7.0.3 Windows - problem with directory cleanup on upgrade

>py -m pip install -U cffi
Collecting cffi
  Using cached cffi-1.1.0-cp27-none-win32.whl
Requirement already up-to-date: pycparser in c:\python27\lib\site-packages (from cffi)
Installing collected packages: cffi
  Found existing installation: cffi 1.0.2
    Uninstalling cffi-1.0.2:
      Successfully uninstalled cffi-1.0.2
Exception:
Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\pip\basecommand.py", line 223, in main
    status = self.run(options, args)
  File "C:\Python27\lib\site-packages\pip\commands\install.py", line 297, in run
    root=options.root_path,
  File "C:\Python27\lib\site-packages\pip\req\req_set.py", line 633, in install
    requirement.commit_uninstall()
  File "C:\Python27\lib\site-packages\pip\req\req_install.py", line 734, in commit_uninstall
    self.uninstalled.commit()
  File "C:\Python27\lib\site-packages\pip\req\req_uninstall.py", line 153, in commit
    rmtree(self.save_dir)
  File "C:\Python27\lib\site-packages\pip\_vendor\retrying.py", line 49, in wrapped_f
    return Retrying(*dargs, **dkw).call(f, *args, **kw)
  File "C:\Python27\lib\site-packages\pip\_vendor\retrying.py", line 212, in call
    raise attempt.get()
  File "C:\Python27\lib\site-packages\pip\_vendor\retrying.py", line 247, in get
    six.reraise(self.value[0], self.value[1], self.value[2])
  File "C:\Python27\lib\site-packages\pip\_vendor\retrying.py", line 200, in call
    attempt = Attempt(fn(*args, **kwargs), attempt_number, False)
  File "C:\Python27\lib\site-packages\pip\utils\__init__.py", line 89, in rmtree
    onerror=rmtree_errorhandler)
  File "C:\Python27\lib\shutil.py", line 247, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "C:\Python27\lib\shutil.py", line 247, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "C:\Python27\lib\shutil.py", line 247, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "C:\Python27\lib\shutil.py", line 252, in rmtree
    onerror(os.remove, fullname, sys.exc_info())
  File "C:\Python27\lib\site-packages\pip\utils\__init__.py", line 101, in rmtree_errorhandler
    func(path)
WindowsError: [Error 5] Access is denied: 'c:\\users\\user\\appdata\\local\\temp\\pip-zzlhal-uninstall\\python27\\lib\\site-packages\\_cffi_backend.pyd'

[dstufft]

Closing this, we’ve fixed this by not importing any C extensions in the install/upgrade path of pip on Windows.

[techtonik]

And there are a large number of technical issues with doing this robustly in pip. So pip does not do this (that’s simply a fact). The reason it’s unlikely that pip will do so in the foreseeable future is that none of the pip developers feels that the cost of developing a solution is worth the benefits, and nobody outside of the pip development team has come up with a mergeable PR that does this (even if someone did, that’s no guarantee it would be applied, as there are other considerations such as maintainability, but without the first step of a usable PR, nothing will happen).

Can you be specific about there is the major problem in pip from technical point of view?