[Apple] Lock over the same object in SafeDeleteSslContext to serialize access to the buffers

[kotlarmilos]

Description

OSX SafeDeleteSslContext takes lock on different object in managed callbacks and other methods. In the callbacks, the lock is SafeDeleteSslContext, while in Dispose and other methods the lock is SafeSslHandle.

This PR adds a _lock to ensure synchronization across all methods that access shared buffers. The issue was identified while working on #117950.

[Copilot]

Pull Request Overview

This PR addresses a synchronization issue in the OSX SafeDeleteSslContext class where different methods were using different lock objects, potentially causing race conditions when accessing shared buffer resources. The change introduces a dedicated _lock object to ensure consistent synchronization across all methods that access the _inputBuffer and _outputBuffer fields.

Key changes:

• Introduces a dedicated _lock object for thread synchronization
• Replaces inconsistent lock targets (_sslContext vs SafeDeleteSslContext instance) with the new _lock object
• Ensures all buffer access operations are properly synchronized

[dotnet-policy-service]

Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @ivanpovazan, @steveisok, @akoeplinger
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[ManickaP]

Thank you!

[kotlarmilos]

/backport to release/9.0

[github-actions]

Started backporting to release/9.0 (link to workflow run)

[kotlarmilos]

/backport to release/9.0-staging

[github-actions]

Started backporting to release/9.0-staging (link to workflow run)