server/cluster: avoid repeated region size scans

[lhy1024]

What problem does this PR solve?

When multiple stores are in the Preparing state, PD recalculates the same
region sizes for every store during each checkStores round. With placement
rules enabled, calculateRange also calls GetRegionSizeByRange once for
every matching rule.

For non-empty key ranges, these repeated scans can significantly increase
Region tree lock contention and delay Region heartbeat processing.

Issue Number: ref #9574

What is changed and how does it work?

Cache region sizes by key range within one checkStores round so all Preparing
stores reuse the same calculation.

Load each range size before processing its matching placement rules instead of
scanning the range once per rule.

Discard the cache after every `checkStores` round so the next round reloads
region sizes from the current Region tree.

Limitations

This PR eliminates duplicate range scans across placement rules and Preparing
stores within one checkStores round. Each unique non-empty range is still
scanned in O(N) time once per round. Supporting O(log N) range queries or
incremental range-size aggregation will be handled separately.

Check List

Tests

• Unit test

Related changes

• core: return the region tree size if scan all the regions #9580 optimizes the full key-range lookup.
• Need to cherry-pick to the release branch.

Release note

Reduce repeated Region tree scans when multiple stores are preparing.

Summary by CodeRabbit

• Bug Fixes

  • Improved store readiness and placement-rule threshold calculations by reusing region-size results within each calculation round.
  • Corrected placement-rule range evaluation to begin with the first rule matching the target store.
  • Preserved separate calculations across rounds to prevent stale sizing results.

• Tests

  • Added coverage for cache reuse across stores and placement rules, independent range caching, and calculation-round isolation.

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[coderabbitai]

📝 Walkthrough

Walkthrough

The cluster now caches region sizes by key range for each checkStores round. Store readiness and placement-rule threshold calculations reuse cached values. Tests verify reuse across stores and rules, plus isolation between calculation rounds.

Changes

Region-size threshold calculations

Layer / File(s)	Summary
Per-round cache wiring server/cluster/cluster.go, server/cluster/cluster_test.go	checkStores creates a range-keyed cache and passes it through store and threshold checks. Tests verify shared and isolated cache behavior.
Cached threshold and rule evaluation server/cluster/cluster.go, server/cluster/cluster_test.go	Threshold and weighted range calculations use cached region sizes. Placement-rule evaluation starts at the first matching rule and continues through later applicable rules.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: avoiding repeated region-size scans in the cluster package.
Description check	✅ Passed	The description explains the problem, implementation, limitations, tests, related changes, release impact, and issue reference.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Fix failing CI checks

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[codecov]

Codecov Report

❌ Patch coverage is 94.28571% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.24%. Comparing base (070828c) to head (ccc20b6).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11072      +/-   ##
==========================================
+ Coverage   79.23%   79.24%   +0.01%     
==========================================
  Files         540      541       +1     
  Lines       76010    76509     +499     
==========================================
+ Hits        60226    60631     +405     
- Misses      11533    11595      +62     
- Partials     4251     4283      +32     

Flag	Coverage Δ
unittests	79.24% <94.28%> (+0.01%)	⬆️

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[ti-chi-bot]

@lhy1024: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-unit-test-next-gen-2	ccc20b6	link	true	/test pull-unit-test-next-gen-2

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