ddl: support session level tidb_ddl_reorg_worker_cnt and batch_size

[tangenta]

What problem does this PR solve?

Issue Number: close #55335

Problem Summary: see #55335

What changed and how does it work?

• Add session scope support for system variables tidb_ddl_reorg_worker_cnt and tidb_ddl_reorg_batch_size.
• For session scope variables, the value cannot be changed during DDL job execution.
• For global scope variables, the they only provide the default values of session variables.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  Execute following SQLs on root user:

  create user 'aa'@'%' identified by '';
  create database aa;
  grant create, alter, insert on aa.* to 'aa'@'%';
  

  Switch to user 'aa'@'%':

  mysql> use aa;
  Database changed
  mysql> show tables;
  Empty set (0.00 sec)
  
  mysql> create table t (a int, b int);
  Query OK, 0 rows affected (0.07 sec)
  
  mysql> insert into t values (1, 1);
  Query OK, 1 row affected (0.03 sec)
  
  mysql> select @@global.tidb_ddl_reorg_worker_cnt;
  +------------------------------------+
  | @@global.tidb_ddl_reorg_worker_cnt |
  +------------------------------------+
  |                                  4 |
  +------------------------------------+
  1 row in set (0.00 sec)
  
  mysql> set  @@global.tidb_ddl_reorg_worker_cnt = 8;
  ERROR 1227 (42000): Access denied; you need (at least one of) the SUPER or SYSTEM_VARIABLES_ADMIN privilege(s) for this operation
  mysql> set @@tidb_ddl_reorg_worker_cnt = 8;
  Query OK, 0 rows affected (0.00 sec)
  
  mysql> alter table t add index idx(a);
  Query OK, 0 rows affected (2.20 sec)
  

  Check TiDB log:

  [INFO] [executor.go:4780] ["initialize reorg meta"] [category=ddl] [jobSchema=aa] [jobTable=t] [jobType="add index"] [enableDistTask=true] [enableFastReorg=true] [targetScope=] [concurrency=8] [batchSize=256]
  

• 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.

None

[tiprow]

Hi @tangenta. Thanks for your PR.

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[codecov]

Codecov Report

Attention: Patch coverage is 95.40230% with 4 lines in your changes missing coverage. Please review.

Project coverage is 75.2401%. Comparing base (cf2c703) to head (d02c6ac).
Report is 35 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #55334        +/-   ##
================================================
+ Coverage   72.9400%   75.2401%   +2.3001%     
================================================
  Files          1570       1580        +10     
  Lines        439546     455968     +16422     
================================================
+ Hits         320605     343071     +22466     
+ Misses        99276      92037      -7239     
- Partials      19665      20860      +1195     

Flag	Coverage Δ
integration	50.9464% <94.1176%> (?)
unit	71.8481% <91.8604%> (-0.1319%)	⬇️

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

Components	Coverage Δ
dumpling	52.9567% <ø> (ø)
parser	∅ <ø> (∅)
br	63.1731% <ø> (+17.2976%)	⬆️

[D3Hunter]

rest lgtm

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-12 02:25:10.73600946 +0000 UTC m=+148395.439479104: ☑️ agreed by lance6716.
• 2024-08-12 04:27:12.06393595 +0000 UTC m=+155716.767405608: ☑️ agreed by D3Hunter.

[tangenta]

/retest

[tiprow]

@tangenta: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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[tangenta]

/ok-to-test

[easonn7]

/approve

[tangenta]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: D3Hunter, easonn7, lance6716, tangenta

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Needs approval from an approver in each of these files:

• OWNERS [D3Hunter,lance6716,tangenta]
• pkg/ddl/OWNERS [tangenta]
• pkg/parser/OWNERS [D3Hunter,tangenta]
• pkg/sessionctx/variable/OWNERS [easonn7]

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[tangenta]

/retest