Support ${:_profile_base_location_}.

src/zc/buildout/buildout.py

@@ -1652,6 +1652,17 @@ def _open(base, filename, seen, dl_options, override, downloaded):
             'No-longer supported "extended-by" option found in %s.' %
             filename)
 
+    # find and expose _profile_base_location_
+    for section, value in result.items():
+        _profile_base_location_ = None
+        for k,v in value.items():
+          if '${:_profile_base_location_}' in v:
+            _profile_base_location_ = base
+          if _profile_base_location_ is not None:
+            break
+        if _profile_base_location_ is not None:
+          value['_profile_base_location_'] = _profile_base_location_
+
     result = _annotate(result, filename)
 
     if root_config_file and 'buildout' in result:

src/zc/buildout/buildout.txt

@@ -1005,6 +1005,102 @@ the current section.  We can also use the special option,
     my_name debug
     recipe recipes:debug
 
+It is possible to have access to profile base url from section by
+using ${:_profile_base_location_}:
+
+    >>> write(sample_buildout, 'buildout.cfg',
+    ... """
+    ... [buildout]
+    ... develop = recipes
+    ... parts = data-dir debug
+    ... log-level = INFO
+    ...
+    ... [debug]
+    ... recipe = recipes:debug
+    ... profile_base_location = ${:_profile_base_location_}
+    ...
+    ... [data-dir]
+    ... recipe = recipes:mkdir
+    ... path = mydata
+    ... """)
+
+    >>> print_(system(buildout), end='')
+    Develop: '/sample-buildout/recipes'
+    Uninstalling debug.
+    Updating data-dir.
+    Installing debug.
+    _profile_base_location_ /sample-buildout
+    profile_base_location /sample-buildout
+    recipe recipes:debug
+
+Keep in mind that in case of sections spaning across multiple profiles,
+the topmost value will be presented:
+
+    >>> write(sample_buildout, 'extended.cfg',
+    ... """
+    ... [debug]
+    ... profile_base_location = ${:_profile_base_location_}
+    ... """)
+
+    >>> write(sample_buildout, 'buildout.cfg',
+    ... """
+    ... [buildout]
+    ... extends = extended.cfg
+    ... develop = recipes
+    ... parts = data-dir debug
+    ... log-level = INFO
+    ...
+    ... [debug]
+    ... recipe = recipes:debug
+    ... profile_base_location = ${:_profile_base_location_}
+    ...
+    ... [data-dir]
+    ... recipe = recipes:mkdir
+    ... path = mydata
+    ... """)
+
+    >>> print_(system(buildout), end='')
+    Develop: '/sample-buildout/recipes'
+    Updating data-dir.
+    Updating debug.
+    _profile_base_location_ /sample-buildout
+    profile_base_location /sample-buildout
+    recipe recipes:debug
+
+But of course, in case if accessing happens in extended profile's section,
+this profile's location will be exposed:
+
+    >>> write(sample_buildout, 'extended.cfg',
+    ... """
+    ... [debug]
+    ... profile_base_location = ${:_profile_base_location_}
+    ... """)
+
+    >>> write(sample_buildout, 'buildout.cfg',
+    ... """
+    ... [buildout]
+    ... extends = extended.cfg
+    ... develop = recipes
+    ... parts = data-dir debug
+    ... log-level = INFO
+    ...
+    ... [debug]
+    ... recipe = recipes:debug
+    ...
+    ... [data-dir]
+    ... recipe = recipes:mkdir
+    ... path = mydata
+    ... """)
+
+    >>> print_(system(buildout), end='')
+    Develop: '/sample-buildout/recipes'
+    Updating data-dir.
+    Updating debug.
+    _profile_base_location_ /sample-buildout
+    profile_base_location /sample-buildout
+    recipe recipes:debug
+    >>> remove(sample_buildout, 'extended.cfg')
+
 Automatic part selection and ordering
 -------------------------------------