*: fix nil pointer panic of some operations when skip-grant-table is true (#29358)

[ti-srebot]

cherry-pick #29358 to release-5.3
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/29397

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tidb.git pr/29397:release-5.3-dc67b57c5e6a

---

What problem does this PR solve?

Issue Number: close #29317

Problem Summary:

When skip-grant-table is configured, some operation panic.

What is changed and how it works?

Check config.GetGlobalConfig().Security.SkipGrantTable in NotifyUpdatePrivilege, if it's set, do nothing to avoid nil pointer panic.

Check List

Tests

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

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

Fix nil pointer panic of some operations when `skip-grant-table` is true

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• crazycs520
• wjhuang2016

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[ti-srebot]

/run-all-tests

[ti-srebot]

@tiancaiamao you're already a collaborator in bot's repo.

[tiancaiamao]

/run-check_dev

[tiancaiamao]

Known unstable test #29283

idb_ghpr_check/go/src/github.com/pingcap/tidb/expression/builtin_json_test.go:290\ntesting.tRunner\n\t/usr/local/go/src/testing/testing.go:1193"]
[2021-11-03T07:31:54.617Z] --- FAIL: TestWeightString (0.00s)
[2021-11-03T07:31:54.617Z]     builtin_string_test.go:2634: 
[2021-11-03T07:31:54.617Z]         	Error Trace:	builtin_string_test.go:2634
[2021-11-03T07:31:54.617Z]         	Error:      	Not equal: 
[2021-11-03T07:31:54.617Z]         	            	expected: "a "
[2021-11-03T07:31:54.617Z]         	            	actual  : "a"
[2021-11-03T07:31:54.617Z]         	            	
[2021-11-03T07:31:54.617Z]         	            	Diff:
[2021-11-03T07:31:54.617Z]         	            	--- Expected
[2021-11-03T07:31:54.617Z]         	            	+++ Actual
[2021-11-03T07:31:54.617Z]         	            	@@ -1 +1 @@
[2021-11-03T07:31:54.617Z]         	            	-a 
[2021-11-03T07:31:54.617Z]         	            	+a
[2021-11-03T07:31:54.617Z]         	Test:       	TestWeightString

[tiancaiamao]

/run-check_dev

[zhouqiang-cl]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 9fd3aab

[ti-chi-bot]

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