New Options.barrier() to set a default requirement for not-yet processed parts

src/zc/buildout/buildout.py

@@ -375,6 +375,7 @@ class Buildout(DictMixin):
         self._parts = []
         self._initializing = []
         self._signature_cache = {}
+        self._default_requirement = None
 
         # provide some defaults before options are parsed
         # because while parsing options those attributes might be
@@ -1416,6 +1417,8 @@ class Options(DictMixin):
         if '<' in self._raw:
             self._raw = self._do_extend_raw(name, self._raw, [])
 
+        default = self.buildout._default_requirement
+
         # force substitutions
         for k, v in sorted(self._raw.items()):
             if '${' in v:
@@ -1425,6 +1428,8 @@ class Options(DictMixin):
             return # buildout section can never be a part
 
         if self.get('recipe'):
+            if default:
+                self.depends.add(default)
             self.recipe = self.buildout.initialize(self, *_recipe(self._data))
             self.buildout._parts.append(name)
 
@@ -1649,6 +1654,19 @@ class Options(DictMixin):
                 self.name)
         return self._created
 
+    def barrier(self):
+        """Set self as a default requirement for not-yet processed parts
+
+        This method must be called if this part may alter the processing
+        of other parts in any way, like modifying environment variables.
+        In other words, it sets an implicit dependency for these parts.
+        """
+        buildout = self.buildout
+        if not buildout._initializing:
+            raise zc.buildout.UserError(
+                "Options.barrier() shall only be used during initialization")
+        buildout._default_requirement = self.name
+
     def __repr__(self):
         return repr(dict(self))