Make XModuleDescriptor hashable again (#22501)

common/lib/xmodule/xmodule/x_module.py

@@ -1263,6 +1263,17 @@ class XModuleDescriptor(XModuleDescriptorToXBlockMixin, HTMLSnippet, ResourceTem
                 all(getattr(self, field.name) == getattr(other, field.name)
                     for field in self.fields.values()))
 
+    def __hash__(self):  # pylint: disable=useless-super-delegation
+        """
+        This isn't technically appropriate since descriptors are actually mutable,
+        but in practice we rarely modify them after creation or instantiate two
+        equivalent descriptors in the same process.  And we perform graph
+        operations on large collections of XBlocks that have simply unacceptable
+        performance if we have to rely on lists and equality rather than sets,
+        dictionaries, and identity-based hash functions.
+        """
+        return super(XModuleDescriptor, self).__hash__()
+
     def __repr__(self):
         return (
             "{0.__class__.__name__}("

openedx/core/lib/graph_traversals.py

@@ -127,7 +127,7 @@ def traverse_topologically(
     used to limit which nodes are actually yielded.
 
     Arguments:
-        start_node (any type) - The starting node for the
+        start_node (any hashable type) - The starting node for the
         traversal.
 
         get_parents (node->[node]) - Function that returns a list of
@@ -204,7 +204,7 @@ def traverse_post_order(start_node, get_children, filter_func=None):
     stack = deque([_Node(start_node, get_children)])
 
     # Keep track of which nodes have been visited.
-    visited = []
+    visited = set()
 
     while stack:
         # Peek at the current node at the top of the stack.
@@ -226,7 +226,7 @@ def traverse_post_order(start_node, get_children, filter_func=None):
             # Since there are no children left, visit the node and
             # remove it from the stack.
             yield current.node
-            visited.append(current.node)
+            visited.add(current.node)
             stack.pop()
 
         else:
@@ -264,8 +264,7 @@ def _traverse_generic(
 
     # Keep track of which nodes have been visited and whether they
     # were in fact yielded.
-    visited = []  # nodes
-    yield_results = []  # booleans
+    yield_results = {}  # dict(node:boolean)
 
     # While there are more nodes on the stack...
     while stack:
@@ -281,17 +280,16 @@ def _traverse_generic(
             parents = get_parents(current_node)
 
             # If all of the parents have not yet been visited, continue.
-            if not all(parent in visited for parent in parents):
+            if not all(parent in yield_results for parent in parents):
                 continue
 
             # If none of the parents have yielded, continue, unless
             # specified otherwise (via yield_descendants_of_unyielded).
-            elif not yield_descendants_of_unyielded and not any(
-                    yield_results[visited.index(parent)] for parent in parents):
+            elif not yield_descendants_of_unyielded and not any(yield_results[parent] for parent in parents):
                 continue
 
         # If the current node has already been visited, continue.
-        if current_node not in visited:
+        if current_node not in yield_results:
 
             # For a topological sort, it's important that we visit
             # the children even if the parent isn't yielded, in case
@@ -318,7 +316,7 @@ def _traverse_generic(
                 unvisited_children = list(
                     child
                     for child in get_children(current_node)
-                    if child not in visited
+                    if child not in yield_results
                 )
 
             # Add the node's unvisited children to the stack in reverse
@@ -334,5 +332,4 @@ def _traverse_generic(
 
             # Keep track of whether or not the node was yielded so we
             # know whether or not to yield its children.
-            visited.append(current_node)
-            yield_results.append(should_yield_node)
+            yield_results[current_node] = should_yield_node