schedule: fix panic when switching placement rules (#7415)

[ti-chi-bot]

This is an automated cherry-pick of #7415

What problem does this PR solve?

Issue Number: Close #7414.

What is changed and how does it work?

When we going to disable the placement rule, we might first check:

pd/pkg/schedule/checker/checker_controller.go

Line 89 in 8e6dcfb

if c.conf.IsPlacementRulesEnabled() {

Before we change it, the value is true. But when we call:

pd/pkg/schedule/checker/checker_controller.go

Line 102 in 8e6dcfb

fit := c.priorityInspector.Inspect(region)

pd/pkg/schedule/checker/priority_inspector.go

Lines 64 to 74 in 8e6dcfb

	func (p *PriorityInspector) Inspect(region *core.RegionInfo) (fit *placement.RegionFit) {
	var makeupCount int
	if p.conf.IsPlacementRulesEnabled() {
	makeupCount, fit = p.inspectRegionInPlacementRule(region)
	} else {
	makeupCount = p.inspectRegionInReplica(region)
	}
	priority := 0 - makeupCount
	p.addOrRemoveRegion(priority, region.GetID())
	return
	}

The value can be changed to false. So the fit will be nil in this case.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• HuSharp
• lhy1024

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[rleungx]

/merge

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This pull request has been accepted and is ready to merge.

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Commit hash: fd4ef1c