extends downloads multiple times from same url

[Rafael Monnerat]

When a URL is used into extends (into some cfg), zc.buildout downloads multiple times the same configuration in the same run if the url is extended into 2 other diferent configuration files.

Example:

-- buildout.cfg content --
[buildout]
extends =
  http://url/to/file.cfg
  http://url/to/file2.cfg

-- http://url/to/file.cfg content --
[buildout]
extends =
  http://url/to/sample1.cfg

-- http://url/to/file2.cfg content --
[buildout]
extends =
  http://url/to/sample1.cfg

The file at http://url/to/sample1.cfg is downloaded twice by zc.buildout for every buildout run, and the download should happens only once per url.

This problem seems to happens due recursivity implemented into zc.buildout.buildout._open that does not consider that a url was already downloaded in same buildout run. I attached a patch which fix this issue and "download only once each url" per run.

[Rafael Monnerat (rafael-nexedi)]

Include a patch with a test for this bug.

This test also test https://bugs.launchpad.net/zc.buildout/+bug/717730

[Rafael Monnerat (rafael-nexedi)]

Included a fix and test for when a relative url is used in extends.

Example:

-- buildout.cfg content --
[buildout]
extends =
  http://url/to/file.cfg
  http://url/to/file2.cfg
  http://url/to/file3.cfg

-- http://url/to/file.cfg content --
[buildout]
extends =
  http://url/to/sample1.cfg

-- http://url/to/file2.cfg content --
[buildout]
extends =
  http://url/to/sample1.cfg

-- http://url/to/file3.cfg content --
[buildout]
extends =
  sample1.cfg

In this case, http://url/to/sample1.cfg should be downloaded only once too.