

Add prototype of returns msg.Services via the Transfer function. Leave
expanding this to RRs out of scope for a bit.

* plugin/kubernetes: axfr

Respond to AXFR queries

* Fixing race condition

* Fixing instantiation of dnstest writer

* Updates from review

plugin/kubernetes/xfr_test.go

@@ -0,0 +1,111 @@
+package kubernetes
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/test"
+	"golang.org/x/net/context"
+
+	"github.com/miekg/dns"
+)
+
+func TestKubernetesXFR(t *testing.T) {
+	k := New([]string{"cluster.local."})
+	k.APIConn = &APIConnServeTest{}
+	k.TransferTo = []string{"127.0.0.1"}
+
+	ctx := context.TODO()
+	w := dnstest.NewMultiRecorder(&test.ResponseWriter{})
+	dnsmsg := &dns.Msg{}
+	dnsmsg.SetAxfr(k.Zones[0])
+
+	_, err := k.ServeDNS(ctx, w, dnsmsg)
+	if err != nil {
+		t.Error(err)
+	}
+
+	if len(w.Msgs) == 0 {
+		t.Logf("%+v\n", w)
+		t.Error("Did not get back a zone response")
+	}
+
+	// Ensure xfr starts with SOA
+	if w.Msgs[0].Answer[0].Header().Rrtype != dns.TypeSOA {
+		t.Error("Invalid XFR, does not start with SOA record")
+	}
+
+	// Ensure xfr starts with SOA
+	// Last message is empty, so we need to go back one further
+	if w.Msgs[len(w.Msgs)-2].Answer[len(w.Msgs[len(w.Msgs)-2].Answer)-1].Header().Rrtype != dns.TypeSOA {
+		t.Error("Invalid XFR, does not end with SOA record")
+	}
+
+	testRRs := []dns.RR{}
+	for _, tc := range dnsTestCases {
+		if tc.Rcode != dns.RcodeSuccess {
+			continue
+		}
+
+		for _, ans := range tc.Answer {
+			// Exclude wildcard searches
+			if strings.Contains(ans.Header().Name, "*") {
+				continue
+			}
+
+			// Exclude TXT records
+			if ans.Header().Rrtype == dns.TypeTXT {
+				continue
+			}
+			testRRs = append(testRRs, ans)
+		}
+	}
+
+	gotRRs := []dns.RR{}
+	for _, resp := range w.Msgs {
+		for _, ans := range resp.Answer {
+			// Skip SOA records since these
+			// test cases do not exist
+			if ans.Header().Rrtype == dns.TypeSOA {
+				continue
+			}
+
+			gotRRs = append(gotRRs, ans)
+		}
+
+	}
+
+	diff := difference(testRRs, gotRRs)
+	if len(diff) != 0 {
+		t.Errorf("Got back %d records that do not exist in test cases, should be 0:", len(diff))
+		for _, rec := range diff {
+			t.Errorf("%+v", rec)
+		}
+	}
+
+	diff = difference(gotRRs, testRRs)
+	if len(diff) != 0 {
+		t.Errorf("Found %d records we're missing tham test cases, should be 0:", len(diff))
+		for _, rec := range diff {
+			t.Errorf("%+v", rec)
+		}
+	}
+
+}
+
+// difference shows what we're missing when comparing two RR slices
+func difference(testRRs []dns.RR, gotRRs []dns.RR) []dns.RR {
+	expectedRRs := map[string]bool{}
+	for _, rr := range testRRs {
+		expectedRRs[rr.String()] = true
+	}
+
+	foundRRs := []dns.RR{}
+	for _, rr := range gotRRs {
+		if _, ok := expectedRRs[rr.String()]; !ok {
+			foundRRs = append(foundRRs, rr)
+		}
+	}
+	return foundRRs
+}