Use detached plugins as a cache for the Update Center

[basil]

While doing manual testing recently I was frustrated by how long it took to get through the setup wizard. This change makes the setup wizard a little faster by using the detached plugins directory as a cache for the update center. This is a best-effort optimization that only kicks in if we happen to have the file we are about to download in the WEB-INF/detached-plugins directory and if its checksum happens to match. If so we simply save ourselves a trip to the download site by using what we already have. If anything goes wrong here we ignore the error and proceed along the normal download path. This brings the number of plugins that need to be downloaded during the setup wizard from 89 to 50 and (on my slow internet connection) drops the setup wizard time from 1 minute 52 seconds to 1 minute 9 seconds.

Testing done

Manual testing:

• Install ASM library plugin via UI; verify that cached copy was used.
• Install Text Finder plugin via UI; verify that cached copy was not used.
• Install ASM library plugin via UI after corrupting the copy in WAR; verify that checksum failed and cached copy was not used.
• Ran Setup Wizard before and after these changes; confirmed the same set of plugins was installed but the installation took about a minute less time.

Automated testing:

mvn clean verify -Dtest=hudson.ClassicPluginStrategyTest,hudson.cli.DisablePluginCommandTest,hudson.cli.EnablePluginCommandTest,hudson.cli.InstallPluginCommandTest,hudson.cli.ListPluginsCommandTest,hudson.core.PluginManagerOverrideTest,hudson.CustomPluginManagerTest,hudson.ExtensionListListenerTest,hudson.lifecycle.LifecycleTest,hudson.model.DownloadServiceTest,hudson.model.UpdateCenter2Test,hudson.model.UpdateCenterConnectionStatusTest,hudson.model.UpdateCenterCustomTest,hudson.model.UpdateCenterPluginInstallTest,hudson.model.UpdateCenterTest,hudson.model.UpdateSiteTest,hudson.PluginManagerCheckUpdateCenterTest,hudson.PluginManagerInstalledGUITest,hudson.PluginManagerTest,hudson.PluginManagerUtil,hudson.PluginManagerWhichTest,hudson.PluginTest,hudson.PluginWrapperTest,hudson.slaves.JNLPLauncherRealTest,jenkins.install.InstallUtilTest,jenkins.install.LoadDetachedPluginsTest,jenkins.install.SetupWizardTest,jenkins.model.JenkinsManagePermissionTest,jenkins.plugins.DetachedPluginsUtilTest,jenkins.security.CustomClassFilterTest,jenkins.security.SecurityContextExecutorServiceTest,jenkins.security.stapler.Security914Test

Proposed changelog entries

• Avoid unnecessary download of bundled plugins during the setup wizard.

Proposed upgrade guidelines

N/A

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@mention

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[basil]

@jtnord I don't think this should affect FIPS support since this is a best-effort optimization and since CloudBees doesn't ship a WEB-INF/detached-plugins directory, but you may be interested nonetheless.

[jtnord]

Thanks @basil, I think this should be fine as plugin installation is limited to a specific set of plugins and this does not appear to interfere with that.
@olamy as you wrote the code can you check please?

[MarkEWaite]

Thanks!

[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