dfs traversal: simplify edges in reverse mode (#33481)

In the dfs code, flip edges so that `parent` means `from` and
`spec` means `to` in the direction of traversal. This makes it slightly
easier to write generic/composable code. For example when using visitors
where one visitor reverses direction, and another only cares about
accepting particular edges or not depending on whether the target node
is seen before, it would be good if the second visitor didn't have to
know whether the order was changed or not.

lib/spack/spack/spec.py

@@ -734,6 +734,9 @@ def __str__(self):
     def canonical(self):
         return self.parent.dag_hash(), self.spec.dag_hash(), self.deptypes
 
+    def flip(self):
+        return DependencySpec(parent=self.spec, spec=self.parent, deptypes=self.deptypes)
+
 
 _valid_compiler_flags = ["cflags", "cxxflags", "fflags", "ldflags", "ldlibs", "cppflags"]
 
@@ -812,6 +815,10 @@ def _sort_by_dep_types(dspec):
     return tuple(t not in dspec.deptypes for t in ("link", "run", "build", "test"))
 
 
+def _sort_edges_by_pkg_name(edges):
+    edges.sort(key=lambda edge: edge.spec.name)
+
+
 #: Enum for edge directions
 EdgeDirection = lang.enum(parent=0, child=1)
 
@@ -1604,29 +1611,28 @@ def installed_upstream(self):
         return upstream
 
     def traverse(self, **kwargs):
-        direction = kwargs.get("direction", "children")
         depth = kwargs.get("depth", False)
 
-        get_spec = lambda s: s.spec
-        if direction == "parents":
-            get_spec = lambda s: s.parent
-
         if depth:
-            for d, dspec in self.traverse_edges(**kwargs):
-                yield d, get_spec(dspec)
+            for d, edge in self.traverse_edges(**kwargs):
+                yield d, edge.spec
         else:
-            for dspec in self.traverse_edges(**kwargs):
-                yield get_spec(dspec)
+            for edge in self.traverse_edges(**kwargs):
+                yield edge.spec
 
     def traverse_edges(self, visited=None, d=0, deptype="all", dep_spec=None, **kwargs):
         """Generic traversal of the DAG represented by this spec.
 
         This yields ``DependencySpec`` objects as they are traversed.
 
-        When traversing top-down, an imaginary incoming edge to the root
-        is yielded first as ``DependencySpec(None, root, ())``. When
-        traversing bottom-up, imaginary edges to leaves are yielded first
-        as ``DependencySpec(left, None, ())`` objects.
+        An imaginary incoming edge to the root is yielded first as
+        ``DependencySpec(None, root, ())``.
+
+        Note: the edges are traversed from ``edge.parent`` to ``edge.spec``,
+        even if the direction is reversed. When ``direction="children"`` the
+        parent points to the dependent, and spec to the dependency. Conversely
+        when ``direction="parents"`` parent points to the dependency, and spec
+        to the dependent.
 
         Options:
 
@@ -1711,10 +1717,7 @@ def validate(name, val, allowed_values):
         def return_val(dspec):
             if not dspec:
                 # make a fake dspec for the root.
-                if direction == "parents":
-                    dspec = DependencySpec(self, None, ())
-                else:
-                    dspec = DependencySpec(None, self, ())
+                dspec = DependencySpec(None, self, ())
             return (d, dspec) if depth else dspec
 
         yield_me = yield_root or d > 0
@@ -1729,22 +1732,18 @@ def return_val(dspec):
 
             # This code determines direction and yields the children/parents
             if direction == "children":
-                edges = self.edges_to_dependencies
-                key_fn = lambda dspec: dspec.spec.name
-                succ = lambda dspec: dspec.spec
-            elif direction == "parents":
-                edges = self.edges_from_dependents
-                key_fn = lambda dspec: dspec.parent.name
-                succ = lambda dspec: dspec.parent
+                edges = self.edges_to_dependencies()
             else:
-                raise ValueError("Invalid traversal direction: %s" % direction)
+                edges = [edge.flip() for edge in self.edges_from_dependents()]
 
-            for dspec in sorted(edges(), key=key_fn):
+            _sort_edges_by_pkg_name(edges)
+
+            for dspec in edges:
                 dt = dspec.deptypes
                 if dt and not any(d in deptype for d in dt):
                     continue
 
-                for child in succ(dspec).traverse_edges(visited, d + 1, deptype, dspec, **kwargs):
+                for child in dspec.spec.traverse_edges(visited, d + 1, deptype, dspec, **kwargs):
                     yield child
 
         # Postorder traversal yields after successors