pkg/executor: fix concurrent SessionVars.systems map access during ANALYZE

[mjonss]

What problem does this PR solve?

Issue Number: close #68457

Problem Summary:

Partition-analyze workers share one SessionVars and each independently
called GetSessionOrGlobalSystemVar(tidb_build_sampling_stats_concurrency)
inside analyzeColumnsPushDown. The session's systems map is lazily
populated on first read and is not protected by any lock, so when multiple
workers raced to fill the same entry the Go runtime reported fatal error: concurrent map read and map write and killed the TiDB server. The original
crash was observed during auto-analyze of an 8-partition table.

What changed and how does it work?

Resolve tidb_build_sampling_stats_concurrency once on the main goroutine
in AnalyzeExec.Next, before the partition workers fan out, and thread the
value through a new AnalyzeColumnsExec.samplingStatsConcurrency field.
Workers now read a plain struct field, so they never touch
SessionVars.systems. The number of sysvar lookups per ANALYZE drops from
one per partition to one per statement.

Regression test: a failpoint inside getBuildSamplingStatsConcurrency
records the calling goroutine's stack on every invocation. The test asserts
the call still happens (resolution is not silently dropped) and that no
call originates from (*AnalyzeExec).analyzeWorker. Reintroducing the
worker-side call would put analyzeWorker on the captured stack and fail
the test.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fixed a TiDB server crash ("fatal error: concurrent map read and map write")
that could occur during ANALYZE on partitioned tables, including auto-analyze,
when worker goroutines concurrently resolved
tidb_build_sampling_stats_concurrency.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR moves the computation of samplingStatsConcurrency from concurrent worker goroutines to a single pre-fanout step on the main goroutine, eliminating unsafe concurrent access to SessionVars.systems. The value is then pre-assigned to each column executor task before workers start, and test instrumentation verifies the fix works correctly.

Changes

ANALYZE sampling concurrency race fix

Layer / File(s)	Summary
Move concurrency resolution to main goroutine pkg/executor/analyze_col.go, pkg/executor/analyze.go, pkg/executor/analyze_col_sampling.go	AnalyzeColumnsExec gains a samplingStatsConcurrency field. AnalyzeExec.Next computes this value once on the main goroutine before workers fan out and assigns it to each task. analyzeColumnsPushDown uses the pre-set value directly instead of computing concurrency on workers, removing the error path from concurrent computation.
Test instrumentation and verification pkg/executor/analyze_utils.go, pkg/executor/analyze_test.go	Failpoint import and hook added to getBuildSamplingStatsConcurrency for test interception. New test TestAnalyzeSamplingConcurrencyResolvedOffWorker installs a failpoint callback to capture stack traces and verify the function is called exactly once from the main goroutine, not from worker threads.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested labels

sig/planner, component/statistics, size/M

Suggested reviewers

• henrybw
• terry1purcell
• guo-shaoge

Poem

🐰 A race upon the goroutine,
Where systems maps clashed unseen,
Now resolved before fans spread,
One computation, safely thread-led! 🌟

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: fixing concurrent access to SessionVars.systems map during ANALYZE operations by resolving sampling stats concurrency on the main goroutine.
Linked Issues check	✅ Passed	The PR fully addresses issue #68457 by resolving tidb_build_sampling_stats_concurrency once on the main goroutine before workers spawn, eliminating the concurrent SessionVars.systems map access that caused the panic.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the concurrent map access issue: adding samplingStatsConcurrency field to AnalyzeColumnsExec, resolving it early in AnalyzeExec.Next, and using pre-resolved values in workers. Changes are focused and within scope.
Description check	✅ Passed	The PR description follows the required template structure with all essential sections completed: Issue Number, Problem Summary, What Changed and How It Works, Check List (with Unit test marked), Side effects assessment, and Release note.

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[tiprow]

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[Copilot]

Pull request overview

Fixes a TiDB server crash ("fatal error: concurrent map read and map write") that occurred during ANALYZE on partitioned tables. The root cause was that multiple partition-analyze worker goroutines shared one SessionVars and each independently invoked GetSessionOrGlobalSystemVar(tidb_build_sampling_stats_concurrency) from analyzeColumnsPushDown, racing on the lazily-populated, unsynchronised SessionVars.systems map. The fix resolves the sysvar once on the main goroutine in AnalyzeExec.Next before workers fan out, and threads the value to workers via a new AnalyzeColumnsExec.samplingStatsConcurrency field.

Changes:

• Resolve tidb_build_sampling_stats_concurrency once in AnalyzeExec.Next and assign it to each task's colExec before starting workers.
• Add samplingStatsConcurrency field to AnalyzeColumnsExec and consume it in analyzeColumnsPushDown instead of calling getBuildSamplingStatsConcurrency per worker.
• Add failpoint getBuildSamplingStatsConcurrencyCalled and a regression test asserting the sysvar is resolved exactly once and never from an analyzeWorker goroutine.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/executor/analyze.go	Resolves sampling concurrency once on the main goroutine and propagates it to each colExec task before worker fan-out.
pkg/executor/analyze_col.go	Adds samplingStatsConcurrency field on AnalyzeColumnsExec to carry the pre-resolved value to workers.
pkg/executor/analyze_col_sampling.go	Replaces in-worker sysvar lookup with the pre-resolved e.samplingStatsConcurrency field.
pkg/executor/analyze_utils.go	Adds a failpoint.InjectCall in getBuildSamplingStatsConcurrency to enable the regression test.
pkg/executor/analyze_test.go	Adds TestAnalyzeSamplingConcurrencyResolvedOffWorker verifying the sysvar is resolved exactly once and not from an analyzeWorker stack.

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[codecov]

Codecov Report

❌ Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.7178%. Comparing base (8c17ce1) to head (7272ae0).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #68465        +/-   ##
================================================
- Coverage   77.2761%   75.7178%   -1.5584%     
================================================
  Files          2010       2008         -2     
  Lines        555473     563343      +7870     
================================================
- Hits         429248     426551      -2697     
- Misses       125305     136741     +11436     
+ Partials        920         51       -869     

Flag	Coverage Δ
integration	41.5522% <76.9230%> (+1.7581%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	60.4679% <ø> (ø)
parser	∅ <ø> (∅)
br	49.9725% <ø> (-13.0354%)	⬇️

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[mjonss]

/retest

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[0xPoe]

Thanks!

rest LGTM

[0xPoe]

The change is pretty straightforward. I think this test is overkill. I guess the only thing we need to do is add an intest.Assert() in analyzeColumnsPushDown to make sure samplingStatsConcurrency is initialized.