data race at TestAddGlobalIndexInIngest

[D3Hunter]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

the backend should only be unregistered once.

WARNING: DATA RACE
Read at 0x00c00bd59e30 by goroutine 24412:
  runtime.mapaccess2_fast64()
      GOROOT/src/runtime/map_fast64.go:62 +0x0
  github.com/pingcap/tidb/pkg/ddl/ingest.(*MockBackendCtxMgr).Unregister()
      pkg/ddl/ingest/mock.go:77 +0x88
  github.com/pingcap/tidb/pkg/ddl.convertAddIdxJob2RollbackJob()
      pkg/ddl/rollingback.go:98 +0x90c
  github.com/pingcap/tidb/pkg/ddl.runReorgJobAndHandleErr()
      pkg/ddl/index.go:1542 +0x1188
  github.com/pingcap/tidb/pkg/ddl.runIngestReorgJobDist()
      pkg/ddl/index.go:1425 +0xb7
  github.com/pingcap/tidb/pkg/ddl.doReorgWorkForCreateIndex()
      pkg/ddl/index.go:1360 +0x1557
  github.com/pingcap/tidb/pkg/ddl.doReorgWorkForCreateIndexMultiSchema()
      pkg/ddl/index.go:1314 +0x105
  github.com/pingcap/tidb/pkg/ddl.(*worker).onCreateIndex()
      pkg/ddl/index.go:1053 +0xb2d
  github.com/pingcap/tidb/pkg/ddl.(*worker).runOneJobStep()
      pkg/ddl/job_worker.go:931 +0xed6
  github.com/pingcap/tidb/pkg/ddl.onMultiSchemaChange()
      pkg/ddl/multi_schema_change.go:64 +0x713
  github.com/pingcap/tidb/pkg/ddl.(*worker).runOneJobStep()
      pkg/ddl/job_worker.go:1007 +0x1b44
  github.com/pingcap/tidb/pkg/ddl.(*worker).transitOneJobStep()
      pkg/ddl/job_worker.go:574 +0x585
  github.com/pingcap/tidb/pkg/ddl.(*jobScheduler).transitOneJobStepAndWaitSync()
      pkg/ddl/job_scheduler.go:588 +0x6c6
  github.com/pingcap/tidb/pkg/ddl.(*jobScheduler).deliveryJob.func1()
      pkg/ddl/job_scheduler.go:498 +0x364
  github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).Run.func1()
      pkg/util/wait_group_wrapper.go:157 +0x97

Previous write at 0x00c00bd59e30 by goroutine 25640:
  runtime.mapdelete_fast64()
      GOROOT/src/runtime/map_fast64.go:304 +0x0
  github.com/pingcap/tidb/pkg/ddl/ingest.(*MockBackendCtxMgr).Unregister()
      pkg/ddl/ingest/mock.go:81 +0x3b1
  github.com/pingcap/tidb/pkg/ddl.(*readIndexExecutor).Cleanup()
      pkg/ddl/backfilling_read_index.go:155 +0xbe
  github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*BaseTaskExecutor).cleanStepExecutor()
      pkg/disttask/framework/taskexecutor/task_executor.go:376 +0xbc
  github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*BaseTaskExecutor).Run.func1()
      pkg/disttask/framework/taskexecutor/task_executor.go:223 +0x424
  runtime.deferreturn()
      GOROOT/src/runtime/panic.go:605 +0x5d
  github.com/pingcap/tidb/pkg/ddl.(*backfillDistExecutor).Run()
      <autogenerated>:1 +0x3e
  github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*Manager).startTaskExecutor.func2()
      pkg/disttask/framework/taskexecutor/manager.go:377 +0x109
  github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).RunWithLog.func1()
      pkg/util/wait_group_wrapper.go:171 +0xe3

Goroutine 24412 (running) created at:
  github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).Run()
      pkg/util/wait_group_wrapper.go:155 +0xf0
  github.com/pingcap/tidb/pkg/ddl.(*jobScheduler).deliveryJob()
      pkg/ddl/job_scheduler.go:477 +0x5e4
  github.com/pingcap/tidb/pkg/ddl.(*jobScheduler).loadAndDeliverJobs()
      pkg/ddl/job_scheduler.go:439 +0xaf1
  github.com/pingcap/tidb/pkg/ddl.(*jobScheduler).schedule()
      pkg/ddl/job_scheduler.go:331 +0xbbb
  github.com/pingcap/tidb/pkg/ddl.(*jobScheduler).scheduleLoop()
      pkg/ddl/job_scheduler.go:266 +0x75
  github.com/pingcap/tidb/pkg/ddl.(*jobScheduler).scheduleLoop-fm()
      <autogenerated>:1 +0x33
  github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).RunWithLog.func1()
      pkg/util/wait_group_wrapper.go:171 +0xe3

Goroutine 25640 (finished) created at:
  github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).RunWithLog()
      pkg/util/wait_group_wrapper.go:164 +0xf0
  github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*Manager).startTaskExecutor()
      pkg/disttask/framework/taskexecutor/manager.go:369 +0xc98
  github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*Manager).handleExecutableTasks()
      pkg/disttask/framework/taskexecutor/manager.go:254 +0x256
  github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*Manager).handleTasks()
      pkg/disttask/framework/taskexecutor/manager.go:231 +0x557
  github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*Manager).handleTasksLoop()
      pkg/disttask/framework/taskexecutor/manager.go:185 +0x2c8
  github.com/pingcap/tidb/pkg/disttask/framework/taskexecutor.(*Manager).handleTasksLoop-fm()
      <autogenerated>:1 +0x33
  github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).Run.func1()
      pkg/util/wait_group_wrapper.go:157 +0x97
==================

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master

[D3Hunter]

we will remove this backend mgr in #57770, so seems no need to fix this one