Exit DynamoDB tryToLock when stop channel is closed (#6640)

* Exit DynamoDB tryToLock when stop channel is closed

If the stop channel is closed (e.g. an error is returned which triggers
close(stop) in Lock), this loop will spin and use 100% CPU.

* Ensure ticker is stopped

physical/dynamodb/dynamodb.go

@@ -600,11 +600,12 @@ func (l *DynamoDBLock) Value() (bool, string, error) {
 // channel is closed.
 func (l *DynamoDBLock) tryToLock(stop, success chan struct{}, errors chan error) {
 	ticker := time.NewTicker(DynamoDBLockRetryInterval)
+	defer ticker.Stop()
 
 	for {
 		select {
 		case <-stop:
-			ticker.Stop()
+			return
 		case <-ticker.C:
 			err := l.updateItem(true)
 			if err != nil {
@@ -620,7 +621,6 @@ func (l *DynamoDBLock) tryToLock(stop, success chan struct{}, errors chan error)
 					return
 				}
 			} else {
-				ticker.Stop()
 				close(success)
 				return
 			}