Allow deque length limits to be removed consistently

Constraint: Preserve existing supported Python syntax and public API behavior.
Confidence: high
Scope-risk: narrow
Tested: 180 core and persistent tests passed; real database reopen and bounded/unbounded transitions
Not-tested: Other Python versions and operating systems.

Author: oyeong011

diskcache/persistent.py

@@ -136,6 +136,7 @@ def maxlen(self, value):
         """Set max length of the deque.
 
         Pops items from left while length greater than max.
+        Set to None to remove the length limit.
 
         >>> deque = Deque()
         >>> deque.extendleft('abcde')
@@ -146,7 +147,7 @@ def maxlen(self, value):
         :param value: max length
 
         """
-        self._maxlen = value
+        self._maxlen = float('inf') if value is None else value
         with self._cache.transact(retry=True):
             while len(self._cache) > self._maxlen:
                 self._popleft()

tests/test_deque.py

@@ -312,3 +312,14 @@ def test_peek(deque):
     assert len(deque) == 1
     assert deque.peek() == value
     assert len(deque) == 1
+
+
+def test_maxlen_can_be_reset_to_unlimited(tmp_path):
+    with dc.Cache(str(tmp_path), eviction_policy='none') as cache:
+        deque = dc.Deque.fromcache(cache, 'abc', maxlen=3)
+
+        deque.maxlen = None
+
+        deque.extend('def')
+        assert list(deque) == list('abcdef')
+        assert deque.maxlen == float('inf')