Source code for plainbox.impl.depmgr

# This file is part of Checkbox.
#
# Copyright 2012-2015 Canonical Ltd.
# Written by:
#   Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
#
# Checkbox is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License version 3,
# as published by the Free Software Foundation.
#
# Checkbox 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 Checkbox.  If not, see <http://www.gnu.org/licenses/>.

"""
Job Dependency Solver.


:mod:`plainbox.impl.depmgr`
===========================

.. warning::

    THIS MODULE DOES NOT HAVE STABLE PUBLIC API
"""

from abc import ABCMeta
from abc import abstractproperty
from logging import getLogger
import enum

from plainbox.i18n import gettext as _


logger = getLogger("plainbox.depmgr")


[docs]class DependencyError(Exception, metaclass=ABCMeta): """ Exception raised when a dependency error is detected. """ @abstractproperty def affected_job(self): """ job that is affected by the dependency error. """ @abstractproperty def affecting_job(self): """ job that is affecting the :attr:`affected_job`. This may be None in certain cases (eg, when the job does not exist and is merely referred to by id). If this job exists removing it SHOULD fix this problem from occurring. This may be the same as :attr:`affected_job` """
[docs]class DependencyUnknownError(DependencyError): """ Exception raised when an unknown job is mentioned. .. note:: This class differs from :class:`DependencyMissingError` in that the unknown job is not a dependency of anything. It can only happen when the job is explicitly mentioned in the list of jobs to visit. """ def __init__(self, job): """ Initialize a new DependencyUnknownError with a given job. """ self.job = job @property def affected_job(self): """ job that is affected by the dependency error. Here it's a job that on the ``visit_list`` but not on the ``job_list``. """ return self.job @property def affecting_job(self): """ job that is affecting the :attr:`affected_job`. Here, it is always None. """ def __str__(self): """ Get a printable description of an error. """ return _("unknown job referenced: {!a}").format(self.job.id) def __repr__(self): """ Get a debugging representation of an error. """ return "<{} job:{!r}>".format(self.__class__.__name__, self.job) def __eq__(self, other): """ Check if one error is equal to another. """ if not isinstance(other, DependencyUnknownError): return NotImplemented return self.job == other.job def __hash__(self): """ Calculate the hash of an error. """ return hash((self.job,))
[docs]class DependencyCycleError(DependencyError): """ Exception raised when a cyclic dependency is detected. """ def __init__(self, job_list): """ Initialize with a list of jobs that form a dependency loop. The dependencies satisfy the given expression: job_list[n - 1] depends-on job_list[n] The error exists because job_list[0] is job_list[-1]. Each item is a JobDefinition instance. """ assert len(job_list) > 1 assert job_list[0] is job_list[-1] self.job_list = job_list @property def affected_job(self): """ job that is affected by the dependency error. Here it is the job that has a cyclic dependency on itself. """ return self.job_list[0] @property def affecting_job(self): """ job that is affecting the :attr:`affected_job`. Here it's always the same as :attr:`~DependencyCycleError.affected_job` """ return self.affected_job def __str__(self): """ Get a printable description of an error. """ return _("dependency cycle detected: {}").format( " -> ".join([job.id for job in self.job_list])) def __repr__(self): """ Get a debugging representation of an error. """ return "<{} job_list:{!r}>".format( self.__class__.__name__, self.job_list)
[docs]class DependencyMissingError(DependencyError): """ Exception raised when a job has an unsatisfied dependency. """ DEP_TYPE_RESOURCE = "resource" DEP_TYPE_DIRECT = "direct" DEP_TYPE_ORDERING = "ordering" def __init__(self, job, missing_job_id, dep_type): """ Initialize a new error with given data. """ self.job = job self.missing_job_id = missing_job_id self.dep_type = dep_type @property def affected_job(self): """ job that is affected by the dependency error. Here it is the job that has a missing dependency. """ return self.job @property def affecting_job(self): """ job that is affecting the :attr:`affected_job`. Here it is always None as we have not seen this job at all and that's what's causing the problem in the first place. """ def __str__(self): """ Get a printable description of an error. """ return _("missing dependency: {!r} ({})").format( self.missing_job_id, self.dep_type) def __repr__(self): """ Get a debugging representation of an error. """ return "<{} job:{!r} missing_job_id:{!r} dep_type:{!r}>".format( self.__class__.__name__, self.job, self.missing_job_id, self.dep_type) def __eq__(self, other): """ Check if one error is equal to another. """ if not isinstance(other, DependencyMissingError): return NotImplemented return (self.job == other.job and self.missing_job_id == other.missing_job_id and self.dep_type == other.dep_type) def __hash__(self): """ Calculate the hash of an error. """ return hash((self.job, self.missing_job_id, self.dep_type))
