Add middleware/erratic (#471)

This middleware allows playing with responses. Only one type is
implemented: it allows you to drop queries. I.e. withhold the response
from the client.

middleware/erratic/erratic_test.go

@@ -0,0 +1,45 @@
+package erratic
+
+import (
+	"testing"
+
+	"github.com/miekg/coredns/middleware/pkg/dnsrecorder"
+	"github.com/miekg/coredns/middleware/test"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+func TestErraticDrop(t *testing.T) {
+	e := &Erratic{amount: 2} // 50% drops
+
+	tests := []struct {
+		expectedCode int
+		expectedErr  error
+		drop         bool
+	}{
+		{expectedCode: dns.RcodeSuccess, expectedErr: nil, drop: true},
+		{expectedCode: dns.RcodeSuccess, expectedErr: nil, drop: false},
+	}
+
+	ctx := context.TODO()
+
+	for i, tc := range tests {
+		req := new(dns.Msg)
+		req.SetQuestion("example.org.", dns.TypeA)
+
+		rec := dnsrecorder.New(&test.ResponseWriter{})
+		code, err := e.ServeDNS(ctx, rec, req)
+
+		if err != tc.expectedErr {
+			t.Errorf("Test %d: Expected error %q, but got %q", i, tc.expectedErr, err)
+		}
+		if code != int(tc.expectedCode) {
+			t.Errorf("Test %d: Expected status code %d, but got %d", i, tc.expectedCode, code)
+		}
+
+		if tc.drop && rec.Msg != nil {
+			t.Errorf("Test %d: Expected dropped packet, but got %q", i, rec.Msg.Question[0].Name)
+		}
+	}
+}