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  1. Jenkins
  2. JENKINS-48725

Jenkins startup workflow can deadlock if exceptions occur during extensions lookup

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      Description

      Underlying issue

      Reactor run method (https://github.com/jenkinsci/lib-task-reactor/blob/master/src/main/java/org/jvnet/hudson/reactor/Reactor.java#L115) only catches TunnelException.

      If another exception occurs in the task run method,

      • done is set to true
      • fatal is not set
      • but pending variable is not decremented

      This makes Reactor execute method wait for ever (https://github.com/jenkinsci/lib-task-reactor/blob/master/src/main/java/org/jvnet/hudson/reactor/Reactor.java#L266) as pending will never reach 0.


      How can task.run throw something else than a TunnelException?

      task run method (https://github.com/jenkinsci/lib-task-reactor/blob/master/src/main/java/org/jvnet/hudson/reactor/Reactor.java#L207) catches exceptions from the runTask method and wraps them in a TunnelException.

      But listener methods are not protected by try/catch blocks (except onTaskCompleted for technical reasons)

      So if an exception occurs in any of the onTaskStarted or onTaskFailed, it won't be wrapped in a TunnelException, making the problem arise.

      For milestones, another type of run method is used (https://github.com/jenkinsci/lib-task-reactor/blob/master/src/main/java/org/jvnet/hudson/reactor/Reactor.java#L176), calling the listener's onAttained method, unprotected by try/catch.

       Even if it can make sense not to protect listener methods, I am wondering if this is a good idea to have different behaviors depending on the listener method as one of them is in fact protected?


      Where does a listener throw an exception?

      InitReactorRunner provides a listener implementation calling InitReactorRunner's onInitMilestoneAttained method, when onAttained is called on a milestone of type InitMilestone.

      https://github.com/jenkinsci/jenkins/blob/a82a47a8f2a90727d873c637c99aa0d087500dfe/core/src/main/java/jenkins/InitReactorRunner.java#L83

      onInitMilestoneAttained implementation in Jenkins class (https://github.com/jenkinsci/jenkins/blob/a82a47a8f2a90727d873c637c99aa0d087500dfe/core/src/main/java/jenkins/model/Jenkins.java#L1092) can throw if ExtensionList.lookup(ExtensionFinder.class).getComponents(); throws.

      Exceptions can occur through a static call to a plugin class in an exception point if this same plugin will not start for a dependency reason

       Note that even if this seems an advanced scenario, other implementations of _ReactorListener_s could be provided through https://github.com/jenkinsci/jenkins/blob/a82a47a8f2a90727d873c637c99aa0d087500dfe/core/src/main/java/jenkins/InitReactorRunner.java#L63, and they might provide implementations which throw exception.


      How to correct?

      I would have proposed a fix, but I'm not sure of the direction to take:

      • enhance the Reactor component
        • by protecting listeners calls ?
        • by adding a way for listener methods to mark the task as fatal for milestones?
      • work around the limitations on listeners error-handling in Jenkins core code 
        • by try catching, logging, and swallowing the exception (not sure of the consequences) ?
        • or by adding back to the reactor a task designed to throw the exception in case of errors (will occur at some unknown point in the future, but at least will stop Jenkins startup) ?
      • just fix all plugins ?
      • some other way?

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            • Assignee:
              greybird Arnaud TAMAILLON
              Reporter:
              greybird Arnaud TAMAILLON
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