RFR (XXXS) 8173693 disable post_class_unload() for non JavaThread initiators

[Daniel D. Daugherty]

Greetings,

I have a very small work around for a JVM/TI event posting bug:

     JDK-8173693 disable post_class_unload() for non JavaThread initiators
     https://bugs.openjdk.java.net/browse/JDK-8173693

This work around disables the JVM/TI post_class_unload() function
when the thread requesting the event is not a JavaThread. See:

     JDK-8173658 JvmtiExport::post_class_unload() is broken for 
non-JavaThread initiators
     https://bugs.openjdk.java.net/browse/JDK-8173658

for the gory details of what can (and has) gone wrong.

Fixing JDK-8173658 is non-trivial so this work around is proposed
to avoid allowing a Class Unload event to crash the VM or silently
corrupt the non JavaThread that tried to post the event.

Webrev URL: http://cr.openjdk.java.net/~dcubed/8173693-webrev/0-jdk9-hs/

Testing: Oracle internal VM/NSK JVM/TI test suite
          JPRT testing (in progress)


Thanks, in advance, for any comments, questions or suggestions.

Dan