RFR: 8280422: thread_from_jni_environment can never return NULL [v2]

[Johannes Bechberger]

On Mon, 24 Jan 2022 02:21:34 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Analysing the code and its use (see JBS issue for gory details) we can see that `thread_from_jni_environment` can never actually return NULL, so we change it to not appear to do so and thus keep static analysis tools happy.  We also always validate the incoming JNIEnv for debug builds and scrap the unused `VerifyJNIEnvThread` flag.
>> 
>> Testing: tiers 1-5
>> 
>> Thanks,
>> David
>
> David Holmes has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update copyright

I would suggest to add a method for checking the termination status in cases where it might happen:

```c++
  // Returns whether current thread as indicated by the given JNIEnv
  // is terminated.
  // We don't assert it is Thread::current here as that is done at the
  // external JNI entry points where the JNIEnv is passed into the VM.
  static bool is_thread_from_jni_environment_terminated(JNIEnv* env) {
    JavaThread* current = (JavaThread*)((intptr_t)env - in_bytes(jni_environment_offset()));
    // We can't get here in a thread that has completed its execution and so
    // "is_terminated", but a thread is also considered terminated if the VM
    // has exited, so we have to check this and block in case this is a daemon
    // thread returning to the VM (the JNI DirectBuffer entry points rely on
    // this).
    return current->is_terminated();
  }

-------------

PR: https://git.openjdk.java.net/jdk/pull/7193