Fix race condition during initial admin account creation

[Pnkcaht]

Fixes #26027

Testing done

This change was tested by reproducing the reported race condition with
multiple concurrent POST requests to /securityRealm/createFirstAccount
during initial setup.
Before the fix, concurrent requests could create multiple users with
administrative privileges. After synchronizing the account creation
logic, only the first request succeeds and all subsequent concurrent
requests fail with an unauthorized error.
Due to the nature of the issue (concurrent HTTP requests during the
initial setup phase), automated test coverage is non-trivial and was
not added as part of this change.

Screenshots (UI changes only)

Before

After

Proposed changelog entries

• Prevent race condition during initial administrator account creation.

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

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[Copilot]

Pull request overview

This PR addresses a critical race condition that could allow multiple concurrent POST requests to /securityRealm/createFirstAccount during initial setup to create multiple users with administrative privileges. The fix introduces a synchronized block using a static lock object to ensure that the check for existing users and the subsequent account creation are performed atomically.

Key Changes

• Added a static final lock object (CREATE_FIRST_ACCOUNT_LOCK) to synchronize access to the first account creation logic
• Wrapped the entire check-then-act sequence in doCreateFirstAccount within a synchronized block to prevent concurrent executions

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[MarkEWaite]

Thanks! The change looks good to me.

I performed a few simple interactive tests of the setup wizard with this pull request and found no issues. I tested creating a new user in the setup wizard and skipping the user creation in the setup wizard.

Thanks very much for the excellent description in the pull request and for your testing to confirm it is well behaved after the change.

GitHub Copilot review noted that a similar race condition exists in the _doCreateAccount method, but that method only attempts to create a new user if signup is allowed and will only grant admin to the created user if it is the first user created. The initial configuration does not allow signup, so there is no chance for the race condition in _doCreateAccount to be exercised.

I was impressed that GitHub Copilot looked outside the diff and even more pleased that it decided to not make any visible comment outside the diff.

[MarkEWaite]

As a minor comment for consideration in future pull requests, please use a branch name other than master when preparing your pull request. When submitters use the branch named master, it complicates the review process a little and increases the chances that you'll make a mistake on your local copy of the repository.

[Pnkcaht]

Thanks everyone for the review and feedback.
I’ve addressed the minor formatting suggestion and updated the pull request accordingly.

I really appreciate the time spent reviewing and testing this change, as well as the additional context shared during the discussion. I’m happy to continue contributing and appreciate the work you all do maintaining Jenkins.

[MarkEWaite]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback.

/label ready-for-merge

[MarkEWaite]

Fixed in https://www.jenkins.io/changelog/2.545/