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# Copyright 2014 Canonical Ltd. This software is licensed under the
# GNU Affero General Public License version 3 (see the file LICENSE).
"""MAAS-specific import fascist.
This is designed to stop the unwary from importing modules from where
they ought not.
We do this to help separate concerns. For example, ``provisioningserver`` is
meant to run on cluster controllers, perhaps using code from ``apiclient``,
but it must *not* import from ``maas``, ``maasserver``, or ``metadataserver``
because these are solely the preserve of the region controller, and because
they contain Django applications which need some extra environmental support
in order to run.
See https://docs.python.org/2/library/sys.html#sys.meta_path and
https://docs.python.org/2.7/reference/simple_stmts.html#the-import-statement
for more information on how this module is able to work.
"""
from __future__ import (
absolute_import,
print_function,
unicode_literals,
)
str = None
__metaclass__ = type
__all__ = []
from inspect import (
getmodule,
getsourcefile,
)
from sys import (
_getframe as getframe,
meta_path,
)
class FascistFinder:
"""Provides the ``find_module`` method.
``find_module`` is a Python-specified hook for overriding the normal
import mechanisms. Put an instance of this class into ``sys.meta_hook``
and Python will consult it when looking to import a module.
If ``find_module`` returns an object other than ``None`` it is then used
to load the module. However, it's also possible to use this to *prevent*
module loading, which is what this class does in cooperation with
`FascistLoader`.
"""
# module to import: forbidden from
rules = {
"maas": {
"apiclient",
"maascli",
"maastesting",
"provisioningserver",
},
"maasserver": {
"apiclient",
"maascli",
"maastesting",
"provisioningserver",
},
"metadataserver": {
"apiclient",
"maascli",
"maastesting",
"provisioningserver",
},
}
def find_rule(self, name):
"""Search `rules` for `name`.
If `name` isn't found but `name` is a dot-separated name, another
search is attempted with the right-most part of the name removed.
For example, given `foo.bar.baz` it will search in order::
foo.bar.baz
foo.bar
foo
If no name matches, it returns `None`.
"""
if name in self.rules:
return self.rules[name]
elif "." in name:
name, _, _ = name.rpartition(".")
return self.find_rule(name)
else:
return None
def is_forbidden(self, forbidden, name):
"""Search `forbidden` for `name`.
If `name` isn't found but `name` is a dot-separated name, another
search is attempted with the right-most part of the name removed.
See `find_rule` for details.
"""
if name in forbidden:
return True
elif "." in name:
name, _, _ = name.rpartition(".")
return self.is_forbidden(forbidden, name)
else:
return False
def find_module(self, fullname, path=None):
"""Consult `rules` to see if `fullname` can be loaded.
This ignores `path`.
"""
forbidden = self.find_rule(fullname)
if forbidden is not None:
origin_frame = getframe(1)
origin_module = getmodule(origin_frame)
if origin_module is not None:
origin_module_name = origin_module.__name__
if self.is_forbidden(forbidden, origin_module_name):
return FascistLoader(origin_module_name)
# Good. Out of the door, line on the left, one cross each.
return None
def install(self):
"""Install this at the front of `meta_path`.
If it's already installed, it is moved to the front.
"""
if self in meta_path:
meta_path.remove(self)
meta_path.insert(0, self)
class FascistLoader:
"""Prevent import of the specified module.
With a message explaining what has been prevented and why.
"""
rules_location = getsourcefile(FascistFinder)
if rules_location is None:
rules_location = FascistFinder.__name__
def __init__(self, whence):
super(FascistLoader, self).__init__()
self.whence = whence
def load_module(self, name):
"""Raises `ImportError` to prevent loading of `name`."""
raise ImportError(
"Don't import %r from %r. This is MAAS policy. If you "
"think this is wrong, amend the rules in %r." % (
name, self.whence, self.rules_location))
fascist = FascistFinder()
fascist.install()
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