[2.2.x] Refs #30179 -- Moved topological sort functions to django.utils.

Backport of e04209e181 from master.

django/db/migrations/autodetector.py

@@ -12,8 +12,7 @@ from django.db.migrations.questioner import MigrationQuestioner
 from django.db.migrations.utils import (
     COMPILED_REGEX_TYPE, RegexObject, get_migration_name_timestamp,
 )
-
-from .topological_sort import stable_topological_sort
+from django.utils.topological_sort import stable_topological_sort
 
 
 class MigrationAutodetector:

django/utils/topological_sort.py

@@ -1,3 +1,7 @@
+class CyclicDependencyError(ValueError):
+    pass
+
+
 def topological_sort_as_sets(dependency_graph):
     """
     Variation of Kahn's algorithm (1962) that returns sets.
@@ -13,7 +17,7 @@ def topological_sort_as_sets(dependency_graph):
         current = {node for node, deps in todo.items() if not deps}
 
         if not current:
-            raise ValueError('Cyclic dependency in graph: {}'.format(
+            raise CyclicDependencyError('Cyclic dependency in graph: {}'.format(
                 ', '.join(repr(x) for x in todo.items())))
 
         yield current

tests/utils_tests/test_topological_sort.py

@@ -0,0 +1,24 @@
+from django.test import SimpleTestCase
+from django.utils.topological_sort import (
+    CyclicDependencyError, stable_topological_sort, topological_sort_as_sets,
+)
+
+
+class TopologicalSortTests(SimpleTestCase):
+
+    def test_basic(self):
+        dependency_graph = {
+            1: {2, 3},
+            2: set(),
+            3: set(),
+            4: {5, 6},
+            5: set(),
+            6: {5},
+        }
+        self.assertEqual(list(topological_sort_as_sets(dependency_graph)), [{2, 3, 5}, {1, 6}, {4}])
+        self.assertEqual(stable_topological_sort([1, 2, 3, 4, 5, 6], dependency_graph), [2, 3, 5, 1, 6, 4])
+
+    def test_cyclic_dependency(self):
+        msg = 'Cyclic dependency in graph: '
+        with self.assertRaisesMessage(CyclicDependencyError, msg):
+            list(topological_sort_as_sets({1: {2}, 2: {1}}))