Fix to LocalCache with limit=0 still stores items rather than disabling the cache (#7663)

* Fix cell-var-from-loop in _send_catch_log_deferred

Replace lambda capturing receiver by reference with a default
argument to capture it by value, fixing a potential bug where
all deferred callbacks could reference the last receiver in the
loop instead of their respective receivers.

Removes the pylint disable comment and TODO that were suppressing
this issue.

* chore: trigger CI rerun for mypy network error

* Fix mypy error: pass receiver via addBoth args instead of lambda default

* style: apply pre-commit ruff formatting

* 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.

* test: add resolver-level regression tests for DNSCACHE_ENABLED=False

Add two regression tests that verify DNS results are not stored in
dnscache when DNSCACHE_ENABLED=False (cache_size=0):

- test_caching_hostname_resolver_dnscache_disabled_rejects_storage:
  verifies _CachingResolutionReceiver does not write to dnscache
  when CachingHostnameResolver is initialized with cache_size=0

- test_caching_threaded_resolver_dnscache_disabled_rejects_storage:
  verifies dnscache rejects storage at the LocalCache level
  when limit=0

* test: drop misleading threaded resolver test

it was writing directly to dnscache, not actually going through
CachingThreadedResolver at all. the threaded resolver already has
its own if dnscache.limit: guard so the bug doesnt affect it anyway.

keeping only the hostname resolver test which covers the actual
bug path through _CachingResolutionReceiver

* test: clean up comments in resolver test

* test: remove unnecessary comment

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_resolver.py

@@ -53,3 +53,21 @@ def test_caching_hostname_resolver_no_addresses_not_cached():
     resolver.resolveHostName(Mock(), "example.com")
 
     assert "example.com" not in dnscache
+
+
+def test_caching_hostname_resolver_dnscache_disabled_rejects_storage():
+
+    def fake_resolve(receiver, *_):
+        receiver.resolutionBegan(Mock())
+        receiver.addressResolved(Mock())
+        receiver.resolutionComplete()
+        return receiver
+
+    reactor = Mock()
+    reactor.nameResolver.resolveHostName.side_effect = fake_resolve
+
+    resolver = CachingHostnameResolver(reactor, cache_size=0)
+    resolver.resolveHostName(Mock(), "example.com")
+
+    assert "example.com" not in dnscache
+    assert len(dnscache) == 0

tests/test_utils_datatypes.py

@@ -309,6 +309,16 @@ class TestLocalCache:
             assert str(x) in cache
             assert cache[str(x)] == x
 
+    def test_cache_with_zero_limit(self):
+        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):