If something goes wrong in the middle of an update, Hudson gets stuck and never recovers without manual intervention.

After a failed build / update last night, the following exception was thrown on every subsequent build. The fix was to delete the files in /var/tomcat0/.hudson/updates/* Ideally, Hudson would be able to recover from this on its own.

FATAL: A JSONObject text must begin with '{' at character 0 of
net.sf.json.JSONException: A JSONObject text must begin with '{' at character 0 of
at net.sf.json.util.JSONTokener.syntaxError(JSONTokener.java:512)
at net.sf.json.JSONObject._fromJSONTokener(JSONObject.java:839)
at net.sf.json.JSONObject._fromString(JSONObject.java:1060)
at net.sf.json.JSONObject.fromObject(JSONObject.java:176)
at net.sf.json.JSONObject.fromObject(JSONObject.java:147)
at hudson.model.DownloadService$Downloadable.getData(DownloadService.java:197)
at hudson.tools.DownloadFromUrlInstaller$DescriptorImpl.getInstallables(DownloadFromUrlInstaller.java:149)
at hudson.tools.DownloadFromUrlInstaller.getInstallable(DownloadFromUrlInstaller.java:54)
at hudson.tools.DownloadFromUrlInstaller.performInstallation(DownloadFromUrlInstaller.java:63)
at hudson.tools.InstallerTranslator.getToolHome(InstallerTranslator.java:61)
at hudson.tools.ToolLocationNodeProperty.getToolHome(ToolLocationNodeProperty.java:99)
at hudson.tools.ToolInstallation.translateFor(ToolInstallation.java:149)
at hudson.tasks.Ant$AntInstallation.forNode(Ant.java:376)
at hudson.tasks.Ant.perform(Ant.java:140)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:601)
at hudson.model.Build$RunnerImpl.build(Build.java:174)
at hudson.model.Build$RunnerImpl.doRun(Build.java:138)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:416)
at hudson.model.Run.run(Run.java:1257)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:129)

I have no idea which component this is actually because of.