From e04209e181c99ac16ca769d115ac640015a83757 Mon Sep 17 00:00:00 2001
From: Matthias Kestenholz <mk@feinheit.ch>
Date: Sun, 24 Feb 2019 13:08:59 +0100
Subject: [PATCH] Refs #30179 -- Moved topological sort functions to
 django.utils.

---
 django/db/migrations/autodetector.py          |  3 +--
 .../migrations => utils}/topological_sort.py  |  6 ++++-
 tests/utils_tests/test_topological_sort.py    | 24 +++++++++++++++++++
 3 files changed, 30 insertions(+), 3 deletions(-)
 rename django/{db/migrations => utils}/topological_sort.py (88%)
 create mode 100644 tests/utils_tests/test_topological_sort.py

diff --git a/django/db/migrations/autodetector.py b/django/db/migrations/autodetector.py
index 3d3eeb9230b..0dc1c77c53b 100644
--- a/django/db/migrations/autodetector.py
+++ b/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:
diff --git a/django/db/migrations/topological_sort.py b/django/utils/topological_sort.py
similarity index 88%
rename from django/db/migrations/topological_sort.py
rename to django/utils/topological_sort.py
index e0a22c92362..3f8ea0f2e4e 100644
--- a/django/db/migrations/topological_sort.py
+++ b/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
diff --git a/tests/utils_tests/test_topological_sort.py b/tests/utils_tests/test_topological_sort.py
new file mode 100644
index 00000000000..25e0cf4ce99
--- /dev/null
+++ b/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: (1, {2}), (2, {1})'
+        with self.assertRaisesMessage(CyclicDependencyError, msg):
+            list(topological_sort_as_sets({1: {2}, 2: {1}}))