Make sure every node is in the values returned from links_and_reverse_links_dict (#827)

Because we're not returning defaultdicts from that function (it's
cached), it's a bit annoying for consumers as they have to `.get`
everything out of it. By ensuring that every node is present in the
returned value, consumers can safely use `[]` again.

Author: Eijebong

src/taskgraph/graph.py

@@ -83,7 +83,7 @@ def _visit(self, reverse):
         dependencies = reverse_links if reverse else forward_links
         dependents = forward_links if reverse else reverse_links
 
-        indegree = {node: len(dependencies.get(node, ())) for node in self.nodes}
+        indegree = {node: len(dependencies[node]) for node in self.nodes}
 
         queue = collections.deque(
             node for node, degree in indegree.items() if degree == 0
@@ -93,7 +93,7 @@ def _visit(self, reverse):
             node = queue.popleft()
             yield node
 
-            for dependent in dependents.get(node, ()):
+            for dependent in dependents[node]:
                 indegree[dependent] -= 1
                 if indegree[dependent] == 0:
                     queue.append(dependent)
@@ -128,8 +128,8 @@ def links_and_reverse_links_dict(self):
         `reverse_links_dict` counterparts to avoid consumers modifying the
         cached value by mistake.
         """
-        forward = collections.defaultdict(set)
-        reverse = collections.defaultdict(set)
+        forward = {node: set() for node in self.nodes}
+        reverse = {node: set() for node in self.nodes}
         for left, right, _ in self.edges:
             forward[left].add(right)
             reverse[right].add(left)