Add debug.Hexdump (#1902)

Allow plugins to dump messages in text pcap to the log. The forward
plugin does this when a reply does not much the query.

If the debug plugin isn't loaded Hexdump and Hexdumpf are noop.

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

plugin/debug/pcap_test.go

@@ -0,0 +1,74 @@
+package debug
+
+import (
+	"bytes"
+	"fmt"
+	golog "log"
+	"strings"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/pkg/log"
+
+	"github.com/miekg/dns"
+)
+
+func msg() *dns.Msg {
+	m := new(dns.Msg)
+	m.SetQuestion("example.local.", dns.TypeA)
+	m.SetEdns0(4096, true)
+	m.Id = 10
+
+	return m
+}
+
+func ExampleLogHexdump() {
+	buf, _ := msg().Pack()
+	h := hexdump(buf)
+	fmt.Println(string(h))
+
+	// Output:
+	// debug: 000000 00 0a 01 00 00 01 00 00 00 00 00 01 07 65 78 61
+	// debug: 000010 6d 70 6c 65 05 6c 6f 63 61 6c 00 00 01 00 01 00
+	// debug: 000020 00 29 10 00 00 00 80 00 00 00
+	// debug: 00002a
+}
+
+func TestHexdump(t *testing.T) {
+	var f bytes.Buffer
+	golog.SetOutput(&f)
+	log.D = true
+
+	str := "Hi There!"
+	Hexdump(msg(), str)
+	logged := f.String()
+
+	if !strings.Contains(logged, "[DEBUG] "+str) {
+		t.Errorf("The string %s, is not contained in the logged output: %s", str, logged)
+	}
+}
+
+func TestHexdumpf(t *testing.T) {
+	var f bytes.Buffer
+	golog.SetOutput(&f)
+	log.D = true
+
+	str := "Hi There!"
+	Hexdumpf(msg(), "%s %d", str, 10)
+	logged := f.String()
+
+	if !strings.Contains(logged, "[DEBUG] "+fmt.Sprintf("%s %d", str, 10)) {
+		t.Errorf("The string %s %d, is not contained in the logged output: %s", str, 10, logged)
+	}
+}
+
+func TestNoDebug(t *testing.T) {
+	var f bytes.Buffer
+	golog.SetOutput(&f)
+	log.D = false
+
+	str := "Hi There!"
+	Hexdumpf(msg(), "%s %d", str, 10)
+	if len(f.Bytes()) != 0 {
+		t.Errorf("Expected no output, got %d bytes", len(f.Bytes()))
+	}
+}