Fix pex force local to handle PEP-420.

Previously, pex would blow up trying to adjust import paths if a PEP-420
implicit namespace package was encountered. Add a test of the path
adjustment functionality both to confirm it was needed (it was) and that
the fix technique works with all forms of namespace packages by only
assuming they support `append`.

Fixes #598

.travis.yml

@@ -92,101 +92,101 @@ matrix:
     - <<: *x-linux-shard
       name: TOXENV=style
       env: *x-py27
-      script: ./scripts/ci.sh style
+      script: tox -ve style
 
     - <<: *x-linux-shard
       name: TOXENV=isort-check
       env: *x-py27
-      script: ./scripts/ci.sh isort-check
+      script: tox -ve isort-check
 
     - <<: *x-linux-shard
       name: TOXENV=py27
       env: *x-py27
-      script: ./scripts/ci.sh py27
+      script: tox -ve py27
 
     - <<: *x-linux-shard
       name: TOXENV=py27-subprocess
       env: *x-py27
-      script: ./scripts/ci.sh py27-subprocess
+      script: tox -ve py27-subprocess
 
     - <<: *x-linux-shard
       name: TOXENV=py27-requests
       env: *x-py27
-      script: ./scripts/ci.sh py27-requests
+      script: tox -ve py27-requests
 
     - <<: *x-linux-shard
       name: TOXENV=py27-requests-cachecontrol
       env: *x-py27
-      script: ./scripts/ci.sh py27-requests-cachecontrol
+      script: tox -ve py27-requests-cachecontrol
 
     - <<: *x-linux-shard
       name: TOXENV=py34
       env:
         - *env
         - PYENV_VERSION=3.4.9
-      script: ./scripts/ci.sh py34
+      script: tox -ve py34
 
     - <<: *x-linux-shard
       name: TOXENV=py35
       env:
         - *env
         - PYENV_VERSION=3.5.6
-      script: ./scripts/ci.sh py35
+      script: tox -ve py35
 
     - <<: *x-linux-shard
       name: TOXENV=py36
       env:
         - *env
         - PYENV_VERSION=3.6.6
-      script: ./scripts/ci.sh py36
+      script: tox -ve py36
 
     - <<: *x-linux-37-shard
       name: TOXENV=py37
-      script: ./scripts/ci.sh py37
+      script: tox -ve py37
 
     - <<: *x-linux-37-shard
       name: TOXENV=py37-requests
-      script: ./scripts/ci.sh py37-requests
+      script: tox -ve py37-requests
 
     - <<: *x-linux-37-shard
       name: TOXENV=py37-requests-cachecontrol
-      script: ./scripts/ci.sh py37-requests-cachecontrol
+      script: tox -ve py37-requests-cachecontrol
 
     - <<: *x-linux-shard
       name: TOXENV=pypy
       env: *x-pypy
-      script: ./scripts/ci.sh pypy
+      script: tox -ve pypy
 
     - <<: *x-linux-shard
       name: TOXENV=py27-integration
       env: *x-py27
-      script: ./scripts/ci.sh py27-integration
+      script: tox -ve py27-integration
 
     - <<: *x-linux-37-shard
       name: TOXENV=py37-integration
-      script: ./scripts/ci.sh py37-integration
+      script: tox -ve py37-integration
 
     - <<: *x-linux-shard
       name: TOXENV=pypy-integration
       env: *x-pypy
-      script: ./scripts/ci.sh pypy-integration
+      script: tox -ve pypy-integration
 
     - <<: *x-osx-shard
       name: TOXENV=py27-requests
       env: *x-py27
-      script: ./scripts/ci.sh py27-requests
+      script: tox -ve py27-requests
 
     - <<: *x-osx-shard
       name: TOXENV=py37-requests
       env: *x-py37
-      script: ./scripts/ci.sh py37-requests
+      script: tox -ve py37-requests
 
     - <<: *x-osx-shard
       name: TOXENV=py27-integration
       env: *x-py27
-      script: ./scripts/ci.sh py27-integration
+      script: tox -ve py27-integration
 
     - <<: *x-osx-shard
       name: TOXENV=py37-integration
       env: *x-py37
-      script: ./scripts/ci.sh py37-integration
+      script: tox -ve py37-integration

pex/bootstrap.py

@@ -0,0 +1,69 @@
+# coding=utf-8
+# Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+import os
+
+
+class Bootstrap(object):
+  """Supports introspection of the PEX bootstrap code."""
+
+  _INSTANCE = None
+
+  @classmethod
+  def locate(cls):
+    """Locates the active PEX bootstrap.
+
+    :rtype: :class:`Bootstrap`
+    """
+    if cls._INSTANCE is None:
+      bootstrap_path = __file__
+      module_import_path = __name__.split('.')
+
+      # For example, our __file__ might be requests.pex/.bootstrap/pex/bootstrap.pyc and our import
+      # path pex.bootstrap; so we walk back through all the module components of our import path to
+      # find the base sys.path entry where we were found (requests.pex/.bootstrap in this example).
+      for _ in module_import_path:
+        bootstrap_path = os.path.dirname(bootstrap_path)
+
+      cls._INSTANCE = cls(path_entry=bootstrap_path)
+    return cls._INSTANCE
+
+  def __init__(self, path_entry):
+    self._path_entry = path_entry
+
+  def demote(self, log):
+    """Demote the bootstrap code to the end of the `sys.path` so it is found last."""
+    import sys  # Grab a hold of `sys` early since we'll be unimporting our module in this process.
+
+    log('Demoting bootstrap code %s' % self._path_entry, V=2)
+    for name, module in reversed(sorted(sys.modules.items())):
+      if self.imported_from_bootstrap(module):
+        sys.modules.pop(name)
+        log('un-imported {}'.format(name), V=9)
+
+    sys.path.remove(self._path_entry)
+    sys.path.append(self._path_entry)
+
+  def imported_from_bootstrap(self, module):
+    """Return ``True`` if the given ``module_name`` was imported from bootstrap code.
+
+    :rtype: bool
+    """
+    # Otherwise we may have an exposed vendored module whose name is not in the root set, but whose
+    # import comes from our path entry. EG: `pkg_resources` imported from
+    # `.bootstrap/pex/vendor/_vendored/setuptools/pkg_resources/__init__.py`
+
+    # A vendored module.
+    path = getattr(module, '__file__', None)
+    if path and path.startswith(self._path_entry):
+      return True
+
+    # A vendored package.
+    path = getattr(module, '__path__', None)
+    if path:
+      for path in path:
+        if path.startswith(self._path_entry):
+          return True
+
+    return False