[Distutils] setuptools bug, inconsistent replacement of eggs when installing from file:// urls.

[Fadhley Salim]

I think I've found a bug in the way that setuptools selects whether an
egg specified by file:// url should be installed over an existing
(identical) egg:
 
To re-create the fault:
 
* Install the egg of your choice.
* make two local "egg repositories", one with a very long path (e.g.
c:/a/) and one with a very long path
* In each of the egg repositories place a copy of the egg you just
installed, along with an HTML file containing a link to that egg... just
it's filename.
* Try to easy_install the egg with the options "-mv" and set "-f" to
either one of the repositories
 
When you select a short path for the repository you will notice that the
egg in your site_packages folder is always replaced, you see something
like this:
 
"Removing
d:\devtools\python24\lib\site-packages\calyon-1.0_r2009_03_12-py2.4-win3
2.egg"
 
When you select a very long path for the repository you will not see
this message, this is because easy_install leaves the egg unmodified.
 
I've found out why this happens: The pkg_resources.Environment uses a
hash-tuple to work out the order in which to consider packages to
install. If you are trying to install two identical eggs, the 4th
element of the tuple -len(self.location or '') becomes significant...
longer URLs get preference to shorter URLs. 
 
That on it's own might not seem too bad, but consider the the 2nd value
of the hash-tuple (precedence) is given an identical value of 3 both for
a package which is already installed in pkg_resources and a package
which can already be downloaded from a file:// link. I think a better
default behaviour would be for it to give stuff that is already
installed a higher precedence than stuff which could potentially be
installed. 
 
Hope all of this makes sense!
 
Sal
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