Backport of discovery: correctly handle IPv6 addresses from go-discover into release/1.9.x

.changelog/24649.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+discovery: Fixed a bug where IPv6 addresses would not be accepted from cloud autojoin
+```

client/rpc.go

@@ -445,14 +445,30 @@ func (c *Client) handleStreamingConn(conn net.Conn) {
 // net.Addr or an error.
 func resolveServer(s string) (net.Addr, error) {
 	const defaultClientPort = "4647" // default client RPC port
+
 	host, port, err := net.SplitHostPort(s)
 	if err != nil {
-		if strings.Contains(err.Error(), "missing port") {
+		switch {
+		case strings.Contains(err.Error(), "missing port"):
 			// with IPv6 addresses the `host` variable will have brackets
 			// removed, so send the original value thru again with only the
 			// correct port suffix
 			return resolveServer(s + ":" + defaultClientPort)
-		} else {
+		case strings.Contains(err.Error(), "too many colons"):
+			// note: we expect IPv6 address strings to be RFC5952 compliant to
+			// disambiguate port numbers from the address. Because the port number
+			// is typically 4 decimal digits, the same size as an IPv6 address
+			// segment, there's no way to disambiguate this. See
+			// https://www.rfc-editor.org/rfc/rfc5952
+			ip := net.ParseIP(s)
+			if ip.To4() == nil && ip.To16() != nil {
+				if !strings.HasPrefix(s, "[") {
+					return resolveServer("[" + s + "]:" + defaultClientPort)
+				}
+			}
+			return nil, err
+
+		default:
 			return nil, err
 		}
 	} else if port == "" {

client/rpc_test.go

@@ -130,9 +130,15 @@ func Test_resolveServer(t *testing.T) {
 		expectErr string
 	}{
 		{
-			name:      "ipv6 no brackets",
-			addr:      "2001:db8::1",
-			expectErr: "address 2001:db8::1: too many colons in address",
+			name:   "ipv6 no brackets",
+			addr:   "2001:db8::1",
+			expect: "[2001:db8::1]:4647",
+		},
+		{
+			// expected bad result
+			name:   "ambiguous ipv6 no brackets with port",
+			addr:   "2001:db8::1:4647",
+			expect: "[2001:db8::1:4647]:4647",
 		},
 		{
 			name:   "ipv6 no port",