Cycle detection and topological sorting

base/task.py

@@ -1,7 +1,6 @@
-from functools import total_ordering
+from common.exceptions import TaskListError
 
 
-@total_ordering
 class Task(object):
 	description = None
 	
@@ -42,15 +41,10 @@ class Task(object):
 				msg = ("The task {self} is specified as running before {other}, "
 				       "but its phase {phase} lies after the phase {other_phase}"
 				       .format(self, other, self.phase, other.phase))
-				raise TaskOrderError(msg)
+				raise TaskListError(msg)
 		for task in self.after:
 			if self.phase < task.phase:
 				msg = ("The task {self} is specified as running after {other}, "
 				       "but its phase {phase} lies before the phase {other_phase}"
 				       .format(self=self, other=other, phase=self.phase, other_phase=other.phase))
-				raise TaskOrderError(msg)
-		conflict = next(iter(set(self.before) &  set(self.after)), None)
-		if conflict is not None:
-				msg = ("The task {self} is specified as running both before and after {conflict}"
-				       .format(self=self, conflict=conflict))
-				raise TaskOrderError(msg)
+				raise TaskListError(msg)

base/tasklist.py

@@ -1,3 +1,4 @@
+from common.exceptions import TaskListError
 import logging
 log = logging.getLogger(__name__)
 
@@ -17,14 +18,89 @@ class TaskList(object):
 		self.remove(task)
 		self.add(replacement)
 
-	def get(self, task):
+	def get(self, ref):
 		return next(task for task in self.tasks if type(task) is ref)
 
 	def run(self, bootstrap_info):
-		log.debug('Tasklist before:\n{list}'.format(list=',\n'.join(str(task) for task in self.tasks) ))
-		task_list = sorted(self.tasks)
-		log.debug('Tasklist:\n{list}'.format(list=',\n'.join(str(task) for task in task_list) ))
-		return
+		task_list = self.create_list()
 		for task in tasks:
 			log.info(task)
 			task.run(bootstrap_info)
+
+	def create_list(self):
+		graph = {}
+		for task in self.tasks:
+			graph[task] = [self.get(succ) for succ in task.before]
+			graph[task].extend([succ for succ in self.tasks if type(task) in succ.after])
+
+		components = self.strongly_connected_components(graph)
+		cycles_found = 0
+		for component in components:
+			if len(component) > 1:
+				cycles_found += 1
+				log.debug('Cycle: {list}\n'.format(list=', '.join(str(task) for task in component)))
+		if cycles_found > 0:
+			msg = ('{0} cycles were found in the tasklist, '
+			       'consult the logfile for more information.'.format(cycles_found))
+			raise TaskListError(msg)
+
+		sorted_tasks = self.topological_sort(graph)
+		log.debug('Tasklist:\n\t{list}\n'.format(list='\n\t'.join(str(task) for task in sorted_tasks)))
+
+
+	def strongly_connected_components(self, graph):
+		# Source: http://www.logarithmic.net/pfh-files/blog/01208083168/sort.py
+		# Find the strongly connected components in a graph using Tarjan's algorithm.
+		# graph should be a dictionary mapping node names to lists of successor nodes.
+
+		result = [ ]
+		stack = [ ]
+		low = { }
+
+		def visit(node):
+			if node in low: return
+
+			num = len(low)
+			low[node] = num
+			stack_pos = len(stack)
+			stack.append(node)
+
+			for successor in graph[node]:
+				# print successor
+				visit(successor)
+				low[node] = min(low[node], low[successor])
+
+			if num == low[node]:
+				component = tuple(stack[stack_pos:])
+				del stack[stack_pos:]
+				result.append(component)
+				for item in component:
+					low[item] = len(graph)
+
+		for node in graph:
+			visit(node)
+
+		return result
+
+	def topological_sort(self, graph):
+		# Source: http://www.logarithmic.net/pfh-files/blog/01208083168/sort.py
+		count = { }
+		for node in graph:
+			count[node] = 0
+		for node in graph:
+			for successor in graph[node]:
+				count[successor] += 1
+
+		ready = [ node for node in graph if count[node] == 0 ]
+
+		result = [ ]
+		while ready:
+			node = ready.pop(-1)
+			result.append(node)
+
+			for successor in graph[node]:
+				count[successor] -= 1
+				if count[successor] == 0:
+					ready.append(successor)
+
+		return result

common/exceptions.py

@@ -9,9 +9,8 @@ class ManifestError(Exception):
 		return "Error in manifest {0}: {1}".format(self.manifest.path, self.message)
 
 
-class TaskOrderError(Exception):
-	def __init__(self, message, task):
+class TaskListError(Exception):
+	def __init__(self, message):
 		self.message = message
-		self.task = task
 	def __str__(self):
-		return "Error in task order: {1}".format(self.message)
+		return "Error in tasklist: {0}".format(self.message)