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

[Y. Srinivas Ramakrishna]

On Wed, 20 Mar 2024 10:16:38 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> A case of flickering of active_generation() was identified, and found to be benign. An assert now checks for this situation. The code has been made robust wrt the flickering.

Testing revealed an issue with this PR, in particular with the clearing of active_generation() via Scopes -- this needs closer examination, so I'll once again take this back to draft. Sorry for the, errr, flickering in this PR :-)

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PR Comment: https://git.openjdk.org/shenandoah/pull/407#issuecomment-2010502031