syncer: optimize safemode mechanism to improve performance (#1906)

[ti-chi-bot]

This is an automated cherry-pick of #1906

What problem does this PR solve?

Implement #1757 method a.

What is changed and how it works?

Reuse exit_safe_binlog_name,exit_safe_binlog_pos,exit_safe_binlog_gtid in checkpoint.

1. When parse events whose location is newer than exit_safe_binlog, flush exit_safe_binlog asap to avoid duplicate in the future.
2. Only when syncer fails due to errors except execution errors or meet some errors when executing, don't update global checkpoint but update safe point to downstream. In other situations, update both global checkpoint and safe point.
3. Set syncDML and syncDDL's context to context.Background() to make sure ddl/dmls can be replicated to downstream as much as possible event when context is canceled.

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change
• Has interface methods change
• Has persistent data change

Side effects

• Increased code complexity

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

[glorv]

/run-all-tests

[glorv]

/merge

[ti-chi-bot]

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

In response to this:

/merge

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.

[lichunzhu]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• lichunzhu

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[lichunzhu]

/merge

[ti-chi-bot]

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

Commit hash: 688b0e6