[docs]class DependencyDuplicateError(DependencyError): """ Exception raised when two jobs have the same id. """ def __init__(self, job, duplicate_job): """ Initialize a new error with given data. """ assert job.id == duplicate_job.id self.job = job self.duplicate_job = duplicate_job @property def affected_job(self): """ job that is affected by the dependency error. Here it is the job that is already known by the system. """ return self.job @property def affecting_job(self): """ job that is affecting the :attr:`affected_job`. Here it is the job that is clashing with another job already present in the system. """ return self.duplicate_job def __str__(self): """ Get a printable description of an error. """ return _("duplicate job id: {!r}").format(self.affected_job.id) def __repr__(self): """ Get a debugging representation of an error. """ return "<{} job:{!r} duplicate_job:{!r}>".format( self.__class__.__name__, self.job, self.duplicate_job)
[docs]class Color(enum.Enum): """ Three classic colors for recursive graph visitor. WHITE: For nodes have not been visited yet. GRAY: For nodes that are currently being visited but the visit is not complete. BLACK: For nodes that have been visited and are complete. """ WHITE = 'white' GRAY = 'gray' BLACK = 'black'
[docs]class DependencySolver: """ Dependency solver for Jobs. Uses a simple depth-first search to discover the sequence of jobs that can run. Use the resolve_dependencies() class method to get the solution. """ COLOR_WHITE = Color.WHITE COLOR_GRAY = Color.GRAY COLOR_BLACK = Color.BLACK
[docs] @classmethod def resolve_dependencies(cls, job_list, visit_list=None): """ Solve the dependency graph expressed as a list of job definitions. :param list job_list: list of known jobs :param list visit_list: (optional) list of jobs to solve The visit_list, if specified, allows to consider only a part of the graph while still having access and knowledge of all jobs. :returns list: the solution (a list of jobs to execute in order) :raises DependencyDuplicateError: if a duplicate job definition is present :raises DependencyCycleError: if a cyclic dependency is present. :raises DependencyMissingErorr: if a required job does not exist. """ return cls(job_list)._solve(visit_list)
def __init__(self, job_list): """ Instantiate a new dependency solver with the specified list of jobs. :raises DependencyDuplicateError: if the initial job_list has any duplicate jobs """ # Remember the jobs that were passed self._job_list = job_list # Build a map of jobs (by id) self._job_map = self._get_job_map(job_list) # Job colors, maps from job.id to COLOR_xxx self._job_color_map = {job.id: self.COLOR_WHITE for job in job_list} # The computed solution, made out of job instances. This is not # necessarily the only solution but the algorithm computes the same # value each time, given the same input. self._solution = [] def _solve(self, visit_list=None): """ Internal method of DependencySolver. Solves the dependency graph and returns the solution. Calls _visit() on each of the initial nodes/jobs """ # Visit the visit list logger.debug(_("Starting solve")) logger.debug(_("Solver job list: %r"), self._job_list) logger.debug(_("Solver visit list: %r"), visit_list) if visit_list is None: visit_list = self._job_list for job in visit_list: self._visit(job) logger.debug(_("Done solving")) # Return the solution return self._solution def _visit(self, job, trail=None): """ Internal method of DependencySolver. Called each time a node is visited. Nodes already seen in _visited are skipped. Attempts to enumerate all dependencies (both direct and resource) and resolve them. Missing jobs cause DependencyMissingError to be raised. Calls _visit recursively on all dependencies. """ try: color = self._job_color_map[job.id] except KeyError: logger.debug(_("Visiting job that's not on the job_list: %r"), job) raise DependencyUnknownError(job) logger.debug(_("Visiting job %s (color %s)"), job.id, color) if color == self.COLOR_WHITE: # This node has not been visited yet. Let's mark it as GRAY (being # visited) and iterate through the list of dependencies self._job_color_map[job.id] = self.COLOR_GRAY # If the trail was not specified start a trail for this node if trail is None: trail = [job] for dep_type, job_id in job.controller.get_dependency_set(job): # Dependency is just an id, we need to resolve it # to a job instance. This can fail (missing dependencies) # so let's guard against that. try: next_job = self._job_map[job_id] except KeyError: logger.debug(_("Found missing dependency: %r from %r"), job_id, job) raise DependencyMissingError(job, job_id, dep_type) else: # For each dependency that we visit let's reuse the trail # to give proper error messages if a dependency loop exists logger.debug(_("Visiting dependency: %r"), next_job) # Update the trail as we visit that node trail.append(next_job) self._visit(next_job, trail) trail.pop() # We've visited (recursively) all dependencies of this node, # let's color it black and append it to the solution list. logger.debug(_("Appending %r to solution"), job) self._job_color_map[job.id] = self.COLOR_BLACK self._solution.append(job) elif color == self.COLOR_GRAY: # This node is not fully traced yet but has been visited already # so we've found a dependency loop. We need to cut the initial # part of the trail so that we only report the part that actually # forms a loop trail = trail[trail.index(job):] logger.debug(_("Found dependency cycle: %r"), trail) raise DependencyCycleError(trail) else: assert color == self.COLOR_BLACK # This node has been visited and is fully traced. # We can just skip it and go back @staticmethod def _get_job_map(job_list): """ Internal method of DependencySolver. Computes a map of job.id => job Raises DependencyDuplicateError if a collision is found """ job_map = {} for job in job_list: if job.id in job_map: raise DependencyDuplicateError(job_map[job.id], job) else: job_map[job.id] = job return job_map
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