Description

With #19040, when ioConfig.serverPriorityToReplicas is set, a supervisor can get stuck throwing
DruidException from computeUnassignedServerPriorities, preventing any new
task replicas with destined replicas from being created until the old tasks rollover. The
exception is as follows:

Found unassignedServerPriorities[[]] of size[0] < total replicas[1] for taskGroupId[0].
Task server priorities[[1, 0]] have already been assigned to tasks[[foo]].

The supervisor can remain in this unhealthy state unable to create additional task replicas until tasks eventually rollover. This can happen when a task spuriously fails after it's created but before it's discovered in the runInternal() loop.

Root cause

createTasksForGroup was an additional writer of group.taskIdToServerPriority,
recording the priority at task-submission time — before the task was ever observed
in group.tasks. The group's other invariant (tasks and taskIdToServerPriority
stay in sync) is enforced by discoverTasks (additions) and TaskGroup.removeTask
(removals), both of which operate on group.tasks.

If a task died after submission but before the next supervisor run picked it up from
the overlord, it never entered group.tasks, so removeTask never fired — the
priority entry was orphaned and made the next computeUnassignedServerPriorities
call throw.

Fix

This patch removes the additional writer, leaving discoverTasks and removeTask
as the sole mutators. group.taskIdToServerPriority should now stay in sync with group.tasks.
The added unit tests fail on master without the fix.

This PR has:

• been self-reviewed.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.