chore: prevent start and stop to run in parallel

[olavloite]

Synchronize the start() and stop() methods of the session maintainer to prevent them from running at the same time. That could cause a data race on the scheduled future that the start() method sets, and the stop() method reads.

[gemini-code-assist]

Summary of Changes

Hello @olavloite, 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 thread-safety of the session management logic by synchronizing the start() and stop() methods of the MultiplexedSessionMaintainer. This crucial change prevents race conditions that could occur when these methods attempt to access or modify the scheduledFuture concurrently, thereby ensuring the stability and reliability of session maintenance operations.

Highlights

• Concurrency Control: The start() and stop() methods within the MultiplexedSessionMaintainer class have been made synchronized to prevent concurrent execution and potential data races on the scheduledFuture.
• Access Modifier Update: The access modifier for both start() and stop() methods has been changed from package-private to private, restricting their visibility and usage to within the MultiplexedSessionMaintainer class itself.

Changelog

• google-cloud-spanner/src/main/java/com/google/cloud/spanner/MultiplexedSessionDatabaseClient.java
  • Modified the start() method to be private synchronized.
  • Modified the stop() method to be private synchronized.

Activity

• No human activity has been recorded on this pull request yet.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a potential data race by synchronizing the start() and stop() methods of the MultiplexedSessionMaintainer. This change ensures thread safety when these methods are called concurrently. I've added a suggestion to further improve the start() method by making it idempotent, which will prevent a potential resource leak where multiple maintenance tasks could be scheduled without canceling the old ones.