abhishekrb19 opened a new pull request, #19405:
URL: https://github.com/apache/druid/pull/19405
### Description
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.
### 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:
- [x] been self-reviewed.
- [x] added Javadocs for most classes and all non-trivial methods. Linked
related entities via Javadoc links.
- [x] added comments explaining the "why" and the intent of the code
wherever would not be obvious for an unfamiliar reader.
- [x] added unit tests or modified existing tests to cover new code paths,
ensuring the threshold for [code
coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md)
is met.
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