Don't allow mutating operations on the underlying graph

Instead of allowing a direct graph return of the underlying
graph_flow graph we should return a frozen version instead so that
users of the returned value can not mutate the graph without going
through the graph_flow pattern (which could have undesired and
harmful side-effects if this occurs).

Change-Id: I38b35119d6e7bd7387b8ab467eba53aee5500629

taskflow/patterns/graph_flow.py

@@ -18,6 +18,7 @@
 
 import collections
 
+import networkx as nx
 from networkx.algorithms import dag
 from networkx.classes import digraph
 
@@ -36,25 +37,47 @@ class Flow(flow.Flow):
 
     def __init__(self, name, uuid=None):
         super(Flow, self).__init__(name, uuid)
-        self._graph = digraph.DiGraph()
+        self._graph = nx.freeze(digraph.DiGraph())
+
+    def _validate(self, graph=None):
+        if graph is None:
+            graph = self._graph
+        # Ensure that there is a valid topological ordering.
+        if not dag.is_directed_acyclic_graph(graph):
+            raise exc.DependencyFailure("No path through the items in the"
+                                        " graph produces an ordering that"
+                                        " will allow for correct dependency"
+                                        " resolution")
 
     def link(self, u, v):
         if not self._graph.has_node(u):
             raise ValueError('Item %s not found to link from' % (u))
         if not self._graph.has_node(v):
             raise ValueError('Item %s not found to link to' % (v))
-        self._graph.add_edge(u, v)
+        if self._graph.has_edge(u, v):
+            return self
 
-        # Ensure that there is a valid topological ordering.
-        if not dag.is_directed_acyclic_graph(self._graph):
-            self._graph.remove_edge(u, v)
-            raise exc.DependencyFailure("No path through the items in the"
-                                        " graph produces an ordering that"
-                                        " will allow for correct dependency"
-                                        " resolution")
+        # NOTE(harlowja): Add an edge to a temporary copy and only if that
+        # copy is valid then do we swap with the underlying graph.
+        tmp_graph = digraph.DiGraph(self._graph)
+        tmp_graph.add_edge(u, v)
+        self._swap(tmp_graph)
+        return self
+
+    def _swap(self, replacement_graph):
+        """Validates the replacement graph and then swaps the underlying graph
+        with a frozen version of the replacement graph (this maintains the
+        invariant that the underlying graph is immutable).
+        """
+        self._validate(replacement_graph)
+        self._graph = nx.freeze(replacement_graph)
 
     def add(self, *items):
         """Adds a given task/tasks/flow/flows to this flow."""
+        items = [i for i in items if not self._graph.has_node(i)]
+        if not items:
+            return self
+
         requirements = collections.defaultdict(list)
         provided = {}
 
@@ -67,42 +90,42 @@ class Flow(flow.Flow):
             for value in node.provides:
                 provided[value] = node
 
-        try:
-            for item in items:
-                self._graph.add_node(item)
-                update_requirements(item)
-                for value in item.provides:
-                    if value in provided:
-                        raise exc.DependencyFailure(
-                            "%(item)s provides %(value)s but is already being"
-                            " provided by %(flow)s and duplicate producers"
-                            " are disallowed"
-                            % dict(item=item.name,
-                                   flow=provided[value].name,
-                                   value=value))
-                    provided[value] = item
+        # NOTE(harlowja): Add items and edges to a temporary copy of the
+        # underlying graph and only if that is successful added to do we then
+        # swap with the underlying graph.
+        tmp_graph = digraph.DiGraph(self._graph)
+        for item in items:
+            tmp_graph.add_node(item)
+            update_requirements(item)
+            for value in item.provides:
+                if value in provided:
+                    raise exc.DependencyFailure(
+                        "%(item)s provides %(value)s but is already being"
+                        " provided by %(flow)s and duplicate producers"
+                        " are disallowed"
+                        % dict(item=item.name,
+                               flow=provided[value].name,
+                               value=value))
+                provided[value] = item
 
-                for value in item.requires:
-                    if value in provided:
-                        self.link(provided[value], item)
+            for value in item.requires:
+                if value in provided:
+                    tmp_graph.add_edge(provided[value], item)
 
-                for value in item.provides:
-                    if value in requirements:
-                        for node in requirements[value]:
-                            self.link(item, node)
-
-        except Exception:
-            self._graph.remove_nodes_from(items)
-            raise
+            for value in item.provides:
+                if value in requirements:
+                    for node in requirements[value]:
+                        tmp_graph.add_edge(item, node)
 
+        self._swap(tmp_graph)
         return self
 
     def __len__(self):
         return self._graph.number_of_nodes()
 
     def __iter__(self):
-        for child in self._graph.nodes_iter():
-            yield child
+        for n in self._graph.nodes_iter():
+            yield n
 
     @property
     def provides(self):