RFR (S) 8212178: Soft reference reclamation race in com.sun.xml.internal.stream.util.ThreadLocalBufferAllocator

[Roman Kennke]

I have verified that the program that failed before doesn't appear to
fail anymore.

The patch looks good to me.

Thanks!
Roman

> Bug:
>   https://bugs.openjdk.java.net/browse/JDK-8212178
> 
> See the details in the bug. We are hitting this race when Shenandoah is running in aggressive mode
> and evacuates lots of objects all the time, processing references very often. This makes weak
> reference reclamation races very visible.
> 
> Fix:
>   http://cr.openjdk.java.net/~shade/8212178/webrev.01/
> 
> I took the liberty of rewriting the entire method to make it more straightforward. New code
> guarantees we never return "null" from this method.
> 
> Testing: failing tests, jdk:tier1, jdk-submit (running)
> 
> Thanks,
> -Aleksey
>