From aa580231e151550ce252ec4dcfdd6a5cba83bdc4 Mon Sep 17 00:00:00 2001
From: Muneeb Sheikh <muneikh@gmail.com>
Date: Sun, 11 Oct 2026 13:11:50 -0400
Subject: [PATCH] Reject negative deque limits before changing persisted data

---
 diskcache/persistent.py |  9 +++++++++
 tests/test_deque.py     | 26 ++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/diskcache/persistent.py b/diskcache/persistent.py
index 522bb74..a51df9d 100644
--- a/diskcache/persistent.py
+++ b/diskcache/persistent.py
@@ -83,8 +83,11 @@ def __init__(self, iterable=(), directory=None, maxlen=None):
 
         :param iterable: iterable of items to append to deque
         :param directory: deque directory (default None)
+        :raises ValueError: if maxlen is negative
 
         """
+        if maxlen is not None and maxlen < 0:
+            raise ValueError('maxlen must be non-negative')
         self._cache = Cache(directory, eviction_policy='none')
         self._maxlen = float('inf') if maxlen is None else maxlen
         self._extend(iterable)
@@ -107,8 +110,11 @@ def fromcache(cls, cache, iterable=(), maxlen=None):
         :param Cache cache: cache to use
         :param iterable: iterable of items
         :return: initialized Deque
+        :raises ValueError: if maxlen is negative
 
         """
+        if maxlen is not None and maxlen < 0:
+            raise ValueError('maxlen must be non-negative')
         # pylint: disable=no-member,protected-access
         self = cls.__new__(cls)
         self._cache = cache
@@ -144,8 +150,11 @@ def maxlen(self, value):
         ['c', 'd', 'e']
 
         :param value: max length
+        :raises ValueError: if value is negative
 
         """
+        if value is not None and value < 0:
+            raise ValueError('maxlen must be non-negative')
         self._maxlen = value
         with self._cache.transact(retry=True):
             while len(self._cache) > self._maxlen:
diff --git a/tests/test_deque.py b/tests/test_deque.py
index f997a86..e49c506 100644
--- a/tests/test_deque.py
+++ b/tests/test_deque.py
@@ -50,6 +50,32 @@ def test_init():
     rmdir(directory)
 
 
+@pytest.mark.parametrize('factory', ['constructor', 'fromcache'])
+def test_negative_maxlen_does_not_change_existing_data(tmp_path, factory):
+    deque = dc.Deque([1, 2, 3], directory=str(tmp_path / 'deque'))
+    try:
+        with pytest.raises(ValueError, match='maxlen'):
+            if factory == 'constructor':
+                dc.Deque([4], directory=deque.directory, maxlen=-1)
+            else:
+                dc.Deque.fromcache(deque.cache, [4], maxlen=-1)
+
+        assert list(deque) == [1, 2, 3]
+    finally:
+        deque.cache.close()
+
+
+def test_negative_maxlen_assignment_keeps_limit_and_data(deque):
+    deque.maxlen = 3
+    deque.extend([1, 2, 3])
+
+    with pytest.raises(ValueError, match='maxlen'):
+        deque.maxlen = -1
+
+    assert deque.maxlen == 3
+    assert list(deque) == [1, 2, 3]
+
+
 def test_getsetdel(deque):
     sequence = list('abcde')
     assert len(deque) == 0