RFR: 8328235: GenShen: Robustify ShenandoahGCSession and fix missing use

[Y. Srinivas Ramakrishna]

On Fri, 15 Mar 2024 01:45:51 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> ShenandoahGCSession is intended to create a scope where the ShenandoahHeap's _gc_cause and _gc_generation field reflect the current gc cycle. We now check that we do not overwrite existing non-default settings (respectively _no_gc and nullptr). The destructor of the scope/stack object also resets these fields to their default settings, ensuring intended uses. This uncovered a situation where the scope was not entered when it should have been, which we have now fixed.
> 
> *Testing*:
> - [ ] code pipeline : in progress
> - [x] specjbb
> - [x] jtreg:hotspot_gc w/fastdebug
> - [x] GHA: existing failures unrelated to this change

Code pipeline testing revealed a further scenario when `active_generation()` was used outside of what should have been a gc session (temporal) scope. I'll revert this to a draft until I fix that issue as well.

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PR Comment: https://git.openjdk.org/shenandoah/pull/407#issuecomment-2000082870