Sharing a connection between threads is not thread safe on JRuby

[JonathanPierce]

If we look at connection.rb and how it handles the sockets, socket, and reset methods, there is an issue that only appears is true concurrency environments like JRuby: multiple threads can try to mutate the cache hashes at once.

Since code calls the socket method multiple times, it means that two calls can return two different sockets if threads clobbered the hash creation/access/etc...

To fix this, a Mutex needs to be defined per-connection and these three methods need to be wrapped in mutex.synchornize blocks.

This is an extreme edge case that requires:

• A shared connection object between threads
• Extreme lack of luck with timing
• A ruby interpreter with actual parallelism (this will never happen on CRuby)

Also, you might want to use ObjectSpace::WeakMap for the per-thread cache to avoid memory leaks.

[geemus]

Thanks for the detailed report. It's too bad we have this issue, but at least it doesn't seem likely to come up all the time.

@JonathanPierce would you have capacity and interest to work on the fixes?

I suspect I could eventually look into it, but don't think I'll have much capacity/bandwidth for the foreseeable future.

[github-actions]

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