Fix thread safety issues in producer-consumer implementation

[bkk07]

This PR fixes thread-safety issues in the producer-consumer implementation.

The previous implementation accessed a shared LinkedList concurrently without proper synchronization. Queue condition checks and wait/notify coordination were performed outside a shared lock, which could lead to race conditions, missed notifications, visibility issues, and potential queue corruption when multiple producers and consumers executed concurrently.

Changes made:

• Moved synchronization responsibility into DataQueue
• Guarded queue operations using synchronized methods
• Replaced unsafe condition checks with guarded wait loops
• Removed separate wait/notify coordination from Producer and Consumer
• Added proper handling for spurious wakeups using while loops
• Added volatile visibility guarantees for running flags

This preserves the original educational intent of demonstrating wait/notify based producer-consumer coordination while ensuring thread-safe behavior.

[lor6]

Thanks @bkk07 The PR looks good, but the fork is outdated so I'm not able to merge it. Could you update your fork with the latest changes from origin?

[bkk07]

Hi @lor6,
I've updated my branch with the latest changes from upstream/master as requested. All checks are now passing. Could you please take another look when you have a chance?
Thank you!

[lor6]

Thanks @bkk07 I've now merged the PR here. I will check what changes are needed in the related article.