Detecting if the installer is being run to install buildout itself.

A working_set passed into install() is the sign that we're called from
buildout.py to install buildout, setuptools, an extension or a recipe.
Those need the global pkg_resources.working_set, for instance to load entry
points.

So... conflicts with global packages are possible, but they should be ignored
so that we can grab the right versions ourselves. The paths that end up in the
bin scripts that we generate are OK.

src/zc/buildout/easy_install.py

@@ -465,8 +465,7 @@ class Installer:
 
         return dist.clone(location=new_location)
 
-    def _get_dist(self, requirement, ws):
-
+    def _get_dist(self, requirement, ws, for_buildout_run=False):
         __doing__ = 'Getting distribution for %r.', str(requirement)
 
         # Maybe an existing dist is already the best dist that satisfies the
@@ -513,8 +512,6 @@ class Installer:
                         # obtained locally.  Just copy it.
                         shutil.copytree(dist.location, newloc)
                     else:
-
-
                         setuptools.archive_util.unpack_archive(
                             dist.location, newloc)
 
@@ -532,6 +529,13 @@ class Installer:
                         dist.location, ws, self._dest, dist)
                     for dist in dists:
                         redo_pyc(dist.location)
+                        if for_buildout_run:
+                            # ws is the global working set and we're
+                            # installing buildout, setuptools, extensions or
+                            # recipes. Make sure that whatever correct version
+                            # we've just installed is the active version,
+                            # hence the ``replace=True``.
+                            ws.add(dist, replace=True)
 
             finally:
                 if tmp != self._download_cache:
@@ -608,6 +612,7 @@ class Installer:
 
         logger.debug('Installing %s.', repr(specs)[1:-1])
 
+        for_buildout_run = bool(working_set)
         path = self._path
         dest = self._dest
         if dest is not None and dest not in path:
@@ -622,7 +627,8 @@ class Installer:
             ws = working_set
 
         for requirement in requirements:
-            for dist in self._get_dist(requirement, ws):
+            for dist in self._get_dist(requirement, ws,
+                                       for_buildout_run=for_buildout_run):
                 ws.add(dist)
                 self._maybe_add_setuptools(ws, dist)
 
@@ -640,6 +646,7 @@ class Installer:
         # to look for new eggs unless what we have is the best that
         # matches the requirement.
         env = pkg_resources.Environment(ws.entries)
+
         while requirements:
             # Process dependencies breadth-first.
             current_requirement = requirements.pop(0)
@@ -656,12 +663,13 @@ class Installer:
                         "Version conflict while processing requirement %s "
                         "(constrained to %s)",
                         current_requirement, req)
-                    # When bootstrapping zc.buildout, we might be doing it
-                    # with a different version than the one we specified in
-                    # our versions list. Reason: the bootstrap grabs the
-                    # latest buildout. Same with setuptools.
-                    # So ignore the version conflict for those two packages.
-                    if req.key not in ['zc.buildout', 'setuptools']:
+                    # Installing buildout itself and its extensions and
+                    # recipes requires the global
+                    # ``pkg_resources.working_set`` to be active, which also
+                    # includes all system packages. So there might be
+                    # conflicts, which are fine to ignore. We'll grab the
+                    # correct version a few lines down.
+                    if not for_buildout_run:
                         raise VersionConflict(err, ws)
             if dist is None:
                 if dest:
@@ -669,7 +677,8 @@ class Installer:
                 else:
                     logger.debug('Adding required %r', str(req))
                 _log_requirement(ws, req)
-                for dist in self._get_dist(req, ws,):
+                for dist in self._get_dist(req, ws,
+                                           for_buildout_run=for_buildout_run):
                     ws.add(dist)
                     self._maybe_add_setuptools(ws, dist)
             if dist not in req: