@Override
  public void registerTestHook(MembershipTestHook mth) {
    getDistribution().doWithViewLocked(() -> {
      if (membershipTestHooks == null) {
        membershipTestHooks = Collections.singletonList(mth);
      } else {
        List<MembershipTestHook> l = new ArrayList<>(membershipTestHooks);
        l.add(mth);
        membershipTestHooks = l;
      }
      return null;
    });
  }
This is an event reflecting that a InternalDistributedMember may be missing but has not yet
left the system.
geode-core/src/main/java/org/apache/geode/distributed/internal/ClusterDistributionManager.java