DATA RACE in the memoryStoreCore

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

==================
WARNING: DATA RACE
Write at 0x00c0001e7620 by goroutine 7:
  runtime.mapassign_faststr()
      GOROOT/src/runtime/map_faststr.go:203 +0x0
  github.com/pingcap/tidb/timer/api.(*memoryStoreCore).Update()
      timer/api/mem_store.go:136 +0x2f3
  github.com/pingcap/tidb/timer/api.(*defaultTimerClient).CloseTimerEvent()
      timer/api/client.go:189 +0x521
  github.com/pingcap/tidb/timer/runtime.TestTimerFullProcess()
      timer/runtime/runtime_test.go:774 +0x15fd
  testing.tRunner()
      GOROOT/src/testing/testing.go:1576 +0x216
  testing.(*T).Run.func1()
      GOROOT/src/testing/testing.go:1629 +0x47
Previous read at 0x00c0001e7620 by goroutine 8:
  runtime.mapiterinit()
      GOROOT/src/runtime/map.go:815 +0x0
  github.com/pingcap/tidb/timer/api.(*memoryStoreCore).List()
      timer/api/mem_store.go:103 +0x164
  github.com/pingcap/tidb/timer/runtime.(*TimerGroupRuntime).partialRefreshTimers()
      timer/runtime/runtime.go:337 +0x61c
  github.com/pingcap/tidb/timer/runtime.(*TimerGroupRuntime).batchHandleWatchResponses()
      timer/runtime/runtime.go:386 +0x204
  github.com/pingcap/tidb/timer/runtime.(*TimerGroupRuntime).loop()
      timer/runtime/runtime.go:179 +0xcf4
  github.com/pingcap/tidb/timer/runtime.(*TimerGroupRuntime).Start.func2()
      timer/runtime/runtime.go:115 +0x39
Goroutine 7 (running) created at:
  testing.(*T).Run()
      GOROOT/src/testing/testing.go:1629 +0x805
  testing.runTests.func1()
      GOROOT/src/testing/testing.go:2036 +0x8d
  testing.tRunner()
      GOROOT/src/testing/testing.go:1576 +0x216
  testing.runTests()
      GOROOT/src/testing/testing.go:2034 +0x87c
  testing.(*M).Run()
      GOROOT/src/testing/testing.go:1906 +0xb44
  go.uber.org/goleak.VerifyTestMain()
      external/org_uber_go_goleak/testmain.go:53 +0x70
  github.com/pingcap/tidb/timer/runtime.TestMain()
      timer/runtime/main_test.go:32 +0x44
  main.main()
      bazel-out/k8-fastbuild/bin/timer/runtime/runtime_test_/testmain.go:136 +0x5e8
Goroutine 8 (running) created at:
  github.com/pingcap/tidb/timer/runtime.(*TimerGroupRuntime).Start()
      timer/runtime/runtime.go:115 +0x1f6
  github.com/pingcap/tidb/timer/runtime.TestTimerFullProcess()
      timer/runtime/runtime_test.go:733 +0xb86
  testing.tRunner()
      GOROOT/src/testing/testing.go:1576 +0x216
  testing.(*T).Run.func1()
      GOROOT/src/testing/testing.go:1629 +0x47
================== 

4. What is your TiDB version? (Required)