Minor Reported on IRC:
[2016-04-13 07:14:46] <benh57> rc1 update failed updating Multibranch [2016-04-13 07:14:47] <benh57> Pipeline: Multibranch [2016-04-13 07:14:47] <benh57> Failure - [2016-04-13 07:14:47] <benh57> java.lang.NoClassDefFoundError: jenkins/scm/api/SCMNavigatorDescriptor [2016-04-13 07:14:47] <benh57> at jenkins.branch.CustomOrganizationFolderDescriptor.doAddSpecificDesc [2016-04-13 07:15:47] <benh57> it seems to have actually installed fine though, looks normal after restart [2016-04-13 11:08:40] <@danielbeck> benh57_ Still around? If so, could you please explain what exactly was updated, and where you see the error message? [2016-04-13 20:54:23] <benh57_> danielbeck: that error message was on the initial plugin upgrade screen that appears when updating to 2.0rc1. [2016-04-13 20:55:05] <@danielbeck> benh57_ To clarify, are you upgrading or installing new? [2016-04-13 20:55:35] <benh57_> upgrading from 1.6something [2016-04-13 20:55:47] <benh57_> i clicked the 'click here to finish updating!' screen [2016-04-13 20:56:07] <benh57_> however, after jenkins restarted, the plugin appears to have updated fine [2016-04-13 20:56:31] <benh57_> i can check the log for the exact full details and email that to you [2016-04-13 20:59:29] <benh57_> there are various SCM-related exceptions in there actually [2016-04-13 20:59:51] <benh57_> 1) Error injecting constructor, java.lang.NoClassDefFoundError: jenkins/scm/api/SCMSourceObserver [2016-04-13 20:59:51] <benh57_> at jenkins.branch.OrganizationFolder$DescriptorImpl.<init>(OrganizationFolder.java:266) [2016-04-13 21:00:11] <benh57_> Apr 13, 2016 5:13:52 AM hudson.ExtensionFinder$GuiceFinder$SezpozModule configure [2016-04-13 21:00:11] <benh57_> WARNING: Failed to load jenkins.branch.BranchProjectFactory$HeadByItemImpl [2016-04-13 21:00:11] <benh57_> java.lang.InstantiationException: java.lang.NoClassDefFoundError: jenkins/scm/api/SCMHead$HeadByItem
My guess: The old dependency system (JENKINS-21486 
) does not take into account that dependencies may exist but be outdated when trying to dynamically load a plugin.
In the case of an outdated SCM API plugin (judging from these messages), this results in exceptions upon Pipeline plugin installation.
[JENKINS-34209] Pipeline plugin installs in upgrade wizard throws exceptions
Mercurial Plugin still depends on SCM API 0.1. These extension points were added in 0.3. So it should be possible setting up a Jenkins 1.651.1 install with Mercurial Plugin, manually downgrade SCM API to 0.2, restart, everything works, upgrade to 2.0 RC, install Pipelines, expect it to explode.
Reproducible by installing the latest weekly, then installing 2.0 RC on top of it, then installing these plugins. Will show a similar error for Credentials Plugin.
As Jenkins is showing the update center install progress screen, and tells people to restart, I don't think this is particularly critical. It's certainly not nice, but we're working on this behavior in the context of JENKINS-21486, JENKINS-34073 (which barely got pulled out of 2.0 due to risk of perceived regressions).
FYI kohsuke.
rtyler Something to investigate tomorrow, perhaps? Not trivial to reproduce, likely requires specifically installing an older version of the SCM API plugin.