Thread safety considerations in C code

[mahesh-hegde]

Non-exhaustive list of places where race conditions may occur

• Class and method references are global variables in generated C bindings. These variables are checked during a call and looked up in JNI if they are null. Method/Field IDs do not seem to be problematic since they are not themselves reclaimable, and merely associated with the class reference. The class reference however may leak if 2 threads try to use an un-initialized class concurrently.

Is this a concern and does this operation (check-and-load-if-null) have to be wrapped in a mutex? I think something like double-checked locking can be used to retain a faster path in loaded case.

• Spawn checks if the global JniContext is populated with non-null JVM reference. There's a chance if 2 spawns are called simultaneously. But spawn is supposed to be called at the beginning of the program. I don't think this is a significant concern.

[mahesh-hegde]

This should be a non-issue if #772 is implemented.