Fix data race in error handling. (#459)

The response message must copied regardless of whether there was an
error or not, otherwise two concurrent queries may modify the response
as they write it out.

client.go

@@ -121,12 +121,12 @@ func (c *Client) Exchange(m *Msg, a string) (r *Msg, rtt time.Duration, err erro
 	r, rtt, err, shared := c.group.Do(m.Question[0].Name+t+cl, func() (*Msg, time.Duration, error) {
 		return c.exchange(m, a)
 	})
+	if r != nil && shared {
+		r = r.Copy()
+	}
 	if err != nil {
 		return r, rtt, err
 	}
-	if shared {
-		return r.Copy(), rtt, nil
-	}
 	return r, rtt, nil
 }
 

client_test.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"net"
 	"strconv"
+	"sync"
 	"testing"
 	"time"
 )
@@ -450,3 +451,61 @@ func TestTimeout(t *testing.T) {
 		t.Errorf("exchange took longer (%v) than specified Timeout (%v)", length, timeout)
 	}
 }
+
+// Check that responses from deduplicated requests aren't shared between callers
+func TestConcurrentExchanges(t *testing.T) {
+	cases := make([]*Msg, 2)
+	cases[0] = new(Msg)
+	cases[1] = new(Msg)
+	cases[1].Truncated = true
+	for _, m := range cases {
+		block := make(chan struct{})
+		waiting := make(chan struct{})
+
+		handler := func(w ResponseWriter, req *Msg) {
+			r := m.Copy()
+			r.SetReply(req)
+
+			waiting <- struct{}{}
+			<-block
+			w.WriteMsg(r)
+		}
+
+		HandleFunc("miek.nl.", handler)
+		defer HandleRemove("miek.nl.")
+
+		s, addrstr, err := RunLocalUDPServer("127.0.0.1:0")
+		if err != nil {
+			t.Fatalf("unable to run test server: %s", err)
+		}
+		defer s.Shutdown()
+
+		m := new(Msg)
+		m.SetQuestion("miek.nl.", TypeSRV)
+		c := &Client{
+			SingleInflight: true,
+		}
+		r := make([]*Msg, 2)
+
+		var wg sync.WaitGroup
+		wg.Add(len(r))
+		for i := 0; i < len(r); i++ {
+			go func(i int) {
+				r[i], _, _ = c.Exchange(m.Copy(), addrstr)
+				wg.Done()
+			}(i)
+		}
+		select {
+		case <-waiting:
+		case <-time.After(time.Second):
+			t.FailNow()
+		}
+		close(block)
+		wg.Wait()
+
+		if r[0] == r[1] {
+			t.Log("Got same response object, expected non-shared responses")
+			t.Fail()
+		}
+	}
+}