Fix potential data inconsistency under heavy ddl operation (#5044)

[ti-chi-bot]

This is an automated cherry-pick of #5044

What problem does this PR solve?

Issue Number: close #5032

Problem Summary: Currently we cache decoding schema for decoding raft data if a table schema doesn't change. And we judge whether a table schema has changed based on the table schema version.

But the schema version is not strictly consistent with the actual table schema which can be seen at https://github.com/pingcap/tiflash/blob/master/dbms/src/TiDB/Schema/SchemaBuilder.cpp#L362. That is when applying different schema changes in a diff, the table schema version will be set to the latest schema version after the first schema change is applied.

More concretely, when a lossy ddl change occurs, it will trigger drop column and add column schema changes and also rewrite some data at the same time. 
The schema changes will be applied one by one, and because tiflash updates the table schema version ahead of time when applying schema diff, after applying the drop column schema change, it will update the schema version to the latest schema version.

And if the decode thread tries to obtain the current schema for decoding data before the add column is applied, the current schema and the latest schema version will be cached. Then after the subsequent add column operation is applied, the table schema version will not be updated, so the cache of the decode thread will not be invalidated.

Therefore, the decode thread will decode the new data with an older schema, considering that the new added column is a dropped column and discarding it.

In addition, there is a lower chance of triggering this problem in the case of frequent add column and insert data.

What is changed and how it works?

Add an internal schema version for DecodingStorageSchemaSnapshot.

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 data inconsistency under heavy ddl operation

[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

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: da5731b

[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: d690f05

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