path: root/morphlib/builddependencygraph.py

# Copyright (C) 2012  Codethink Limited
# 
# This program 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; version 2 of the License.
# 
# This program 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 this program; if not, write to the Free Software Foundation, Inc.,
# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.


import collections
import os

import morphlib


class BuildDependencyGraph(object): # pragma: no cover

    '''This class constructs a build dependency graph from an input morphology
       and provides ways to traverse this graph. It also provides a method to
       transform the dependency graph into groups of items that are independent
       and can be built in parallel.'''
    
    def __init__(self, loader, morph):
        self.loader = loader
        self.morph = morph
        self.blobs = set()

    def create_blob(self, morph, parent=None):
        '''Creates a blob from a morphology. The optional parent is used to
           associate chunks with their containing stratum.'''

        if morph.kind == 'stratum':
            return morphlib.blobs.Stratum(parent, morph)
        elif morph.kind == 'chunk':
            return morphlib.blobs.Chunk(parent, morph)
        else:
            return morphlib.blobs.System(parent, morph)

    def resolve(self):
        '''Constructs the dependency graph by resolving dependencies
           recursively.'''

        self.cached_blobs = {}
        self._resolve_strata()
        self._resolve_chunks()

    def build_order(self):
        '''Computes a topological sorting of the dependency graph and
           generates a deque of groups, each of which contains a set
           of items that are independent and can be built in parallel.'''

        sorting = self._compute_topological_sorting()
        groups = collections.deque()

        # create the first group
        group = set()
        groups.append(group)

        # traverse the graph in topological order
        for blob in sorting:
            # add the current item to the current group, or a new group
            # if one of its dependencies is in the current one
            create_group = False
            for dependency in blob.dependencies:
                if dependency in group:
                    create_group = True
            if create_group:
                group = set()
                groups.append(group)
            group.add(blob)

        # return the set of blobs and the build groups
        return set(self.blobs), groups

    def _get_blob(self, info, parent=None):
        '''Takes a (repo, ref, filename) tuple and looks up the blob for it.
           It loads the corresponding morphology and blob on-demand if it is
           not cached yet.'''

        blob = self.cached_blobs.get(info, None)
        if not blob:
            morphology = self.loader.load(info[0], info[1], info[2])
            blob = self.create_blob(morphology, parent)
            self.cached_blobs[info] = blob
        return blob

    def _resolve_strata(self):
        '''This method recursively generates a dependency graph of strata
           for the input morphology using breadth-first search. It loads
           morphologies and blobs on demand.'''

        if self.morph.kind == 'stratum':
            # turn the morphology into a stratum object
            stratum = self.create_blob(self.morph)

            # start the BFS at the input stratum
            queue = collections.deque()
            queue.append(stratum)
            self.blobs.add(stratum)
            
            while len(queue) > 0:
                stratum = queue.popleft()

                # the DFS recursion ends whenever we have a stratum
                # that depends on nothing else
                if not stratum.morph.build_depends:
                    continue
                
                # verify that the build-depends format is valid
                if isinstance(stratum.morph.build_depends, list):
                    for depname in stratum.morph.build_depends:
                        # prepare a tuple for the dependency stratum
                        repo = stratum.morph.repo
                        ref = stratum.morph.ref
                        filename = '%s.morph' % depname
                        info = (repo, ref, filename)

                        # load the dependency stratum on demand
                        depstratum = self._get_blob(info)

                        # add the dependency stratum to the graph
                        stratum.add_dependency(depstratum)
                        queue.append(depstratum)
                        self.blobs.add(depstratum)
                else:
                    raise Exception('%s uses an invalid "build-depends" format'
                                    % stratum)

    def _resolve_chunks(self):
        '''Starting with a dependency graph of strata, this method fills the
           graph with all contained chunks and creates dependencies where 
           appropriate. Chunk morphologies and blobs are loaded on demand.'''

