locate: refactor RegionRequestSender.SendReqCtx

[zyguan]

This PR extracts the iteration of retry loop in SendReqCtx into sendReqState.next. We will reuse sendReqState in async client API.

ref #1586

[cfzjywxk]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, ekexium

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [cfzjywxk,ekexium]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-18 03:48:02.106991621 +0000 UTC m=+933124.503213678: ☑️ agreed by cfzjywxk.
• 2025-02-25 03:34:05.535755435 +0000 UTC m=+326793.488913700: ☑️ agreed by ekexium.

[you06]

This commit break the a test in TiDB.

Reproduce

tiup playground --tiflash=0 nightly --mode tikv-slim --kv=3

go test ./tests/realtikvtest/sessiontest -run TestTiKVClientReadTimeout --tags=intest --with-real-tikv=true

Log

    testkit.go:318: 
                Error Trace:    pkg/testkit/testkit.go:318
                                                        pkg/testkit/testkit.go:243
                                                        pkg/testkit/testkit.go:176
                                                        tests/realtikvtest/sessiontest/session_fail_test.go:287
                Error:          Received unexpected error:
                                [127.0.0.1:20160](1) wait recvLoop timeout, timeout:1ms: context deadline exceeded
                                github.com/pingcap/errors.AddStack
                                        external/com_github_pingcap_errors/errors.go:178
                                github.com/pingcap/errors.Trace
                                        external/com_github_pingcap_errors/juju_adaptor.go:15
                                github.com/pingcap/tidb/pkg/store/driver/error.ToTiDBErr
                                        pkg/store/driver/error/error.go:208
                                github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).handleTaskOnce
                                        pkg/store/copr/coprocessor.go:1426
                                github.com/pingcap/tidb/pkg/store/copr.(*liteCopIteratorWorker).liteSendReq
                                        pkg/store/copr/coprocessor.go:1197
                                github.com/pingcap/tidb/pkg/store/copr.(*copIterator).Next
                                        pkg/store/copr/coprocessor.go:1112
                                github.com/pingcap/tidb/pkg/distsql.(*selectResult).fetchResp
                                        pkg/distsql/select_result.go:317
                                github.com/pingcap/tidb/pkg/distsql.(*selectResult).Next
                                        pkg/distsql/select_result.go:382
                                github.com/pingcap/tidb/pkg/executor.(*tableResultHandler).nextChunk
                                        pkg/executor/table_reader.go:612
                                github.com/pingcap/tidb/pkg/executor.(*TableReaderExecutor).Next
                                        pkg/executor/table_reader.go:331
                                github.com/pingcap/tidb/pkg/executor/internal/exec.Next
                                        pkg/executor/internal/exec/executor.go:460
                                github.com/pingcap/tidb/pkg/executor.(*ExplainExec).executeAnalyzeExec
                                        pkg/executor/explain.go:129
                                github.com/pingcap/tidb/pkg/executor.(*ExplainExec).generateExplainInfo
                                        pkg/executor/explain.go:148
                                github.com/pingcap/tidb/pkg/executor.(*ExplainExec).Next
                                        pkg/executor/explain.go:75
                                github.com/pingcap/tidb/pkg/executor/internal/exec.Next
                                        pkg/executor/internal/exec/executor.go:460
                                github.com/pingcap/tidb/pkg/executor.(*ExecStmt).next
                                        pkg/executor/adapter.go:1269
                                github.com/pingcap/tidb/pkg/executor.(*recordSet).Next
                                        pkg/executor/adapter.go:172
                                github.com/pingcap/tidb/pkg/session.GetRows4Test
                                        pkg/session/tidb.go:343
                                github.com/pingcap/tidb/pkg/session.ResultSetToStringSlice
                                        pkg/session/tidb.go:361
                                github.com/pingcap/tidb/pkg/testkit.(*TestKit).ResultSetToResultWithCtx
                                        pkg/testkit/testkit.go:317
                                github.com/pingcap/tidb/pkg/testkit.(*TestKit).MustQueryWithContext
                                        pkg/testkit/testkit.go:243
                                github.com/pingcap/tidb/pkg/testkit.(*TestKit).MustQuery
                                        pkg/testkit/testkit.go:176
                                tests/realtikvtest/sessiontest/sessiontest_test.TestTiKVClientReadTimeout
                                        tests/realtikvtest/sessiontest/session_fail_test.go:287
                                testing.tRunner
                                        GOROOT/src/testing/testing.go:1690
                                runtime.goexit
                                        src/runtime/asm_amd64.s:1700
                Test:           TestTiKVClientReadTimeout
                Messages:       sql:explain analyze select /*+ set_var(tikv_client_read_timeout=1) */ * from t where b > 1, args:[]