reuse old index file format for empty dmfile and just ignore it's size in statistical data

[lidezhu]

What problem does this PR solve?

Issue Number: close #4708

Problem Summary:
In this pr #4500, we avoid write data into minmax index file for emtpy dmfile. But this kind of dmfile cannot be recogonized by older version. So after degrading, tiflash fails.

What is changed and how it works?

Reuse old index file format and just ignore the size written into minmax index file in this case.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

1. Test that stable_siz_on_disk is 0 for empty segments;
2. Test that after this fix the master branch can be degraded to ver6.0.0;
   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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JinheLin
• Lloyd-Pottiger

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[lidezhu]

/run-all-tests

[sre-bot]

Coverage for changed files

Filename                      Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
DMFileWriter.cpp                  103                 8    92.23%          15                 0   100.00%         438                18    95.89%          70                 9    87.14%
DMFileWriter.h                     24                 1    95.83%          14                 1    92.86%          37                 3    91.89%           6                 0   100.00%
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                             127                 9    92.91%          29                 1    96.55%         475                21    95.58%          76                 9    88.16%

Coverage summary

Functions  MissedFunctions  Executed  Lines   MissedLines  Cover
17132      9492             44.59%    192843  96244        50.09%

full coverage report (for internal network access only)

[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.

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[ti-chi-bot]

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

Commit hash: dc2e99a

[sre-bot]

Coverage for changed files

Filename                      Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
DMFileWriter.cpp                  103                 8    92.23%          15                 0   100.00%         438                18    95.89%          70                 9    87.14%
DMFileWriter.h                     24                 1    95.83%          14                 1    92.86%          37                 3    91.89%           6                 0   100.00%
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                             127                 9    92.91%          29                 1    96.55%         475                21    95.58%          76                 9    88.16%

Coverage summary

Functions  MissedFunctions  Executed  Lines   MissedLines  Cover
17132      9492             44.59%    192856  96262        50.09%

full coverage report (for internal network access only)