Add missing synchronized

1 files changed, 1 insertions, 1 deletions

diff --git a/zkfacade/src/main/java/com/yahoo/vespa/curator/SingletonManager.java b/zkfacade/src/main/java/com/yahoo/vespa/curator/SingletonManager.java
index 3bff9a1c3e0..cf6e51836ec 100644
--- a/zkfacade/src/main/java/com/yahoo/vespa/curator/SingletonManager.java
+++ b/zkfacade/src/main/java/com/yahoo/vespa/curator/SingletonManager.java
@@ -74,7 +74,7 @@ class SingletonManager implements AutoCloseable {
      * The container may abandon the lease early, if deactivation is triggered and completes before the deadline.
      * Unless connection to the underlying ZK cluster is lost, the returned value will regularly move forwards in time.
      */
-    Optional<Instant> activeUntil(String singletonId) {
+    synchronized Optional<Instant> activeUntil(String singletonId) {
         return Optional.ofNullable(janitors.get(singletonId)).map(janitor -> janitor.doom.get());
     }