mesos::internal::master::Slave::tasks can grow unboundedly

So in a Mesos cluster we observed the following

$ jq '.orphan_tasks | length' state.json
1369
$ jq '.unregistered_frameworks | length' state.json
20162

Aside from unregistered_frameworks here being "the list of frameworkIDs for each orphan task" (described in MESOS-4973), the discrepancy between the two values above is surprising.

I think the problem is that we do this in the master:

From source:

    foreachvalue (Slave* slave, slaves.registered) {
      foreachvalue (Task* task, slave->tasks[framework->id()]) {
        framework->addTask(task);
      }
      foreachvalue (const ExecutorInfo& executor,
                    slave->executors[framework->id()]) {
        framework->addExecutor(slave->id, executor);
      }
    }

Here an operator[] is used whenever a framework subscribes regardless of whether this agent has tasks for the framework or not.

If the agent has no such task for this framework, then this {frameworkID: empty hashmap} entry will stay in the map indefinitely! If frameworks are ephemeral and new ones keep come in, the map grows unboundedly.

We should do tasks.contains(frameworkId) before using the [] operator.