log backup: use global checkpoint ts as source of truth

[3pointer]

What problem does this PR solve?

Issue Number: close #58031

Problem Summary:

The previous lag calculation relied on c.lastCheckpoint.TS to compute the lag. However, this approach is unreliable, especially when ownership changes, as c.lastCheckpoint.TS is not guaranteed to increase steadily. This PR addresses the issue by introducing a global checkpoint timestamp that maintains a strictly non-decreasing state.

What changed and how does it work?

The lag calculation now utilizes a global checkpoint timestamp instead of c.lastCheckpoint.TS. This global timestamp ensures consistency and stability, as it always increases or stays the same, even during ownership transitions. This change guarantees a more robust and accurate lag measurement.

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.

Fix this issue that when new advancer owner starts the task unexpected paused due to last checkpoint ts equal to start ts

[tiprow]

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[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Project coverage is 74.9132%. Comparing base (68ac9ec) to head (99c99bb).
Report is 71 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #58135        +/-   ##
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+ Coverage   73.1841%   74.9132%   +1.7291%     
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  Files          1675       1693        +18     
  Lines        461917     466183      +4266     
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+ Hits         338050     349233     +11183     
+ Misses       103127      95400      -7727     
- Partials      20740      21550       +810     

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unit	72.5386% <71.4285%> (+0.2238%)	⬆️

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Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	61.7720% <71.4285%> (+15.8112%)	⬆️

[RidRisR]

Why? If there is a new task and the global checkpoint is never advanced, the task will never be paused even exceed the limit.

This implies that we should never pause a task that never advanced.

[3pointer]

actually it should be if global ts is less than task.start-ts which implies that could have some corner cases when start a new task.

[RidRisR]

	if globalTs <= c.task.StartTs {
		// task is not started yet
		return false, nil
	}

Then maybe here should be < instead of <= ?

[3pointer]

if globalTs == c.task.StartTs this will only happen when new task created.

Since it's not the common case after task running for some time. I think it's better to make this not pause by default

[YuJuncen]

Sometimes the task may be stuck from creating, say, the advancer doesn't work or one of TiKV didn't notice the task.

[3pointer]

After talk with @YuJuncen , we had a agreement to make the check include when globalTs == task.StartTs. I changed it.

Additionally I found the unproper error return logic when add task. I also fixed it this PR, and fix the related test cases.

[3pointer]

/retest

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BornChanger, YuJuncen

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-12-12 05:24:24.307792113 +0000 UTC m=+502454.396594649: ☑️ agreed by YuJuncen.
• 2024-12-13 12:29:15.088349356 +0000 UTC m=+614345.177151898: ☑️ agreed by BornChanger.

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@3pointer: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #58259.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #58265.