net: ignore network failures on some builders

We run the external network tests on builders, but some of our
builders have less-than-ideal DNS connectivity. This change continues
to run the tests on all builders, but marks certain builders as flaky
(network-wise), and only validates their DNS results if they got DNS
results.

Change-Id: I826dc2a6f6da55add89ae9c6db892b3b2f7b526b
Reviewed-on: https://go-review.googlesource.com/22852
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/testenv/testenv.go

@@ -16,6 +16,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"runtime"
+	"strconv"
 	"strings"
 	"testing"
 )
@@ -133,3 +134,9 @@ func SkipFlaky(t *testing.T, issue int) {
 		t.Skipf("skipping known flaky test without the -flaky flag; see golang.org/issue/%d", issue)
 	}
 }
+
+func SkipFlakyNet(t *testing.T) {
+	if v, _ := strconv.ParseBool(os.Getenv("GO_BUILDER_FLAKY_NET")); v {
+		t.Skip("skipping test on builder known to have frequent network failures")
+	}
+}

src/net/lookup_test.go

@@ -498,6 +498,7 @@ func TestLookupDotsWithRemoteSource(t *testing.T) {
 func testDots(t *testing.T, mode string) {
 	names, err := LookupAddr("8.8.8.8") // Google dns server
 	if err != nil {
+		testenv.SkipFlakyNet(t)
 		t.Errorf("LookupAddr(8.8.8.8): %v (mode=%v)", err, mode)
 	} else {
 		for _, name := range names {
@@ -509,12 +510,16 @@ func testDots(t *testing.T, mode string) {
 	}
 
 	cname, err := LookupCNAME("www.mit.edu")
-	if err != nil || !strings.HasSuffix(cname, ".") {
+	if err != nil {
-		t.Errorf("LookupCNAME(www.mit.edu) = %v, %v, want cname ending in . with trailing dot (mode=%v)", cname, err, mode)
+		testenv.SkipFlakyNet(t)
+		t.Errorf("LookupCNAME(www.mit.edu, mode=%v): %v", mode, err)
+	} else if !strings.HasSuffix(cname, ".") {
+		t.Errorf("LookupCNAME(www.mit.edu) = %v, want cname ending in . with trailing dot (mode=%v)", cname, mode)
 	}
 
 	mxs, err := LookupMX("google.com")
 	if err != nil {
+		testenv.SkipFlakyNet(t)
 		t.Errorf("LookupMX(google.com): %v (mode=%v)", err, mode)
 	} else {
 		for _, mx := range mxs {
@@ -527,6 +532,7 @@ func testDots(t *testing.T, mode string) {
 
 	nss, err := LookupNS("google.com")
 	if err != nil {
+		testenv.SkipFlakyNet(t)
 		t.Errorf("LookupNS(google.com): %v (mode=%v)", err, mode)
 	} else {
 		for _, ns := range nss {
@@ -539,6 +545,7 @@ func testDots(t *testing.T, mode string) {
 
 	cname, srvs, err := LookupSRV("xmpp-server", "tcp", "google.com")
 	if err != nil {
+		testenv.SkipFlakyNet(t)
 		t.Errorf("LookupSRV(xmpp-server, tcp, google.com): %v (mode=%v)", err, mode)
 	} else {
 		if !strings.HasSuffix(cname, ".google.com.") {