[JENKINS-35272] Prevent duplicate invocation of Launcher.afterDisconnect

[RajThak-998]

Fixes #JENKINS-35272

Summary

Ensure that SlaveComputer#afterDisconnect is invoked only once for a single
agent disconnect event.

Problem

When a Jenkins agent disconnects unexpectedly (for example due to network loss
or abrupt process termination), the afterDisconnect callback could be invoked
multiple times for the same disconnect event. This could result in duplicate
cleanup logic being executed and inconsistent agent state handling.

Reproduction steps

1. Start a Jenkins controller.
2. Connect a JNLP agent.
3. Forcefully terminate the agent process (for example with SIGKILL or by
   dropping the network connection).
4. Observe that afterDisconnect may be invoked more than once for the same
   disconnect.

Testing done

Interactive testing

• Started a local Jenkins controller.
• Connected a JNLP agent.
• Abruptly terminated the agent process.
• Verified via logging and debugging that afterDisconnect is now invoked
  only once per disconnect event.

Automated tests

Automated tests are being added to cover repeated disconnect scenarios and
to demonstrate that afterDisconnect is invoked only once. The tests will
fail without this change and pass with it.

Screenshots (UI changes only)

N/A

Proposed changelog entries

• Ensure agent disconnect cleanup is executed only once per disconnect event.

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue is well-described.
• The changelog entry is appropriate and in the imperative mood.
• Automated tests are being added to cover this change.
• No new public APIs are introduced.

[RajThak-998]

/label bug

[MarkEWaite]

Please don't sabotage yourself as a new contributor. Be sure that you:

• Duplicate the bug before trying to fix it
• Read and follow the contributing guide
• Test changes interactively
• Create automated tests that show the problem is fixed
• Test changes in the debugger so that you see them execute
• Run automated tests locally before submitting a pull request
• Use the pull request template and provide all the information it requests
• Respond to review comments with changes to address the comment or a clear, concise explanation why you are not making the change
• Wait at least 3 days before reminding that a review or update is needed

As far as I can tell, you skipped several of those items in your pull request.

[Copilot]

Pull request overview

This PR aims to fix JENKINS-35272 by preventing duplicate invocations of Launcher.afterDisconnect() during agent disconnection. The solution introduces a volatile boolean flag with double-checked locking to ensure the method is called exactly once per connection lifecycle.

Changes:

• Added afterDisconnectCalled flag to track whether afterDisconnect() has been called
• Introduced safeAfterDisconnect() method implementing double-checked locking pattern
• Replaced direct afterDisconnect() calls with safeAfterDisconnect() in disconnect paths
• Added flag reset logic when establishing new connections

Comments suppressed due to low confidence (1)

core/src/main/java/hudson/slaves/SlaveComputer.java:668

• The safeAfterDisconnect() method is called at line 659, but then launcher.afterDisconnect() is still being called directly at line 661. This defeats the purpose of the fix and will still result in duplicate calls to afterDisconnect(). The direct call at lines 660-668 should be removed since safeAfterDisconnect() already handles calling afterDisconnect() with proper error handling.

                safeAfterDisconnect(); // changed from direct call to safe method to avoid multiple calls
                try {
                    launcher.afterDisconnect(SlaveComputer.this, taskListener);
                } catch (Throwable t) {
                    LogRecord lr = new LogRecord(Level.SEVERE,
                            "Launcher {0}'s afterDisconnect method propagated an exception when {1}'s connection was closed: {2}");
                    lr.setThrown(t);
                    lr.setParameters(new Object[]{launcher, SlaveComputer.this.getName(), t.getMessage()});
                    logger.log(lr);
                }

---

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[Copilot]

Add spaces around braces to follow standard Java formatting conventions. This block should be formatted as 'try {' instead of 'try{'.

Suggested change

	try{
	try {

[Copilot]

Add space before opening brace to follow standard Java formatting conventions. It should be '} catch (Throwable t) {' instead of '} catch (Throwable t){'.

Suggested change

	} catch (Throwable t){
	} catch (Throwable t) {

[MarkEWaite]

Please don't sabotage yourself as a new contributor. Be sure that you:

• Use the pull request template and provide all the information it requests

Please restore the pull request template and provide all the information that it requests. You didn't use the pull request template and haven't provided the required information that is requested in the pull request template.

• Duplicate the bug before trying to fix it

You've provided no description that you were able to duplicate the issue. Describe the testing that showed your change is necessary.

• Read and follow the contributing guide

You didn't follow the guide. Please read it carefully and make the necessary corrections to your pull request.

• Test changes interactively

You don't describe how you tested interactively. Please describe the interactive testing you performed.

• Create automated tests that show the problem is fixed

The change contains no automated tests. Please provide automated tests.

• Respond to review comments with changes to address the comment or a clear, concise explanation why you are not making the change

You ignored my comment that you might be sabotaging yourself and did not correct the issues. That makes reviewers work harder and that tends to make reviewers less willing to interact with you and more likely to close your pull request.

[RajThak-998]

Thanks for the feedback and apologies for missing the PR template and testing
details earlier.

I have updated the pull request description to fully follow the template,
including reproduction steps and interactive testing details. I am also working
on adding automated tests to demonstrate the issue and the fix.

Thanks for the guidance.

[MarkEWaite]

• Read and follow the contributing guide

You didn't follow the guide. Please read it carefully and make the necessary corrections to your pull request.

The checks are failing because you didn't follow the contributing guide. The guide notes how you can check and fix the formatting of your changes before you submit the pull request.

I'm placing this pull request in "Draft" so that other maintainers do not waste their time reviewing it when it is not yet ready to be reviewed. When you've completed all the items in the list of actions that I provided, you can mark the pull request as "ready for review".

[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[MarkEWaite]

This has been in draft for two weeks with no further progress from the submitter. There is an unresolved merge conflict. I plan to close this after a 24 hour waiting period if there is no further progress on it.