connect: updating a service-defaults config entry should leave an unset protocol alone (#6342)

If the entry is updated for reasons other than protocol it is surprising
that the value is explicitly persisted as 'tcp' rather than leaving it
empty and letting it fall back dynamically on the proxy-defaults value.

agent/consul/config_endpoint_test.go

@@ -23,11 +23,13 @@ func TestConfigEntry_Apply(t *testing.T) {
 	codec := rpcClient(t, s1)
 	defer codec.Close()
 
+	updated := &structs.ServiceConfigEntry{
+		Name: "foo",
+	}
+
 	args := structs.ConfigEntryRequest{
 		Datacenter: "dc1",
-		Entry: &structs.ServiceConfigEntry{
-			Name: "foo",
-		},
+		Entry:      updated,
 	}
 	out := false
 	require.NoError(msgpackrpc.CallWithCodec(codec, "ConfigEntry.Apply", &args, &out))
@@ -42,13 +44,17 @@ func TestConfigEntry_Apply(t *testing.T) {
 	require.Equal("foo", serviceConf.Name)
 	require.Equal(structs.ServiceDefaults, serviceConf.Kind)
 
+	updated = &structs.ServiceConfigEntry{
+		Name: "foo",
+		MeshGateway: structs.MeshGatewayConfig{
+			Mode: structs.MeshGatewayModeLocal,
+		},
+	}
+
 	args = structs.ConfigEntryRequest{
 		Datacenter: "dc1",
 		Op:         structs.ConfigEntryUpsertCAS,
-		Entry: &structs.ServiceConfigEntry{
-			Name:     "foo",
-			Protocol: "tcp",
-		},
+		Entry:      updated,
 	}
 
 	require.NoError(msgpackrpc.CallWithCodec(codec, "ConfigEntry.Apply", &args, &out))
@@ -64,9 +70,10 @@ func TestConfigEntry_Apply(t *testing.T) {
 
 	serviceConf, ok = entry.(*structs.ServiceConfigEntry)
 	require.True(ok)
-	require.Equal("foo", serviceConf.Name)
-	require.Equal("tcp", serviceConf.Protocol)
-	require.Equal(structs.ServiceDefaults, serviceConf.Kind)
+
+	updated.Kind = structs.ServiceDefaults    // the server adds this
+	updated.RaftIndex = serviceConf.RaftIndex // these will always be different
+	require.Equal(updated, serviceConf)
 }
 
 func TestConfigEntry_ProxyDefaultsMeshGateway(t *testing.T) {

agent/structs/config_entry.go

@@ -79,11 +79,7 @@ func (e *ServiceConfigEntry) Normalize() error {
 	}
 
 	e.Kind = ServiceDefaults
-	if e.Protocol == "" {
-		e.Protocol = DefaultServiceProtocol
-	} else {
-		e.Protocol = strings.ToLower(e.Protocol)
-	}
+	e.Protocol = strings.ToLower(e.Protocol)
 
 	return nil
 }