ClatUtils: add test GenerateIpv6AddressFailWithUlaSocketAddress

Used for increasing the test coverage of generateIpv6Address.

For now, generateIpv6Address test coverage: 0% (0/27 lines)
https://blackbox.googleplex.com/testsuite?branch=git_tm-mainline-prod-daily-release&target=mainline_modules_x86_coverage_r_released&testsuite=coverage%2Fmts_tethering_native_t_on_t&type=coverage

This is not a realistic test. The generateIpv6Address functionality is
not real verified. See also test MakeChecksumNeutral.

TODO: remove this test once EthernetTetheringTest can
test on mainline test coverage branch.

Test: atest ClatUtils

Change-Id: I8e4fbc5af2cd0dd0a1bb181388ea0d4b200baed3

service/native/libs/libclat/clatutils_test.cpp

@@ -19,6 +19,7 @@
 #include <gtest/gtest.h>
 #include <linux/if_packet.h>
 #include <linux/if_tun.h>
+#include <netinet/in.h>
 #include "tun_interface.h"
 
 extern "C" {
@@ -182,6 +183,31 @@ TEST_F(ClatUtils, ConfigurePacketSocket) {
     v6Iface.destroy();
 }
 
+// This is not a realistic test because we can't test generateIPv6Address here since it requires
+// manipulating routing, which we can't do without talking to the real netd on the system.
+// See test MakeChecksumNeutral.
+// TODO: remove this test once EthernetTetheringTest can test on mainline test coverage branch.
+TEST_F(ClatUtils, GenerateIpv6AddressFailWithUlaSocketAddress) {
+    // Create an interface for generateIpv6Address to connect to.
+    TunInterface tun;
+    ASSERT_EQ(0, tun.init());
+
+    // Unused because v6 address is ULA and makeChecksumNeutral has never called.
+    in_addr v4 = {inet_addr(kIPv4LocalAddr)};
+    // Not a NAT64 prefix because test can't connect to in generateIpv6Address.
+    // Used to be a fake address from the tun interface for generating an IPv6 socket address.
+    // nat64Prefix won't be used in MakeChecksumNeutral because MakeChecksumNeutral has never
+    // be called.
+    in6_addr nat64Prefix = tun.dstAddr();  // not realistic
+    in6_addr v6;
+    EXPECT_EQ(1, inet_pton(AF_INET6, "::", &v6));  // initialize as zero
+
+    EXPECT_EQ(-ENETUNREACH, generateIpv6Address(tun.name().c_str(), v4, nat64Prefix, &v6));
+    EXPECT_TRUE(IN6_IS_ADDR_ULA(&v6));
+
+    tun.destroy();
+}
+
 }  // namespace clat
 }  // namespace net
 }  // namespace android