There are some issues talking about idle executors populating Jenkins:
There is already a fix to manually tested that seems to avoid that, at least, the steps to reproduce the problem confirm it works:
In the comments of the PR Jesse Glick expressed some concerns about the fix, maybe it's solving the symptoms but not the root cause.
As the fix is valuable by itself, avoiding instances to show hung executors, we have gone forward with it and we have filed this ticket to analyze whether any other improvement could be done to solve the issue.
to know where he is pointing out to.
The way to work on it, I guess, should be, revert the fix on the [PR|https://github.com/jenkinsci/jenkins/pull/4329] and look where/when the finalize2 method is called and analyze if we are forgetting to call it or any other finalization method in some places.