netutil: add url comparison without resolver to URLStringsEqual

If one of the nodes in the cluster has lost a dns record,
restarting the second node will break it.
This PR makes an attempt to add a comparison without using a resolver,
which allows to protect cluster from dns errors and does not break
the current logic of comparing urls in the URLStringsEqual function.
You can read more in the issue etcd-io#7798

Fixes etcd-io#7798

pkg/netutil/netutil.go

@@ -174,29 +174,29 @@ func URLStringsEqual(ctx context.Context, lg *zap.Logger, a []string, b []string
 	if len(a) != len(b) {
 		return false, fmt.Errorf("len(%q) != len(%q)", a, b)
 	}
-	urlsA := make([]url.URL, 0)
-	for _, str := range a {
-		u, err := url.Parse(str)
-		if err != nil {
-			return false, fmt.Errorf("failed to parse %q", str)
-		}
-		urlsA = append(urlsA, *u)
+	urlsA, err := stringsToURLs(a)
+	if err != nil {
+		return false, err
 	}
-	urlsB := make([]url.URL, 0)
-	for _, str := range b {
-		u, err := url.Parse(str)
-		if err != nil {
-			return false, fmt.Errorf("failed to parse %q", str)
-		}
-		urlsB = append(urlsB, *u)
+	urlsB, err := stringsToURLs(b)
+	if err != nil {
+		return false, err
 	}
 	if lg == nil {
 		lg, _ = zap.NewProduction()
 		if lg == nil {
 			lg = zap.NewExample()
 		}
 	}
-	return urlsEqual(ctx, lg, urlsA, urlsB)
+	sort.Sort(types.URLs(urlsA))
+	sort.Sort(types.URLs(urlsB))
+	for i := range urlsA {
+		if !reflect.DeepEqual(urlsA[i], urlsB[i]) {
+			// If urls are not equal, try to resolve it and compare again.
+			return urlsEqual(ctx, lg, urlsA, urlsB)
+		}
+	}
+	return true, nil
 }
 
 func urlsToStrings(us []url.URL) []string {
@@ -207,6 +207,18 @@ func urlsToStrings(us []url.URL) []string {
 	return rs
 }
 
+func stringsToURLs(us []string) ([]url.URL, error) {
+	urls := make([]url.URL, 0, len(us))
+	for _, str := range us {
+		u, err := url.Parse(str)
+		if err != nil {
+			return nil, fmt.Errorf("failed to parse %q", str)
+		}
+		urls = append(urls, *u)
+	}
+	return urls, nil
+}
+
 func IsNetworkTimeoutError(err error) bool {
 	nerr, ok := err.(net.Error)
 	return ok && nerr.Timeout()

pkg/netutil/netutil_test.go

@@ -17,6 +17,7 @@ package netutil
 import (
 	"context"
 	"errors"
+	"fmt"
 	"net"
 	"net/url"
 	"reflect"
@@ -292,11 +293,33 @@ func TestURLsEqual(t *testing.T) {
 	}
 }
 func TestURLStringsEqual(t *testing.T) {
-	result, err := URLStringsEqual(context.TODO(), zap.NewExample(), []string{"http://127.0.0.1:8080"}, []string{"http://127.0.0.1:8080"})
-	if !result {
-		t.Errorf("unexpected result %v", result)
+	defer func() { resolveTCPAddr = resolveTCPAddrDefault }()
+	errOnResolve := func(ctx context.Context, addr string) (*net.TCPAddr, error) {
+		return nil, fmt.Errorf("unexpected attempt to resolve: %q", addr)
+	}
+	cases := []struct {
+		urlsA    []string
+		urlsB    []string
+		resolver func(ctx context.Context, addr string) (*net.TCPAddr, error)
+	}{
+		{[]string{"http://127.0.0.1:8080"}, []string{"http://127.0.0.1:8080"}, resolveTCPAddrDefault},
+		{[]string{
+			"http://host1:8080",
+			"http://host2:8080",
+		}, []string{
+			"http://host1:8080",
+			"http://host2:8080",
+		}, errOnResolve},
 	}
-	if err != nil {
-		t.Errorf("unexpected error %v", err)
+	for idx, c := range cases {
+		t.Logf("TestURLStringsEqual, case #%d", idx)
+		resolveTCPAddr = c.resolver
+		result, err := URLStringsEqual(context.TODO(), zap.NewExample(), c.urlsA, c.urlsB)
+		if !result {
+			t.Errorf("unexpected result %v", result)
+		}
+		if err != nil {
+			t.Errorf("unexpected error %v", err)
+		}
 	}
 }