Allow download-cache and eggs-directory to be relative paths

This implements additional suggestions on issue #171.

CHANGES.rst

@@ -10,6 +10,14 @@ Unreleased
 - Add ``BUILDOUT_HOME`` as an alternate way to control how the user default
   configuration is found.
 
+- Treat ``download-cache`` and ``eggs-directory`` in a special manner:
+
+  a) they can be relative paths, relative to the location of the configuration
+     file that defines them
+
+  b) the can be in the form ``~/subdir``, with the usual convention that the
+     ``~`` char means the home directory of the user executing the buildout
+
 2.2.1 (2013-09-05)
 ==================
 

src/zc/buildout/buildout.py

@@ -253,6 +253,36 @@ class Buildout(DictMixin):
                  if k not in versions
                  ))
 
+        # Absolutize some particular directory, handling also the ~/foo form,
+        # and considering the location of the configuration file that generated
+        # the setting as the base path, falling back to the main configuration
+        # file location
+        for name in ('download-cache', 'eggs-directory'):
+            if name in data['buildout']:
+                origdir, src = data['buildout'][name]
+                if '${' in origdir:
+                    continue
+                if not os.path.isabs(origdir):
+                    if src in ('DEFAULT_VALUE',
+                               'COMPUTED_VALUE',
+                               'COMMAND_LINE_VALUE'):
+                        if 'directory' in data['buildout']:
+                            basedir = data['buildout']['directory'][0]
+                        else:
+                            basedir = self._buildout_dir
+                    else:
+                        if _isurl(src):
+                            raise zc.buildout.UserError(
+                                'Setting "%s" to a non absolute location ("%s") '
+                                'within a\n'
+                                'remote configuration file ("%s") is ambiguous.' % (
+                                    name, origdir, src))
+                        basedir = os.path.dirname(src)
+                    absdir = os.path.expanduser(origdir)
+                    if not os.path.isabs(absdir):
+                        absdir = os.path.join(basedir, absdir)
+                    data['buildout'][name] = (absdir, src)
+
         self._annotated = copy.deepcopy(data)
         self._raw = _unannotate(data)
         self._data = {}

src/zc/buildout/buildout.txt

@@ -810,7 +810,7 @@ COMMAND_LINE_VALUE).
         DEFAULT_VALUE
     directory= /sample-buildout
         COMPUTED_VALUE
-    eggs-directory= eggs
+    eggs-directory= /sample-buildout/eggs
         DEFAULT_VALUE
     executable= ...
         DEFAULT_VALUE

src/zc/buildout/downloadcache.txt

@@ -159,3 +159,58 @@ created, provided it is within an already existing directory::
     >>> ls(cache)
     d  dist
     d  newdir
+
+Using relative paths
+--------------------
+
+You can use a relative path for ``download-cache`` (the same logic is applied
+to ``eggs-directory`` too) and in such case it is considered relative to the
+location of the configuration file that sets its value.
+
+As an example, we create a ``base.cfg`` configuration in a different directory::
+
+    >>> basedir = tmpdir('basecfg')
+    >>> write(basedir, 'base.cfg',
+    ... '''
+    ... [buildout]
+    ... download-cache = cache
+    ... ''')
+
+and a ``buildout.cfg`` that extends from there::
+
+    >>> write('buildout.cfg',
+    ... '''
+    ... [buildout]
+    ... extends = %(basedir)s/base.cfg
+    ... parts =
+    ... ''' % globals())
+
+    >>> dummy = system(buildout)
+    >>> ls(basedir)
+    -  base.cfg
+    d  cache
+
+Of course this cannot be used when the base configuration is not on the local
+filesystem because it wouldn't make any sense having a remote cache::
+
+    >>> server_data = tmpdir('server_data')
+    >>> server_url = start_server(server_data)
+    >>> cd(sample_buildout)
+
+    >>> write(server_data, 'base.cfg', """\
+    ... [buildout]
+    ... download-cache = cache
+    ... """)
+
+    >>> write('buildout.cfg',
+    ... '''
+    ... [buildout]
+    ... extends = %(server_url)s/base.cfg
+    ... parts =
+    ... ''' % globals())
+
+    >>> print_(system(buildout), end='') # doctest: +ELLIPSIS
+    While:
+      Initializing.
+    Error: Setting "download-cache" to a non absolute location ("cache") within a
+    remote configuration file...