fix: address PR review feedback for user config watcher

- agentd.go: add nil check for event before accessing event.User in
  handleUserEvent to prevent panic on nil pointer dereference
- session.go: add continue after nil user check to prevent panic,
  rename lagger to logger (typo fix), rename bytes to userData to
  avoid shadowing standard library package, add continue after
  marshal error
- watcher.go: add error logging when proto.Unmarshal fails for user
  config

Signed-off-by: rakibhossainctr <rakib.hossain.ctr@sumologic.com>

Author: rakibhossainctr

backend/agentd/agentd.go

@@ -322,8 +322,11 @@ func (a *Agentd) handleEvent(event store.WatchEventEntityConfig) error {
 
 // adding the UserConfig updates to the etcd bus for the watcher to consume
 func (a *Agentd) handleUserEvent(event *store.WatchEventUserConfig) error {
+	if event == nil {
+		return errors.New("nil event received from the user config watcher")
+	}
 	if event.User == nil {
-		return errors.New("nil entry received from the user config watcher")
+		return errors.New("nil user received from the user config watcher")
 	}
 
 	topic := messaging.UserConfigTopic(event.User.Username)

backend/agentd/session.go

@@ -371,19 +371,21 @@ func (s *Session) sender() {
 
 			if watchEvent.User == nil {
 				logger.Error("session received nil user in watch event")
+				continue
 			}
-			lagger := logger.WithFields(logrus.Fields{
+			logger := logger.WithFields(logrus.Fields{
 				"action":    watchEvent.Action.String(),
 				"user":      watchEvent.User.Username,
 				"namespace": watchEvent.User.GetMetadata().GetNamespace(),
 			})
-			lagger.Debug("user update received")
+			logger.Debug("user update received")
 
-			bytes, err := s.marshal(watchEvent.User)
+			userData, err := s.marshal(watchEvent.User)
 			if err != nil {
-				lagger.WithError(err).Error("session failed to serialize user config")
+				logger.WithError(err).Error("session failed to serialize user config")
+				continue
 			}
-			msg = transport.NewMessage(transport.MessageTypeUserConfig, bytes)
+			msg = transport.NewMessage(transport.MessageTypeUserConfig, userData)
 
 		// ---- entity ----//
 		case e := <-s.entityConfig.updatesChannel:

backend/agentd/watcher.go

@@ -3,6 +3,7 @@ package agentd
 import (
 	"context"
 	"errors"
+
 	"github.com/gogo/protobuf/proto"
 	corev2 "github.com/sensu/core/v2"
 	corev3 "github.com/sensu/core/v3"
@@ -11,7 +12,7 @@ import (
 	storev2 "github.com/sensu/sensu-go/backend/store/v2"
 	etcdstorev2 "github.com/sensu/sensu-go/backend/store/v2/etcdstore"
 	"github.com/sensu/sensu-go/backend/store/v2/wrap"
-	"go.etcd.io/etcd/client/v3"
+	clientv3 "go.etcd.io/etcd/client/v3"
 )
 
 // GetEntityConfigWatcher watches changes to EntityConfig in etcd and publish them
@@ -98,6 +99,8 @@ func GetUserConfigWatcher(ctx context.Context, client *clientv3.Client) <-chan *
 			// unmarshal the user config
 			var userConfig corev2.User
 			if err := proto.Unmarshal(response.Object, &userConfig); err != nil {
+				logger.WithField("key", response.Key).WithError(err).
+					Error("unable to unmarshal user config from key")
 				continue
 			}