net, net/http: don't trace DNS dials

This fixes change https://go-review.googlesource.com/#/c/23069/, which
assumes all DNS requests are UDP. This is not true -- DNS requests can
be TCP in some cases. See:
https://tip.golang.org/src/net/dnsclient_unix.go#L154
https://en.wikipedia.org/wiki/Domain_Name_System#Protocol_transport

Also, the test code added by the above change doesn't actually test
anything because the test uses a faked DNS resolver that doesn't
actually make any DNS queries. I fixed that by adding another test
that uses the system DNS resolver.

Updates #12580

Change-Id: I6c24c03ebab84d437d3ac610fd6eb5353753c490
Reviewed-on: https://go-review.googlesource.com/23101
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/internal/nettrace/nettrace.go

@@ -32,14 +32,14 @@ type Trace struct {
 	// actually be for circular dependency reasons.
 	DNSDone func(netIPs []interface{}, coalesced bool, err error)
 
-	// ConnectStart is called before a TCPAddr or UnixAddr
-	// Dial. In the case of DualStack (Happy Eyeballs) dialing,
-	// this may be called multiple times, from multiple
+	// ConnectStart is called before a Dial, excluding Dials made
+	// during DNS lookups. In the case of DualStack (Happy Eyeballs)
+	// dialing, this may be called multiple times, from multiple
 	// goroutines.
 	ConnectStart func(network, addr string)
 
-	// ConnectStart is called after a TCPAddr or UnixAddr Dial
-	// with the results. It may also be called multiple times,
-	// like ConnectStart.
+	// ConnectStart is called after a Dial with the results, excluding
+	// Dials made during DNS lookups. It may also be called multiple
+	// times, like ConnectStart.
 	ConnectDone func(network, addr string, err error)
 }

src/net/dial.go

@@ -317,7 +317,16 @@ func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn
 		ctx = subCtx
 	}
 
-	addrs, err := resolveAddrList(ctx, "dial", network, address, d.LocalAddr)
+	// Shadow the nettrace (if any) during resolve so Connect events don't fire for DNS lookups.
+	resolveCtx := ctx
+	if trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace); trace != nil {
+		shadow := *trace
+		shadow.ConnectStart = nil
+		shadow.ConnectDone = nil
+		resolveCtx = context.WithValue(resolveCtx, nettrace.TraceKey{}, &shadow)
+	}
+
+	addrs, err := resolveAddrList(resolveCtx, "dial", network, address, d.LocalAddr)
 	if err != nil {
 		return nil, &OpError{Op: "dial", Net: network, Source: nil, Addr: nil, Err: err}
 	}
@@ -472,21 +481,11 @@ func dialSerial(ctx context.Context, dp *dialParam, ras addrList) (Conn, error)
 	return nil, firstErr
 }
 
-// traceDialType reports whether ra is an address type for which
-// nettrace.Trace should trace.
-func traceDialType(ra Addr) bool {
-	switch ra.(type) {
-	case *TCPAddr, *UnixAddr:
-		return true
-	}
-	return false
-}
-
 // dialSingle attempts to establish and returns a single connection to
 // the destination address.
 func dialSingle(ctx context.Context, dp *dialParam, ra Addr) (c Conn, err error) {
 	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
-	if trace != nil && traceDialType(ra) {
+	if trace != nil {
 		raStr := ra.String()
 		if trace.ConnectStart != nil {
 			trace.ConnectStart(dp.network, raStr)

src/net/http/transport_test.go

@@ -3308,6 +3308,52 @@ func testTransportEventTrace(t *testing.T, noHooks bool) {
 	}
 }
 
+func TestTransportEventTraceRealDNS(t *testing.T) {
+	defer afterTest(t)
+	tr := &Transport{}
+	defer tr.CloseIdleConnections()
+	c := &Client{Transport: tr}
+
+	var mu sync.Mutex
+	var buf bytes.Buffer
+	logf := func(format string, args ...interface{}) {
+		mu.Lock()
+		defer mu.Unlock()
+		fmt.Fprintf(&buf, format, args...)
+		buf.WriteByte('\n')
+	}
+
+	req, _ := NewRequest("GET", "http://dns-should-not-resolve.golang:80", nil)
+	trace := &httptrace.ClientTrace{
+		DNSStart:     func(e httptrace.DNSStartInfo) { logf("DNSStart: %+v", e) },
+		DNSDone:      func(e httptrace.DNSDoneInfo) { logf("DNSDone: %+v", e) },
+		ConnectStart: func(network, addr string) { logf("ConnectStart: %s %s", network, addr) },
+		ConnectDone:  func(network, addr string, err error) { logf("ConnectDone: %s %s %v", network, addr, err) },
+	}
+	req = req.WithContext(httptrace.WithClientTrace(context.Background(), trace))
+
+	resp, err := c.Do(req)
+	if err == nil {
+		resp.Body.Close()
+		t.Fatal("expected error during DNS lookup")
+	}
+
+	got := buf.String()
+	wantSub := func(sub string) {
+		if !strings.Contains(got, sub) {
+			t.Errorf("expected substring %q in output.", sub)
+		}
+	}
+	wantSub("DNSStart: {Host:dns-should-not-resolve.golang}")
+	wantSub("DNSDone: {Addrs:[] Err:")
+	if strings.Contains(got, "ConnectStart") || strings.Contains(got, "ConnectDone") {
+		t.Errorf("should not see Connect events")
+	}
+	if t.Failed() {
+		t.Errorf("Output:\n%s", got)
+	}
+}
+
 func TestTransportMaxIdleConns(t *testing.T) {
 	defer afterTest(t)
 	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {