Memory leak in Index for principal null

[ruslanys]

I believe this should be a solution for the reported issue.

The explanation is the following. There are only two places where we add sessionId into an index: first, second.

And the first one is under the validation: if (principal != null).
So, we do not create an index if a principal is null.

When a session changes its ID, we try to change it in the index as well.
But we do not check here that the principal is null. So, we do not check if the index exists.

In the end, instead of replacing sessionId in Index, we create a new one. And this leads to a memory leak.

Close #1987

[ruslanys]

The bug goes from #1941 (eb9f62a)

[quaff]

Verified this PR will fix unexpected key spring:session:index:org.springframework.session.FindByIndexNameSessionRepository.PRINCIPAL_NAME_INDEX_NAME:null creation.

[eleftherias]

Thanks for the PR @ruslanys!
The changes look good to me.
Could you please squash the commits into one, and add a descriptive commit message, for example

Fix memory leak with null principal in Redis

Closes gh-1987

[ruslanys]

@eleftherias Thank you for the feedback! Sure. Will do right now.

[eleftherias]

Thanks @ruslanys!
I have merged this into the main branch and I will also be backporting the fix to the 2.6.x branch, making it available in 2.6.2.

[ruslanys]

Thank you @eleftherias !

[jebeaudet]

@eleftherias Since this regression also reached the 2.5.x branch via #1941, could you backport it there too please? I can open a PR if you prefer.

[eleftherias]

Thanks for pointing that out @jebeaudet. I will backport it to 2.5.x as well.