Add hacks to allow Mock to be imported.

docs/shame.rst

@@ -0,0 +1,44 @@
+
+Module Import Wall Of Shame
+---------------------------
+
+The following modules and packages run magic during ``__init.py__`` that makes
+life hard for Mitogen. Executing code during module import is always bad, and
+Mitogen is a concrete benchmark for why it's bad.
+
+
+``pkg_resources``
+=================
+
+Anything that imports ``pkg_resources`` will eventually cause
+pkg_resources to try and import and scan ``__main__`` for its ``__requires__``
+attribute (``pkg_resources/__init__.py::_build_master()``). This breaks any app
+that is not expecting its ``__main__`` to suddenly be sucked over a network and
+injected into a remote process, like py.test.
+
+A future version of Mitogen might have a more general hack that doesn't import
+the master's ``__main__`` as ``__main__`` in the slave, avoiding all kinds of
+issues like these.
+
+**What could it do instead?**
+
+* Explicit is better than implicit: wait until the magical behaviour is
+  explicitly requested (i.e. an API call).
+
+* Use ``get("__main__")`` on :py:attr:`sys.modules` rather than ``import``, but
+  this method isn't general enough, it only really helps tools like Mitogen.
+
+
+``pbr``
+=======
+
+It claims to use ``pkg_resources`` to read version information
+(``_get_version_from_pkg_metadata()``), which would result in PEP-302 being
+reused and everything just working wonderfully, but instead it actually does
+direct filesystem access. So we smodge the environment with a ``PBR_VERSION``
+environment variable to override any version that was defined. This will
+probably break code I haven't seen yet.
+
+**What could it do instead?**
+
+* ``pkg_resources.get_resource_stream()``

docs/toc.rst

@@ -11,3 +11,4 @@ Table Of Contents
    getting_started
    api
    internals
+   shame

mitogen/core.py

@@ -368,8 +368,31 @@ class Importer(object):
         finally:
             del self.tls.running
 
+    def _load_module_hacks(self, fullname):
+        f = sys._getframe(2)
+        requestee = f.f_globals['__name__']
+
+        if fullname == '__main__' and requestee == 'pkg_resources':
+            # Anything that imports pkg_resources will eventually cause
+            # pkg_resources to try and scan __main__ for its __requires__
+            # attribute (pkg_resources/__init__.py::_build_master()). This
+            # breaks any app that is not expecting its __main__ to suddenly be
+            # sucked over a network and injected into a remote process, like
+            # py.test.
+            raise ImportError('Refused')
+
+        if fullname == 'pbr':
+            # It claims to use pkg_resources to read version information, which
+            # would result in PEP-302 being used, but it actually does direct
+            # filesystem access. So instead smodge the environment to override
+            # any version that was defined. This will probably break something
+            # later.
+            os.environ['PBR_VERSION'] = '0.0.0'
+
     def load_module(self, fullname):
         LOG.debug('Importer.load_module(%r)', fullname)
+        self._load_module_hacks(fullname)
+
         try:
             ret = self._cache[fullname]
         except KeyError: