RFR (XXS): 8172969: JVMTI spec: GetCurrentThread may return NULL in the early start phase

[serguei.spitsyn at oracle.com]

Please, review a JDK 9 fix for the bug:
   https://bugs.openjdk.java.net/browse/JDK-8172969

Webrev:
http://cr.openjdk.java.net/~sspitsyn/webrevs/2017/hotspot/8172969-jvmti-spec.hs1/


Summary:

   It was identified during the review of 8172261 that the GetCurrentThread
   may return NULL during the early start phase. This issue existed with
   the normal start phase in the initial JVM TI implementation (JDK 1.5).
   Now the GetCurrentThread is Ok to call during the normal start phase 
as the
   VMStart event is shifted to the point where the java.base is fully 
initialized.
   But now the early VMStart event is at the same point where the VMStart
   was in all previous releases.

   The fix is a spec update saying that the GetCurrentThread may return 
NULL in the
   early start phase when the java.lang.Thread class has not been 
initialized yet.

Thanks,
Serguei