Add Client.Timeout to allow limiting total exchange duration (#345)

client.go

@@ -28,9 +28,10 @@ type Client struct {
 	Net            string            // if "tcp" or "tcp-tls" (DNS over TLS) a TCP query will be initiated, otherwise an UDP one (default is "" for UDP)
 	UDPSize        uint16            // minimum receive buffer for UDP messages
 	TLSConfig      *tls.Config       // TLS connection configuration
-	DialTimeout    time.Duration     // net.DialTimeout, defaults to 2 seconds
-	ReadTimeout    time.Duration     // net.Conn.SetReadTimeout value for connections, defaults to 2 seconds
-	WriteTimeout   time.Duration     // net.Conn.SetWriteTimeout value for connections, defaults to 2 seconds
+	Timeout        time.Duration     // a cumulative timeout for dial, write and read, defaults to 0 (disabled) - overrides DialTimeout, ReadTimeout and WriteTimeout when non-zero
+	DialTimeout    time.Duration     // net.DialTimeout, defaults to 2 seconds - overridden by Timeout when that value is non-zero
+	ReadTimeout    time.Duration     // net.Conn.SetReadTimeout value for connections, defaults to 2 seconds - overridden by Timeout when that value is non-zero
+	WriteTimeout   time.Duration     // net.Conn.SetWriteTimeout value for connections, defaults to 2 seconds - overridden by Timeout when that value is non-zero
 	TsigSecret     map[string]string // secret(s) for Tsig map[<zonename>]<base64 secret>, zonename must be fully qualified
 	SingleInflight bool              // if true suppress multiple outstanding queries for the same Qname, Qtype and Qclass
 	group          singleflight
@@ -129,6 +130,9 @@ func (c *Client) Exchange(m *Msg, a string) (r *Msg, rtt time.Duration, err erro
 }
 
 func (c *Client) dialTimeout() time.Duration {
+	if c.Timeout != 0 {
+		return c.Timeout
+	}
 	if c.DialTimeout != 0 {
 		return c.DialTimeout
 	}
@@ -170,6 +174,11 @@ func (c *Client) exchange(m *Msg, a string) (r *Msg, rtt time.Duration, err erro
 		}
 	}
 
+	var deadline time.Time
+	if c.Timeout != 0 {
+		deadline = time.Now().Add(c.Timeout)
+	}
+
 	if tls {
 		co, err = DialTimeoutWithTLS(network, a, c.TLSConfig, c.dialTimeout())
 	} else {
@@ -192,12 +201,12 @@ func (c *Client) exchange(m *Msg, a string) (r *Msg, rtt time.Duration, err erro
 	}
 
 	co.TsigSecret = c.TsigSecret
-	co.SetWriteDeadline(time.Now().Add(c.writeTimeout()))
+	co.SetWriteDeadline(deadlineOrTimeout(deadline, c.writeTimeout()))
 	if err = co.WriteMsg(m); err != nil {
 		return nil, 0, err
 	}
 
-	co.SetReadDeadline(time.Now().Add(c.readTimeout()))
+	co.SetReadDeadline(deadlineOrTimeout(deadline, c.readTimeout()))
 	r, err = co.ReadMsg()
 	if err == nil && r.Id != m.Id {
 		err = ErrId
@@ -434,3 +443,10 @@ func DialTimeoutWithTLS(network, address string, tlsConfig *tls.Config, timeout
 	}
 	return conn, nil
 }
+
+func deadlineOrTimeout(deadline time.Time, timeout time.Duration) time.Time {
+	if deadline.IsZero() {
+		return time.Now().Add(timeout)
+	}
+	return deadline
+}

client_test.go

@@ -419,3 +419,51 @@ func TestTruncatedMsg(t *testing.T) {
 		t.Fail()
 	}
 }
+
+func TestTimeout(t *testing.T) {
+	// Set up a dummy UDP server that won't respond
+	addr, err := net.ResolveUDPAddr("udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("unable to resolve local udp address: %v", err)
+	}
+	conn, err := net.ListenUDP("udp", addr)
+	if err != nil {
+		t.Fatalf("unable to run test server: %v", err)
+	}
+	defer conn.Close()
+	addrstr := conn.LocalAddr().String()
+
+	// Message to send
+	m := new(Msg)
+	m.SetQuestion("miek.nl.", TypeTXT)
+
+	// Use a channel + timeout to ensure we don't get stuck if the
+	// Client Timeout is not working properly
+	done := make(chan struct{})
+
+	timeout := time.Millisecond
+	allowable := timeout + (10 * time.Millisecond)
+	abortAfter := timeout + (100 * time.Millisecond)
+
+	start := time.Now()
+
+	go func() {
+		c := &Client{Timeout: timeout}
+		_, _, err := c.Exchange(m, addrstr)
+		if err == nil {
+			t.Error("no timeout using Client")
+		}
+		done <- struct{}{}
+	}()
+
+	select {
+	case <-done:
+	case <-time.After(abortAfter):
+	}
+
+	length := time.Since(start)
+
+	if length > allowable {
+		t.Errorf("exchange took longer (%v) than specified Timeout (%v)", length, timeout)
+	}
+}