active-active: add failpoint to construct data loss, data redundant and lww violation scenarios

[Leavrth]

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[gemini-code-assist]

Summary of Changes

Hello @Leavrth, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the testing capabilities for active-active replication scenarios by introducing several failpoints. These failpoints allow developers to deliberately inject data loss, data redundancy/mutation, and Last-Write-Wins (LWW) violations into the replication process. This is crucial for validating the robustness and consistency checking mechanisms of the system under various failure conditions, ensuring that the active-active setup behaves as expected when inconsistencies arise.

Highlights

• Failpoint for Data Loss Simulation: Introduced a failpoint (cloudStorageSinkDropMessage) in the cloud storage sink writer to simulate data loss by conditionally dropping messages before they are written to data files. This allows testing scenarios where messages might be lost during replication.
• Failpoint for Data Redundancy/Mutation Simulation: Added a failpoint (cloudStorageSinkMutateValue) and a helper function (mutateMessageValueForFailpoint) to the cloud storage sink writer. This failpoint mutates the last CSV field of a message's value, allowing the multi-cluster-consistency-checker to identify the original row as 'lost' and the mutated row as 'redundant', simulating data inconsistency.
• Failpoint for LWW Violation Simulation: Implemented a failpoint (mysqlSinkBypassLWW) in the MySQL DML writer. This failpoint bypasses the Last-Write-Wins (LWW) UPSERT logic, forcing the system to use normal SQL (REPLACE INTO or plain INSERT). This creates a genuine LWW violation that can be observed flowing through the replication pipeline.

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Changelog

• downstreamadapter/sink/cloudstorage/writer.go
  • Added mutateMessageValueForFailpoint function to corrupt message values for testing.
  • Introduced cloudStorageSinkDropMessage failpoint to simulate data loss by dropping messages.
  • Added cloudStorageSinkMutateValue failpoint to simulate data inconsistency by mutating message values.
• pkg/sink/mysql/mysql_writer_dml.go
  • Imported failpoint, log, and zap packages.
  • Added mysqlSinkBypassLWW failpoint to bypass LWW UPSERT logic and simulate LWW violations.

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[gemini-code-assist]

Code Review

This pull request introduces several failpoints to simulate different failure scenarios in active-active replication, such as data loss, data redundancy, and LWW violations. The changes in pkg/sink/mysql/mysql_writer_dml.go correctly add a failpoint to bypass LWW logic. However, there's an issue in downstreamadapter/sink/cloudstorage/writer.go where the failpoint for dropping messages doesn't work as intended due to incorrect usage of failpoint.Continue(). I've left a specific comment with a suggestion for a fix.

[gemini-code-assist]

failpoint.Continue() does not skip the current loop iteration as a continue statement would. Instead, it jumps to the statement immediately following the failpoint.Inject block. This means the message processing logic below will still execute, causing the message not to be dropped and its callback to be appended twice.

To correctly skip processing the current message, you should use failpoint.Goto("continueLoop") and add a continueLoop: label at the end of the for loop body.