flush cache before segment merge (#4955)

[ti-chi-bot]

This is an automated cherry-pick of #4955

What problem does this PR solve?

Issue Number: close #4956

Problem Summary:
When do segment split, we try to copy the tail column files in the delta layer of the original segment to the new result
segments. So the new segments may contain data that doesn't belong to its segment range.

And this is ok for most cases, because the redundant data will be filtered out by the segment range when serve the read requests to the segment. So the redundant is invisible in almost all cases.

But when do segment merge later, if the previous redundant data is still not flushed to disk, it will be directly copied to the new merged segment again.

So the redundant data in each segment become visible again after segment merge which may cause potential data incorrectness.

What is changed and how it works?

Flush cache before every merge operation. So the potential unsaved data will be filtered out by the segment range when do merge.

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 potential wrong result after a lot of insert and delete operations

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JaySon-Huang
• lidezhu

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[JaySon-Huang]

LGTM

[lidezhu]

/merge

[ti-chi-bot]

@lidezhu: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

@lidezhu: /merge in this pull request requires 2 approval(s).

In response to this:

/merge

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[lidezhu]

/merge

[ti-chi-bot]

@lidezhu: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

If you have any questions about the PR merge process, please refer to pr process.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[ti-chi-bot]

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

Commit hash: e6341b6

[sre-bot]

Coverage for changed files

Filename                       Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
DeltaMergeStore.cpp               1450               499    65.59%          67                 5    92.54%        2034               455    77.63%         838               385    54.06%
Segment.cpp                        683               212    68.96%          47                 3    93.62%        1162               266    77.11%         362               195    46.13%
Segment.h                           34                 5    85.29%          22                 3    86.36%          31                 4    87.10%           6                 3    50.00%
tests/gtest_dm_segment.cpp        3404               814    76.09%          38                 1    97.37%        1505                21    98.60%        1130               471    58.32%
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                             5571              1530    72.54%         174                12    93.10%        4732               746    84.23%        2336              1054    54.88%

Coverage summary

Functions  MissedFunctions  Executed  Lines   MissedLines  Cover
18224      9785             46.31%    204169  97910        52.04%

full coverage report (for internal network access only)