[srp-server] fix adding service with subtypes to existing host (#6901)

When a new service with subtypes is added to an existing
host, the SRP server incorrectly merges the subtype service
copies so that the TXT information is lost, i.e. 0-length
TXT record is created.

Moreover, the OT DNS client cannot parse services with
0-length TXT record (instead of ignoring the TXT record
itself) causing that such a service cannot be examined
at all.

The problem is caused by incorrect merging of the service
description structure which is shared across all services
with the same instance name (i.e. among the base type
service and its subtypes).

Signed-off-by: Damian Krolik <[email protected]>

tests/scripts/thread-cert/test_dnssd.py

@@ -156,6 +156,18 @@ class TestDnssd(thread_cert.TestCase):
             'aaaa_ttl': lambda x: x > 0,
         }
 
+        instance4_verify_info = {
+            'port': 44444,
+            'priority': 4,
+            'weight': 4,
+            'host': 'host3.default.service.arpa.',
+            'address': client3_addrs,
+            'txt_data': 'KEY=414243',  # KEY=ABC
+            'srv_ttl': lambda x: x > 0,
+            'txt_ttl': lambda x: x > 0,
+            'aaaa_ttl': lambda x: x > 0,
+        }
+
         # Browse for main service
         service_instances = client1.dns_browse(f'{SERVICE}.{DOMAIN}', server.get_mleid(), 53)
         self.assertEqual({'ins1', 'ins2', 'ins3'}, set(service_instances.keys()))
@@ -182,6 +194,20 @@ class TestDnssd(thread_cert.TestCase):
         service_instance = client1.dns_resolve_service('ins2', f'{SERVICE}.{DOMAIN}', server.get_mleid(), 53)
         self._assert_service_instance_equal(service_instance, instance2_verify_info)
 
+        #---------------------------------------------------------------
+        # Add another service with TXT entries to the existing host and
+        # verify that it is properly merged.
+
+        client3.srp_client_add_service('ins4', SERVICE + ",_s1", 44444, 4, 4, txt_entries=['KEY=ABC'])
+        self.simulator.go(5)
+
+        service_instances = client1.dns_browse(f'{SERVICE}.{DOMAIN}', server.get_mleid(), 53)
+        self.assertEqual({'ins1', 'ins2', 'ins3', 'ins4'}, set(service_instances.keys()))
+        self._assert_service_instance_equal(service_instances['ins1'], instance1_verify_info)
+        self._assert_service_instance_equal(service_instances['ins2'], instance2_verify_info)
+        self._assert_service_instance_equal(service_instances['ins3'], instance3_verify_info)
+        self._assert_service_instance_equal(service_instances['ins4'], instance4_verify_info)
+
     def _assert_service_instance_equal(self, instance, info):
         for f in ('port', 'priority', 'weight', 'host', 'txt_data'):
             self.assertEqual(instance[f], info[f], instance)