gang: track incomplete_tasks on-demand via bind/unbind hooks

[k82cn]

Summary

Update GangPlugin to track incomplete_tasks dynamically via bind/unbind hooks instead of relying solely on the snapshot value from setup().

Changes

• Decrement incomplete_tasks in on_session_bind() when a task is bound to an executor
• Increment incomplete_tasks in on_session_unbind() when an executor is unbound

Motivation

The incomplete_tasks value was previously calculated once in setup() and remained static throughout the scheduling cycle. This could become stale as tasks are bound/unbound during the cycle. By updating it on-demand via hooks, the gang scheduling decisions (is_ready/is_fulfilled) now reflect the current state.

Testing

All 21 gang plugin tests pass.

[gemini-code-assist]

Code Review

This pull request attempts to update the number of incomplete tasks when sessions are bound or unbound in the gang scheduler plugin. However, the feedback highlights a critical logic bug where modifying incomplete_tasks during binding and unbinding leads to double-counting and premature fulfillment of gang scheduling requirements, as a bound task is still incomplete. It is recommended to revert these changes.

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[gemini-code-assist]

Double-Counting & Premature Fulfillment Bug

Decrementing incomplete_tasks here introduces a critical logic bug:

1. Conceptual Issue: A task bound to an executor is still incomplete (it is in Running or Binding state, not Succeed/Failed/Cancelled). Therefore, binding a task does not reduce the number of incomplete tasks.
2. Double-Counting: The gang plugin evaluates fulfillment using total >= needed, where total = allocated + bound and needed = div_ceil(incomplete_tasks, batch_size) * batch_size. If you decrement incomplete_tasks (reducing needed) and increment bound (increasing total) simultaneously, you double-count the progress.

For example, with batch_size = 2 and 4 pending tasks:

• Initially: needed = 4, total = 0.
• Bind 1 task: needed = 4, total = 1.
• Bind 2 tasks: needed = 2, total = 2 -> is_fulfilled becomes true prematurely, leaving the remaining 2 tasks without executors.

Revert this change to keep incomplete_tasks static during the scheduling cycle.

Suggested change

	state.bound += 1;
	state.incomplete_tasks = state.incomplete_tasks.saturating_sub(1);
	state.bound += 1;

[gemini-code-assist]

Double-Counting & Premature Fulfillment Bug

Incrementing incomplete_tasks here is the inverse of the double-counting bug introduced in on_session_bind. Since unbinding a task does not change its completion status (it remains incomplete), incomplete_tasks should not be modified here. Revert this change to keep incomplete_tasks static during the scheduling cycle.

Suggested change

	state.bound = state.bound.saturating_sub(1);
	state.incomplete_tasks = state.incomplete_tasks.saturating_add(1);
	state.bound = state.bound.saturating_sub(1);