cdist/cdist/resolver.py

# -*- coding: utf-8 -*-
#
# 2011-2012 Steven Armstrong (steven-cdist at armstrong.cc)
#
# This file is part of cdist.
#
# cdist is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# cdist is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with cdist. If not, see <http://www.gnu.org/licenses/>.
#
#

import logging
import os
import itertools
import fnmatch

import cdist

log = logging.getLogger(__name__)


class CircularReferenceError(cdist.Error):
    def __init__(self, cdist_object, required_object):
        self.cdist_object = cdist_object
        self.required_object = required_object

    def __str__(self):
        return 'Circular reference detected: %s -> %s' % (self.cdist_object.name, self.required_object.name)


class RequirementNotFoundError(cdist.Error):
    def __init__(self, requirement):
        self.requirement = requirement

    def __str__(self):
        return 'Requirement could not be found: %s' % self.requirement


class DependencyResolver(object):
    """Cdist's dependency resolver.

    Usage:
    >> resolver = DependencyResolver(list_of_objects)
    # Easy access to the objects we are working with
    >> resolver.objects['__some_type/object_id']
    <CdistObject __some_type/object_id>
    # Easy access to a specific objects dependencies
    >> resolver.dependencies['__some_type/object_id']
    [<CdistObject __other_type/dependency>, <CdistObject __some_type/object_id>]
    # Pretty print the dependency graph
    >> from pprint import pprint
    >> pprint(resolver.dependencies)
    # Iterate over all existing objects in the correct order
    >> for cdist_object in resolver:
    >>     do_something_with(cdist_object)
    """
    def __init__(self, objects, logger=None):
        self.objects = dict((o.name, o) for o in objects)
        self._dependencies = None
        self.log = logger or log

    @property
    def dependencies(self):
        """Build the dependency graph.

        Returns a dict where the keys are the object names and the values are
        lists of all dependencies including the key object itself.
        """
        if self._dependencies is None:
            self._dependencies = d = {}
            self._preprocess_requirements()
            for name,cdist_object in self.objects.items():
                resolved = []
                unresolved = []
                self._resolve_object_dependencies(cdist_object, resolved, unresolved)
                d[name] = resolved
        return self._dependencies

    def find_requirements_by_name(self, requirements):
        """Takes a list of requirement patterns and returns a list of matching object instances.

        Patterns are expected to be Unix shell-style wildcards for use with fnmatch.filter.

        find_requirements_by_name(['__type/object_id', '__other_type/*']) -> 
            [<Object __type/object_id>, <Object __other_type/any>, <Object __other_type/match>]
        """
        object_names = self.objects.keys()
        for pattern in requirements:
            found = False
            for requirement in fnmatch.filter(object_names, pattern):
                found = True
                yield self.objects[requirement]
            if not found:
                # FIXME: get rid of the singleton object_id, it should be invisible to the code -> hide it in Object
                singleton = os.path.join(pattern, 'singleton')
                if singleton in self.objects:
                    yield self.objects[singleton]
                else:
                    raise RequirementNotFoundError(pattern)

    def _preprocess_requirements(self):
        """Find all before, after and autorequire dependencies and merge them
        to be just requirements for further processing.
        """
        for cdist_object in self.objects.values():
            if cdist_object.after:
                cdist_object.requirements.extend(cdist_object.after)
                # As we changed the object on disc, we have to ensure it is not 
                # preprocessed again if someone would call us multiple times.
                cdist_object.after = []
            if cdist_object.before:
                for other_object in self.find_requirements_by_name(cdist_object.before):
                    other_object.requirements.append(cdist_object.name)
                # As we changed the object on disc, we have to ensure it is not 
                # preprocessed again if someone would call us multiple times.
                cdist_object.before = []
            if cdist_object.autorequire:
                # The objects (children) that this cdist_object (parent) defined
                # in it's type manifest shall inherit all explicit requirements 
                # that the parent has so that user defined requirements are 
                # fullfilled and processed in the expected order.
                for auto_requirement in self.find_requirements_by_name(cdist_object.autorequire):
                    for requirement in cdist_object.requirements:
                        if requirement not in auto_requirement.requirements:
                            auto_requirement.requirements.append(requirement)
                # On the other hand the parent shall depend on all the children
                # it created so that the user can setup dependencies on it as a 
                # whole without having to know anything about the parents 
                # internals.
                cdist_object.requirements.extend(cdist_object.autorequire)
                # As we changed the object on disc, we have to ensure it is not 
                # preprocessed again if someone would call us multiple times.
                cdist_object.autorequire = []

    def _resolve_object_dependencies(self, cdist_object, resolved, unresolved):
        """Resolve all dependencies for the given cdist_object and store them
        in the list which is passed as the 'resolved' arguments.

        e.g.
        resolved = []
        unresolved = []
        resolve_object_dependencies(some_object, resolved, unresolved)
        print("Dependencies for %s: %s" % (some_object, resolved))
        """
        self.log.debug('Resolving dependencies for: %s' % cdist_object.name)
        try:
            unresolved.append(cdist_object)
            for required_object in self.find_requirements_by_name(cdist_object.requirements):
                self.log.debug("Object %s requires %s", cdist_object, required_object)
                if required_object not in resolved:
                    if required_object in unresolved:
                        raise CircularReferenceError(cdist_object, required_object)
                    self._resolve_object_dependencies(required_object, resolved, unresolved)
            resolved.append(cdist_object)
            unresolved.remove(cdist_object)
        except RequirementNotFoundError as e:
            raise cdist.CdistObjectError(cdist_object, "requires non-existing " + e.requirement)

    def __iter__(self):
        """Iterate over all unique objects and yield them in the correct order.
        """
        iterable = itertools.chain(*self.dependencies.values())
        # Keep record of objects that have already been seen
        seen = set()
        seen_add = seen.add
        for cdist_object in itertools.filterfalse(seen.__contains__, iterable):
            seen_add(cdist_object)
            yield cdist_object