[JENKINS-69543] Fix thread safety in websockets handling.

[Vlatombe]

Follow-up of #6780, #6801

Session-holding maps are accessed by multiple threads.

Even though some concurrent access version of this would perform better it should be safe to access sessions in a synchronized way.

org.springframework.util.ConcurrentReferenceHashMap could be an alternative if synchronized access is too slow.

See JENKINS-69543.

Proposed changelog entries

• Fix thread safety in websockets handling.

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change) and are in the imperative mood. Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade
• Appropriate autotests or explanation to why this change has no tests
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadoc, as appropriate.
• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO") if applicable.
• New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content-Security-Policy directives (see documentation on jenkins.io).
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@mention

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
• Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[krisstern]

Should I backport this ASAP for the LTS 2.361.1 release?

[timja]

Should I backport this ASAP for the LTS 2.361.1 release?

Generally no it needs real world testing via the weekly line.

Unless others have evidence that this is a critical issue for the release and needs to go in ASAP

[NotMyFault]

Is this intended to be a TODO for today's LTS release @Vlatombe? The release took already place, and backporting outside from weekly's aren't eligible.

[jglick]

Good catch!

[basil]

Very nice! Too bad this did not make it into 2.361.1 LTS, as there is no workaround on the client side. I suppose the best we can do is get this into 2.361.2 LTS. Thanks for the PR!

[basil]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback. Please see the merge process documentation for more information about the merge process. Thanks!