Nil check pointer fields in some watchers before using them (#4248)

- Nil check entity field coming from watcher
- Nil check tessen config coming from watcher
- Nil check check config coming from watcher

Signed-off-by: Cyril Cressent <cyril@sensu.io>

CHANGELOG.md

@@ -16,6 +16,8 @@ round robin checks.
 in OSS builds.
 - Fixed a potential crash in tessend.
 - Fixed a potential deadlock in agentd.
+- Fixed a bug where some Etcd watchers could try to process watch events holding
+invalid pointers.
 
 ## [6.2.2] - 2021-01-14
 

backend/agentd/agentd.go

@@ -3,6 +3,7 @@ package agentd
 import (
 	"context"
 	"crypto/tls"
+	"errors"
 	"fmt"
 	"log"
 	"net"
@@ -246,6 +247,10 @@ func (a *Agentd) runWatcher() {
 }
 
 func (a *Agentd) handleEvent(event store.WatchEventEntityConfig) error {
+	if event.Entity == nil {
+		return errors.New("nil entity received from entity config watcher")
+	}
+
 	topic := messaging.EntityConfigTopic(event.Entity.Metadata.Namespace, event.Entity.Metadata.Name)
 	if err := a.bus.Publish(topic, &event); err != nil {
 		logger.WithField("topic", topic).WithError(err).

backend/schedulerd/check_watcher.go

@@ -129,6 +129,12 @@ func (c *CheckWatcher) startWatcher() {
 
 func (c *CheckWatcher) handleWatchEvent(watchEvent store.WatchEventCheckConfig) {
 	check := watchEvent.CheckConfig
+
+	if check == nil {
+		logger.Error("nil check config received from check config watcher")
+		return
+	}
+
 	key := concatUniqueKey(check.Name, check.Namespace)
 
 	c.mu.Lock()

backend/tessend/tessend.go

@@ -319,6 +319,11 @@ func (t *Tessend) startWatcher() {
 func (t *Tessend) handleWatchEvent(watchEvent store.WatchEventTessenConfig) {
 	tessen := watchEvent.TessenConfig
 
+	if tessen == nil {
+		logger.Error("nil config received from tessen config watcher")
+		return
+	}
+
 	switch watchEvent.Action {
 	case store.WatchCreate:
 		logger.WithField("opt-out", tessen.OptOut).Debug("tessen configuration created")