        if self.morph.kind == 'chunk':
            blob = self.create_blob(self.morph)
            self.blobs.add(blob)

        blobs = list(self.blobs)
        for blob in blobs:
            if isinstance(blob, morphlib.blobs.Stratum):
                self._resolve_stratum_chunks(blob)

    def _resolve_stratum_chunks(self, stratum):
        # the set of chunks contained in the stratum
        stratum_chunks = set()

        # dictionary that maps chunk names to chunks
        name_to_chunk = {}

        # create objects for all chunks in the stratum
        for i in xrange(0, len(stratum.morph.sources)):
            source = stratum.morph.sources[i]

            # construct a tuple for loading the chunk
            repo = source['repo']
            ref = source['ref']
            filename = '%s.morph' % (source['morph']
                                     if 'morph' in source
                                     else source['name'])
            info = (repo, ref, filename)

            # load the chunk on demand
            chunk = self._get_blob(info, stratum)

            # store (name -> chunk) association to avoid loading the chunk twice
            name_to_chunk[source['name']] = chunk

            # read the build-depends information
            build_depends = (source['build-depends']
                             if 'build-depends' in source
                             else None)

            # turn build-depends into proper dependencies in the graph
            if build_depends is None:
                # chunks with no build-depends implicitly depend on all
                # chunks listed earlier in the same stratum
                for dependency in stratum_chunks:
                    chunk.add_dependency(dependency)
            elif isinstance(build_depends, list):
                for depname in build_depends:
                    if depname in name_to_chunk:
                        dependency = name_to_chunk[depname]
                        chunk.add_dependency(dependency)
                    else:
                        filename = os.path.basename(stratum.morph.filename)
                        raise Exception('%s: source %s references %s before it '
                                        'is defined' % (filename,
                                                        source['name'],
                                                        depname))
            else:
                filename = os.path.basename(stratum.morph.filename)
                raise Exception('%s: source %s uses an invalid build-depends '
                                'format' % (filename, source['name']))
            
            # add the chunk to stratum and graph
            stratum_chunks.add(chunk)
            self.blobs.add(chunk)

        # make the chunks in this stratum depend on all
        # strata that need to be built first
        for chunk in stratum_chunks:
            for dependency in stratum.dependencies:
                chunk.add_dependency(dependency)

        # clear the dependencies of the stratum
        stratum.dependencies = set()

        # make the stratum depend on all its chunks
        for chunk in stratum_chunks:
            stratum.add_dependency(chunk)

    def _compute_topological_sorting(self):
        '''Computes a topological sorting of the dependency graph. A topological
           sorting basically is the result of a series of breadth-first searches
           starting at each leaf node (blobs with no dependencies). Blobs are
           added to the sorting as soon as all their dependencies have been
           added (which means that by then, all dependencies are satisfied).

           http://en.wikipedia.org/wiki/Topological_sorting.'''

        # map blobs to sets of satisfied dependencies. this is to detect when
        # we can actually add blobs to the BFS queue. rather than dropping
        # links between nodes, like most topological sorting algorithms do,
        # we simply remember all satisfied dependencies and check if all
        # of them are met repeatedly
        satisfied_dependencies = {}

        # create an empty sorting
        sorting = collections.deque()

        # create a set of leafs to start the DFS from
        leafs = collections.deque()
        for blob in self.blobs:
            satisfied_dependencies[blob] = set()
            if len(blob.dependencies) == 0:
                leafs.append(blob)

        while len(leafs) > 0:
            # fetch a leaf blob from the DFS queue
            blob = leafs.popleft()

            # add it to the sorting
            sorting.append(blob)

            # mark this dependency as resolved
            for dependent in blob.dependents:
                satisfied_dependencies[dependent].add(blob)

                # add the dependent blob as a leaf if all
                # its dependencies have been resolved
                has = len(satisfied_dependencies[dependent])
                needs = len(dependent.dependencies)
                if has == needs:
                    leafs.append(dependent)

        # if not all dependencies were resolved on the way, we
        # have found at least one cyclic dependency
        if len(sorting) < len(self.blobs):
            raise Exception('Cyclic dependencies found in the dependency '
                            'graph of "%s"' % self.morph)

        return sorting