ProgrammableUpcallHandler::on_entry modifies:

context->new_handles = JNIHandleBlock::allocate_block(thread);
thread->clear_pending_exception();

Before transitioning to Java. Which means there can be a concurrent safepoint/handshake.
 
ProgrammableUpcallHandler::on_exit modify and access same fields here:

JNIHandleBlock::release_block(context->new_handles, thread);
assert(!thread->has_pending_exception(), "Upcall can not throw an exception");

After transitioning back to native.

E.g. _free_handle_block, _pending_exception.

This have two issues:
- Is it safe today? I traced some code and FAICT it is.
- These fields are now only allowed to be modified in a safepoint/handshake if target thread is waiting for the safepoint/handshake or by the thread it self. (I.e. it is not enough that thread is safepoint safe, it must actively be waiting for the safepoint/handshake)

Is this a restriction that we want, and am I correct in saying that is safe?