Improve coverage resolver (#7642)

* Improve test coverage for resolver.py

* Make the Twisted code more readable

scrapy/resolver.py

@@ -75,7 +75,7 @@ class HostResolution:
     def __init__(self, name: str):
         self.name: str = name
 
-    def cancel(self) -> None:
+    def cancel(self) -> None:  # pragma: no cover
         raise NotImplementedError
 
 

tests/test_resolver.py

@@ -0,0 +1,55 @@
+from __future__ import annotations
+
+from unittest.mock import Mock
+
+import pytest
+
+from scrapy.resolver import CachingHostnameResolver, CachingThreadedResolver, dnscache
+from scrapy.utils.defer import maybe_deferred_to_future
+from scrapy.utils.test import get_crawler
+from tests.utils.decorators import coroutine_test
+
+
+@pytest.fixture(autouse=True)
+def reset_dnscache():
+    original_limit = dnscache.limit
+    dnscache.clear()
+    yield
+    dnscache.clear()
+    dnscache.limit = original_limit
+
+
+def test_caching_threaded_resolver_dnscache_disabled():
+    crawler = get_crawler(settings_dict={"DNSCACHE_ENABLED": False})
+    CachingThreadedResolver.from_crawler(crawler, Mock())
+    assert dnscache.limit == 0
+
+
+@coroutine_test
+async def test_caching_threaded_resolver_getHostByName_cache_hit():
+    resolver = CachingThreadedResolver(Mock(), cache_size=10, timeout=5.0)
+    dnscache["example.com"] = "1.2.3.4"
+
+    result = await maybe_deferred_to_future(resolver.getHostByName("example.com"))
+    assert result == "1.2.3.4"
+
+
+def test_caching_hostname_resolver_dnscache_disabled():
+    crawler = get_crawler(settings_dict={"DNSCACHE_ENABLED": False})
+    CachingHostnameResolver.from_crawler(crawler, Mock())
+    assert dnscache.limit == 0
+
+
+def test_caching_hostname_resolver_no_addresses_not_cached():
+    def fake_resolve(receiver, *_):
+        receiver.resolutionBegan(Mock())
+        receiver.resolutionComplete()
+        return receiver
+
+    reactor = Mock()
+    reactor.nameResolver.resolveHostName.side_effect = fake_resolve
+
+    resolver = CachingHostnameResolver(reactor, cache_size=10)
+    resolver.resolveHostName(Mock(), "example.com")
+
+    assert "example.com" not in dnscache