RFR (S) 8231197: Shenandoah: JVMTI heap walking cleanup crashes with NULL forwardee

[Zhengyu Gu]

Looks good.

-Zhengyu

On 9/19/19 10:54 AM, Aleksey Shipilev wrote:
> Bug:
>    https://bugs.openjdk.java.net/browse/JDK-8231197
> 
> Fix:
>    https://cr.openjdk.java.net/~shade/8231197/webrev.02/
> 
> This reliably reproduces with vmTestbase_nsk_jvmti. The reason is described in the comment there:
> weird clash between JVMTI-own walk of heap and Shenandoah. ShenandoahCodeRoots assert also needs
> relaxing, and I relaxed it only for JVMTI heap walk.
> 
> Testing: hotspot_gc_shenandoah, vmTestbase_nsk_jvmti
>