Maintenance: updater DNS resolution more resilient

internal/updater/resolver.go

@@ -88,9 +88,9 @@ func resolveRepeat(ctx context.Context, lookupIP lookupIPFunc, host string,
 
 	i := 0
 	for {
-		newIPs, err := lookupIP(ctx, host)
-		if err != nil {
-			return nil, err
+		newIPs, newErr := lookupIP(ctx, host)
+		if err == nil {
+			err = newErr // it's fine to fail some of the resolutions
 		}
 		for _, ip := range newIPs {
 			key := ip.String()
@@ -113,6 +113,10 @@ func resolveRepeat(ctx context.Context, lookupIP lookupIPFunc, host string,
 		}
 	}
 
+	if len(uniqueIPs) == 0 {
+		return nil, err
+	}
+
 	ips = make([]net.IP, 0, len(uniqueIPs))
 	for key := range uniqueIPs {
 		ip := net.ParseIP(key)
@@ -126,5 +130,5 @@ func resolveRepeat(ctx context.Context, lookupIP lookupIPFunc, host string,
 		return bytes.Compare(ips[i], ips[j]) < 1
 	})
 
-	return ips, nil
+	return ips, err
 }

internal/updater/resolver_test.go

@@ -20,12 +20,17 @@ func Test_resolveRepeat(t *testing.T) {
 		ips            []net.IP
 		err            error
 	}{
-		"failure": {
-			lookupIPResult: [][]net.IP{
-				{{1, 1, 1, 1}, {1, 1, 1, 2}},
-			},
+		"failure twice": {
+			lookupIPResult: [][]net.IP{{}, {}},
 			lookupIPErr:    fmt.Errorf("feeling sick"),
-			n:           1,
+			n:              2,
+			err:            fmt.Errorf("feeling sick"),
+		},
+		"failure once": {
+			lookupIPResult: [][]net.IP{{}, {{1, 1, 1, 1}}},
+			lookupIPErr:    fmt.Errorf("feeling sick"),
+			n:              2,
+			ips:            []net.IP{{1, 1, 1, 1}},
 			err:            fmt.Errorf("feeling sick"),
 		},
 		"successful": {