When our JNI support attaches to the current thread, it should have a balancing DetachCurrentThread

[mkustermann]

To me it seems we are lazily attaching to the current thread and caching the result in a thread_local JNIEnv* but there's no corresponding call to DetachCurrentThread.

See JNI tips:

"""
Threads attached through JNI must call DetachCurrentThread() before they exit. If coding this directly is awkward, in Android 2.0 (Eclair) and higher you can use pthread_key_create() to define a destructor function that will be called before the thread exits, and call DetachCurrentThread() from there. (Use that key with pthread_setspecific() to store the JNIEnv in thread-local-storage; that way it'll be passed into your destructor as the argument.)
"""

So it seems that we

• should first detect whether current thread is attached
• use AttachCurrentThread() which may be a NOP if it's already attached
• if we were not attached before, we should use the pthread key approach mentioned above to install a destructor that calls DetachCurrentThread()