[3.13] gh-121160: Add some tests for readline.set_history_length (GH-121326) (GH-121856)

(cherry picked from commit 263c7e611bb24715e513d457a3477a61fff15162)

Co-authored-by: Petr Viktorin <encukou@gmail.com>

2 files changed, 48 insertions, 0 deletions

diff --git a/Lib/test/test_readline.py b/Lib/test/test_readline.py
index 5e0e6f8..91fd7dd 100644
--- a/Lib/test/test_readline.py
+++ b/Lib/test/test_readline.py
@@ -132,6 +132,32 @@ class TestHistoryManipulation (unittest.TestCase):
         self.assertEqual(readline.get_history_item(1), "entrée 1")
         self.assertEqual(readline.get_history_item(2), "entrée 22")
 
+    def test_write_read_limited_history(self):
+        previous_length = readline.get_history_length()
+        self.addCleanup(readline.set_history_length, previous_length)
+
+        readline.clear_history()
+        readline.add_history("first line")
+        readline.add_history("second line")
+        readline.add_history("third line")
+
+        readline.set_history_length(2)
+        self.assertEqual(readline.get_history_length(), 2)
+        readline.write_history_file(TESTFN)
+        self.addCleanup(os.remove, TESTFN)
+
+        readline.clear_history()
+        self.assertEqual(readline.get_current_history_length(), 0)
+        self.assertEqual(readline.get_history_length(), 2)
+
+        readline.read_history_file(TESTFN)
+        self.assertEqual(readline.get_history_item(1), "second line")
+        self.assertEqual(readline.get_history_item(2), "third line")
+        self.assertEqual(readline.get_history_item(3), None)
+
+        # Readline seems to report an additional history element.
+        self.assertIn(readline.get_current_history_length(), (2, 3))
+
 
 class TestReadline(unittest.TestCase):
 
@@ -323,6 +349,26 @@ readline.write_history_file(history_file)
             self.assertEqual(len(lines), history_size)
             self.assertEqual(lines[-1].strip(), b"last input")
 
+    def test_write_read_limited_history(self):
+        previous_length = readline.get_history_length()
+        self.addCleanup(readline.set_history_length, previous_length)
+
+        readline.add_history("first line")
+        readline.add_history("second line")
+        readline.add_history("third line")
+
+        readline.set_history_length(2)
+        self.assertEqual(readline.get_history_length(), 2)
+        readline.write_history_file(TESTFN)
+        self.addCleanup(os.remove, TESTFN)
+
+        readline.read_history_file(TESTFN)
+        # Without clear_history() there's no good way to test if
+        # the correct entries are present (we're combining history limiting and
+        # possible deduplication with arbitrary previous content).
+        # So, we've only tested that the read did not fail.
+        # See TestHistoryManipulation for the full test.
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Tests/2024-07-03-14-41-00.gh-issue-121160.LEtiTd.rst b/Misc/NEWS.d/next/Tests/2024-07-03-14-41-00.gh-issue-121160.LEtiTd.rst
new file mode 100644
index 0000000..2c8c9ac
--- /dev/null
+++ b/Misc/NEWS.d/next/Tests/2024-07-03-14-41-00.gh-issue-121160.LEtiTd.rst
@@ -0,0 +1,2 @@
+Add a test for :func:`readline.set_history_length`. Note that this test may
+fail on readline libraries.