fix race condition when context is canceled

[methane]

Description

Fix #1559.

This PR should be backported to 1.8.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

Summary by CodeRabbit

• Refactor
  • Improved error handling in database connections by adding a check before setting deadlines on operations.
  • Enhanced readability of the cleanup method in the mysqlConn struct for clarity.
  • Clarified comments on the idempotent nature of cleanup and deferred mc.clearResult() for potential concurrent calls.

[coderabbitai]

Walkthrough

The modification in connection.go introduces a safeguard to prevent potential nil pointer dereferences by checking netConn before attempting to set a deadline. This change aims to enhance error handling during the cancellation process, specifically addressing scenarios where pending read/write operations could be interrupted without proper checks, potentially leading to data races or unexpected behavior in database transactions.

Changes

Files	Change Summary
connection.go	Added check for netConn before setting deadline to wake up pending reads/writes; improved error handling comments.

Assessment against linked issues

Objective	Addressed	Explanation
Data race resolution in mysqlConn during context cancellation (#1559)	✅
Synchronization of database transactions with context cancellation to prevent data races (#1559)	✅
Addressing read/write operation conflicts on mysqlConn during context cancellation (#1559)	✅
Compatibility and stability across configurations (driver, Go versions, server versions, OS) (#1559)	❓	The change addresses potential data races but does not explicitly validate across all configurations.

Related issues

• go-sql-driver/mysql#1528: The changes in this PR do not directly address the objectives related to handling BeginTx with timeout settings or connection settings behaviors.
• go-sql-driver/mysql#1538: Although the PR focuses on error handling and data race prevention, it does not specifically address the panic error in startWatcher or issues related to sync/atomic, suggesting no direct link to resolving this issue.

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[coderabbitai]

Review Status

Actionable comments generated: 1

Configuration used: CodeRabbit UI

Commits

Files that changed from the base of the PR and between 8a327a3 and e9b7c9a.

Files selected for processing (1)

• connection.go (1 hunks)

[coderabbitai]

Review Status

Actionable comments generated: 0

Configuration used: CodeRabbit UI

Commits

Files that changed from the base of the PR and between e9b7c9a and a832658.

Files selected for processing (1)

• connection.go (2 hunks)

Files skipped from review as they are similar to previous changes (1)

• connection.go

[coveralls]

coverage: 82.003%. remained the same
when pulling b568757 on methane:fix-race-on-cancel
into 1a64773 on go-sql-driver:master.

[shogo82148]

LGTM

[coderabbitai]

Review Status

Actionable comments generated: 0

Configuration used: CodeRabbit UI

Commits

Files that changed from the base of the PR and between a832658 and b568757.

Files selected for processing (1)

• connection.go (2 hunks)

Files skipped from review as they are similar to previous changes (1)

• connection.go