Export tasks in a deterministic order (#3549)

fix issue #3527

test/CMakeLists.txt

@@ -170,6 +170,7 @@ set (pythonTests
   tw-2575.test.py
   tw-262.test.py
   tw-295.test.py
+  tw-3527.test.py
   uda.test.py
   uda_orphan.test.py
   uda_report.test.py

test/tw-3527.test.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+
+import sys
+import os
+import unittest
+import re
+import json
+import string
+
+# Ensure python finds the local simpletap module
+sys.path.append(os.path.dirname(os.path.abspath(__file__)))
+
+from basetest import Task, TestCase
+
+
+class TestExport(TestCase):
+    def setUp(self):
+        self.t = Task()
+
+        # pretty arbitrary, just need several unique tasks
+        for letter in string.ascii_lowercase:
+            self.t(f"add test_task +{letter}")
+            self.t(f"+{letter} done")
+
+    def test_export_stability_for_multiple_id_0(self):
+        exports = [self.t("export")[1] for _ in range(2)]
+        json_lists = [json.loads(s.strip()) for s in exports]
+        # to rule out a typo causing two failed exports
+        self.assertEqual(len(json_lists[0]), len(string.ascii_lowercase))
+        # for better diff view
+        self.assertEqual(json_lists[0], json_lists[1])
+        # the real test
+        self.assertEqual(exports[0], exports[1])
+
+
+if __name__ == "__main__":
+    from simpletap import TAPTestRunner
+    unittest.main(testRunner=TAPTestRunner())
+