Cover ttl=0 (expire always) _DNSCacheTable case (aio-libs#7014)

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## What do these changes do?
Fix bug when using ttl_dns_cache=0

<!-- Please give a short brief about these changes. -->
When using `ttl_dns_cache=0`, hostname is added to cache and on expire
check there is a KeyError as it is never added into _timestamp
dictionary.
Added a short description to docs ClientSession advising to use
`use_dns_cache` instead of `ttl_dns_cache=0` for always expiring cache.

Alternative to this change is to enforce user to use positive none 0
float or None for `ttl_dns_cache` during init time and in case of using
0 implicitly change `use_dns_cache` to False or raise exception.
## Are there changes in behavior for the user?
No.

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## Related issue number

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change? -->

## Checklist

- [X] I think the code is well written
- [X] Unit tests for the changes exist
- [X] Documentation reflects the changes
- [X] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`
  * The format is &lt;Name&gt; &lt;Surname&gt;.
  * Please keep alphabetical order, the file is sorted by names.
- [X] Add a new news fragment into the `CHANGES` folder
  * name it `<issue_id>.<type>` for example (588.bugfix)
* if you don't have an `issue_id` change it to the pr id after creating
the pr
  * ensure type is one of the following:
    * `.feature`: Signifying a new feature.
    * `.bugfix`: Signifying a bug fix.
    * `.doc`: Signifying a documentation improvement.
    * `.removal`: Signifying a deprecation or removal of public API.
* `.misc`: A ticket has been closed, but it is not of interest to users.
* Make sure to use full sentences with correct case and punctuation, for
example: "Fix issue with non-ascii contents in doctest text files."

Co-authored-by: Sam Bull <aa6bs0@sambull.org>

CHANGES/7014.bugfix

@@ -0,0 +1 @@
+Fixed bug when using ``TCPConnector`` with ``ttl_dns_cache=0``.

CONTRIBUTORS.txt

@@ -153,6 +153,7 @@ Illia Volochii
 Ilya Chichak
 Ilya Gruzinov
 Ingmar Steen
+Ivan Lakovic
 Ivan Larin
 Jacob Champion
 Jaesung Lee

aiohttp/connector.py

@@ -678,13 +678,13 @@ def __contains__(self, host: object) -> bool:
     def add(self, key: Tuple[str, int], addrs: List[Dict[str, Any]]) -> None:
         self._addrs_rr[key] = (cycle(addrs), len(addrs))
 
-        if self._ttl:
+        if self._ttl is not None:
             self._timestamps[key] = monotonic()
 
     def remove(self, key: Tuple[str, int]) -> None:
         self._addrs_rr.pop(key, None)
 
-        if self._ttl:
+        if self._ttl is not None:
             self._timestamps.pop(key, None)
 
     def clear(self) -> None:

tests/test_connector.py

@@ -2179,10 +2179,27 @@ def test_not_expired_ttl(self) -> None:
         dns_cache_table.add("localhost", ["127.0.0.1"])
         assert not dns_cache_table.expired("localhost")
 
-    async def test_expired_ttl(self, loop: Any) -> None:
-        dns_cache_table = _DNSCacheTable(ttl=0.01)
+    def test_expired_ttl(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        dns_cache_table = _DNSCacheTable(ttl=1)
+        monkeypatch.setattr("aiohttp.connector.monotonic", lambda: 1)
         dns_cache_table.add("localhost", ["127.0.0.1"])
-        await asyncio.sleep(0.02)
+        monkeypatch.setattr("aiohttp.connector.monotonic", lambda: 2)
+        assert not dns_cache_table.expired("localhost")
+        monkeypatch.setattr("aiohttp.connector.monotonic", lambda: 3)
+        assert dns_cache_table.expired("localhost")
+
+    def test_never_expire(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        dns_cache_table = _DNSCacheTable(ttl=None)
+        monkeypatch.setattr("aiohttp.connector.monotonic", lambda: 1)
+        dns_cache_table.add("localhost", ["127.0.0.1"])
+        monkeypatch.setattr("aiohttp.connector.monotonic", lambda: 10000000)
+        assert not dns_cache_table.expired("localhost")
+
+    def test_always_expire(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        dns_cache_table = _DNSCacheTable(ttl=0)
+        monkeypatch.setattr("aiohttp.connector.monotonic", lambda: 1)
+        dns_cache_table.add("localhost", ["127.0.0.1"])
+        monkeypatch.setattr("aiohttp.connector.monotonic", lambda: 1.00001)
         assert dns_cache_table.expired("localhost")
 
     def test_next_addrs(self, dns_cache_table: Any) -> None: