fix: LocalCache with limit=0 incorrectly stores items

When limit=0 is passed to LocalCache (e.g. when DNSCACHE_ENABLED=False),
the condition 'if self.limit' evaluates to False due to Python's truthiness
rules, causing items to be stored despite the cache being disabled.

This leads to an unbounded memory leak during long crawls when DNS caching
is explicitly disabled.

Fix changes the condition to 'if self.limit is not None' and adds an early
return when limit=0 to correctly handle the disabled cache case.

scrapy/utils/datatypes.py

@@ -152,7 +152,9 @@ class LocalCache(OrderedDict[_KT, _VT]):
         self.limit: int | None = limit
 
     def __setitem__(self, key: _KT, value: _VT) -> None:
-        if self.limit:
+        if self.limit is not None:
+            if self.limit == 0:
+                return
             while len(self) >= self.limit:
                 self.popitem(last=False)
         super().__setitem__(key, value)

tests/test_utils_datatypes.py

@@ -309,6 +309,17 @@ class TestLocalCache:
             assert str(x) in cache
             assert cache[str(x)] == x
 
+    def test_cache_with_zero_limit(self):
+        # limit=0 means cache is disabled, no items should be stored
+        cache = LocalCache(limit=0)
+        cache["a"] = 1
+        cache["b"] = 2
+        cache["c"] = 3
+        assert len(cache) == 0
+        assert "a" not in cache
+        assert "b" not in cache
+        assert "c" not in cache
+
 
 class TestLocalWeakReferencedCache:
     def test_cache_with_limit(self):