Is it safe to have multiple versions of the jni crate in use within a single application?

[rib]

This issue is really just a reminder to take some care to consider how safe it is to have multiple versions of the jni crate linked in a single application.

I don't currently know of any specific problem but it's something that's crossed my mind a number of times recently and I expect that it might not have been considered carefully before, so I don't feel confident that I understand the safety concerns (if any).

The use of TLS for attaching threads is probably the main thing that I wonder about.

Multiple versions of the jni crate will effectively have their own TLS state and maybe they could end up with a muddled understanding of which threads are attached to the JVM? - is that a safety concern?

Is there any static state, or other TLS usage to consider?

[rib]

Track this issue/discussion around thread attachment, where it's easy to see how the use of thread-local storage across different versions of the jni crate could be hazardous: #441