Description of problem: 'yum update' generate an blocked error


Version-Release number of selected component (if applicable): yum-3.4.3-18.fc17.noarch


How reproducible: do a 'yum update' with or whithout parameters


Steps to Reproduce:
1. 'yum update'
2. 'y' to accept
3. error !
  
Actual results:

Is this ok [y/N]: y
Downloading Packages:
Setting up and reading Presto delta metadata
Processing delta metadata
Traceback (most recent call last):
  File "/bin/yum", line 29, in <module>
    yummain.user_main(sys.argv[1:], exit_code=True)
  File "/usr/share/yum-cli/yummain.py", line 321, in user_main
    errcode = main(args)
  File "/usr/share/yum-cli/yummain.py", line 229, in main
    return_code = base.doTransaction()
  File "/usr/share/yum-cli/cli.py", line 547, in doTransaction
    problems = self.downloadPkgs(downloadpkgs, callback_total=self.download_callback_total_cb) 
  File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 2147, in downloadPkgs
    self.plugins.run('predownload', pkglist=pkglist)
  File "/usr/lib/python2.7/site-packages/yum/plugins.py", line 187, in run
    func(conduitcls(self, self.base, conf, **kwargs))
  File "/usr/lib/yum-plugins/presto.py", line 633, in predownload_hook
    (problems, more) = downloadPkgs(conduit, pinfo, download_pkgs)
  File "/usr/lib/yum-plugins/presto.py", line 321, in downloadPkgs
    delta = getDelta(po, presto, conduit, conf_mp)
  File "/usr/lib/yum-plugins/presto.py", line 198, in getDelta
    conduit.info(5, -("Could not find delta rpm for package %s.%s.")
TypeError: bad operand type for unary -: 'str'




Expected results:

'downloading packages' -> 'installing packages' -> 'verifying packages'


Additional info:
no ctrl-c possible, thread blocked, need to process to free 'yum' in root mode