privilege: handle username with "@" correctly for role(RBAC) related code

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #59552

Problem Summary:

What changed and how does it work?

The old code use key = username + "@" + host, and also split by "@" to get back the username.
This is not stable enough when the username itself contains "@"

Change key from username + "@" + host to {username, host} struct makes the code more robust.

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.

None

[tiprow]

Hi @tiancaiamao. Thanks for your PR.

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[codecov]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.4831%. Comparing base (4cf94a7) to head (7e88b0c).
Report is 4 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #59739        +/-   ##
================================================
+ Coverage   72.9611%   73.4831%   +0.5219%     
================================================
  Files          1697       1699         +2     
  Lines        468962     469633       +671     
================================================
+ Hits         342160     345101      +2941     
+ Misses       105734     103513      -2221     
+ Partials      21068      21019        -49     

Flag	Coverage Δ
integration	43.0231% <96.7741%> (?)
unit	72.2930% <96.7741%> (+0.1411%)	⬆️

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

Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	45.0701% <ø> (-0.0462%)	⬇️

[tiancaiamao]

/test check-dev2

[tiprow]

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

In response to this:

/test check-dev2

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[zimulala]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lance6716, zimulala

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-25 12:57:41.356126912 +0000 UTC m=+360609.309285178: ☑️ agreed by lance6716.
• 2025-02-26 02:36:21.530089199 +0000 UTC m=+409729.483247464: ☑️ agreed by zimulala.