Fix PermissionError when loading .netrc (aio-libs#7237) (aio-libs#7378)

## What do these changes do?

If no NETRC environment variable is provided and the .netrc path cannot
be accessed due to missing permission, a PermissionError was raised
instead of returning None. See issue aio-libs#7237. This PR fixes the issue.

If the changes look good, I can also prepare backports.

## Are there changes in behavior for the user?

If the .netrc cannot be accessed due to a permission problem (and the
`NETRC` environment variable is unset), no `PermissionError` will be
raised. Instead it will be silently ignored.

## Related issue number

Fixes aio-libs#7237

(cherry picked from commit 0d2e43b)

# Conflicts:
#	CONTRIBUTORS.txt
#	aiohttp/helpers.py
#	tests/test_helpers.py

CHANGES/7237.bugfix

@@ -0,0 +1 @@
+Fixed ``PermissionError`` when .netrc is unreadable due to permissions.

CONTRIBUTORS.txt

@@ -153,6 +153,7 @@ Jake Davis
 Jakob Ackermann
 Jakub Wilk
 Jan Buchar
+Jan Gosmann
 Jashandeep Sohi
 Jens Steinhauser
 Jeonghun Lee

aiohttp/helpers.py

@@ -3,6 +3,7 @@
 import asyncio
 import base64
 import binascii
+import contextlib
 import datetime
 import functools
 import inspect
@@ -226,8 +227,11 @@ def netrc_from_env() -> Optional[netrc.netrc]:
     except netrc.NetrcParseError as e:
         client_logger.warning("Could not parse .netrc file: %s", e)
     except OSError as e:
+        netrc_exists = False
+        with contextlib.suppress(OSError):
+            netrc_exists = netrc_path.is_file()
         # we couldn't read the file (doesn't exist, permissions, etc.)
-        if netrc_env or netrc_path.is_file():
+        if netrc_env or netrc_exists:
             # only warn if the environment wanted us to load it,
             # or it appears like the default file does actually exist
             client_logger.warning("Could not read .netrc file: %s", e)
@@ -742,7 +746,6 @@ def ceil_timeout(delay: Optional[float]) -> async_timeout.Timeout:
 
 
 class HeadersMixin:
-
     ATTRS = frozenset(["_content_type", "_content_dict", "_stored_content_type"])
 
     _content_type: Optional[str] = None

tests/test_helpers.py

@@ -5,6 +5,7 @@
 import platform
 import tempfile
 from math import isclose, modf
+from pathlib import Path
 from unittest import mock
 from urllib.request import getproxies_environment
 
@@ -178,7 +179,6 @@ def test_basic_auth_from_not_url() -> None:
 
 
 class ReifyMixin:
-
     reify = NotImplemented
 
     def test_reify(self) -> None:
@@ -763,3 +763,40 @@ def test_repr(self) -> None:
 )
 def test_parse_http_date(value, expected):
     assert parse_http_date(value) == expected
+
+
+@pytest.mark.parametrize(
+    ["netrc_contents", "expected_username"],
+    [
+        (
+            "machine example.com login username password pass\n",
+            "username",
+        ),
+    ],
+    indirect=("netrc_contents",),
+)
+@pytest.mark.usefixtures("netrc_contents")
+def test_netrc_from_env(expected_username: str):
+    """Test that reading netrc files from env works as expected"""
+    netrc_obj = helpers.netrc_from_env()
+    assert netrc_obj.authenticators("example.com")[0] == expected_username
+
+
+@pytest.fixture
+def protected_dir(tmp_path: Path):
+    protected_dir = tmp_path / "protected"
+    protected_dir.mkdir()
+    try:
+        protected_dir.chmod(0o600)
+        yield protected_dir
+    finally:
+        protected_dir.rmdir()
+
+
+def test_netrc_from_home_does_not_raise_if_access_denied(
+    protected_dir: Path, monkeypatch: pytest.MonkeyPatch
+):
+    monkeypatch.setattr(Path, "home", lambda: protected_dir)
+    monkeypatch.delenv("NETRC", raising=False)
+
+    helpers.netrc_from_env()