locate: skip per-store limits for requests without a store

[wjhuang2016]

What problem does this PR solve?

Issue Number: ref pingcap/tidb#57829

With tidb_store_limit enabled, querying TiDB cluster information-schema tables can panic in getStoreToken. These coprocessor requests use a TiDB endpoint, for which getRPCContext intentionally returns an address without a backing Store. The synchronous send path nevertheless dereferences that missing store to acquire a per-store token.

What changed and how does it work?

Apply the per-store token limit only when the RPC context has a store. Requests to TiDB endpoints continue to the RPC client without acquiring or releasing a store token. TiKV/TiFlash contexts with stores retain the existing limit and release behavior. This matches the existing no-store handling in acquireRequestAttemptToken and does not change public APIs.

Add a regression sending TiDB coprocessor requests with limits 0, 1, and 10, checking that the response is returned and the RPC context has no store. The positive-limit cases panic before the fix; the zero-limit control succeeds.

Tests

• RED: go test -tags=intest ./internal/locate -run '^TestRegionRequestToSingleStore$/^TestTiDBRequestWithStoreLimit$' -count=1
• GREEN, including existing TiKV limiter and async request coverage: go test -tags=intest ./internal/locate -run '^TestRegionRequestToSingleStore$/(TestTiDBRequestWithStoreLimit|TestRequestAttemptLimiter|TestSendReqAsync)$' -count=1
• Race: go test -race -tags=intest ./internal/locate -run '^TestRegionRequestToSingleStore$/(TestTiDBRequestWithStoreLimit|TestRequestAttemptLimiter|TestSendReqAsync)$' -count=1
• golangci-lint run ./internal/locate: zero issues (local v2.11.3; repository CI uses v2.8).
• gofmt on both changed files and git diff --check.

Real TiDB/TiKV cluster queries and the full module test suite were not run locally. This PR fixes client-go; TiDB still needs to consume the updated dependency before its reported issue is fully resolved.

Release note

Fix a panic when per-store request limits are enabled for coprocessor requests sent to TiDB endpoints.

Summary by CodeRabbit

• Bug Fixes

  • Fixed Cop requests to TiDB endpoints when store limits are enabled.
  • TiDB endpoint requests now complete without store-token errors while preserving correct routing and responses.

• Tests

  • Added coverage for TiDB requests under multiple store-limit configurations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Advanced

Run ID: b158447f-8da8-4846-ba59-59ee9737e3ef

📥 Commits

Reviewing files that changed from the base of the PR and between f7a805b and 0020efa.

📒 Files selected for processing (2)

• internal/locate/region_request.go
• internal/locate/region_request_test.go

Included review availability: Your plan provides up to 2 included reviews per hour; 1 remains after this review.

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📝 Walkthrough

Walkthrough

The request path now skips per-store token handling for TiDB endpoints without a backing store. A test validates routing and responses across three store-limit values.

Changes

TiDB endpoint request handling

Layer / File(s)	Summary
Store-limit guard and TiDB request validation internal/locate/region_request.go, internal/locate/region_request_test.go	The sender acquires and releases store tokens only when the RPC context has a backing store. The test validates TiDB routing, response handling, and nil store context behavior for limits 0, 1, and 10.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: ekexium

Merge Risk: ⚪ Minimal · up to 0020e

The TiDB request path avoids the prior nil-store failure while retaining limits for backed stores. No actionable merge risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: skipping per-store limits when requests have no backing store. This matches the implementation and PR objective.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…
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.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Commit to this branch
• Create a new PR

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