cacheprovider: use warnings directly

Allows for filtering of PytestCacheWarning.

Using `_issue_warning_captured` is not necessary here, and was probably
only used because the cacheprovider misses warnings during
`pytest_sessionfinish`, which is also fixed here.

I think the usage of `_issue_warning_captured` can be removed/reduced
further, but also that this is good enough for now.

Ref: pytest-dev#6681.

src/_pytest/cacheprovider.py

@@ -71,10 +71,10 @@ def cache_dir_from_config(config):
         return resolve_from_str(config.getini("cache_dir"), config.rootdir)
 
     def warn(self, fmt, **args):
-        from _pytest.warnings import _issue_warning_captured
+        import warnings
         from _pytest.warning_types import PytestCacheWarning
 
-        _issue_warning_captured(
+        warnings.warn(
             PytestCacheWarning(fmt.format(**args) if args else fmt),
             self._config.hook,
             stacklevel=3,

src/_pytest/warnings.py

@@ -136,6 +136,15 @@ def pytest_terminal_summary(terminalreporter):
         yield
 
 
+@pytest.hookimpl(hookwrapper=True)
+def pytest_sessionfinish(session):
+    config = session.config
+    with catch_warnings_for_item(
+        config=config, ihook=config.hook, when="config", item=None
+    ):
+        yield
+
+
 def _issue_warning_captured(warning, hook, stacklevel):
     """
     This function should be used instead of calling ``warnings.warn`` directly when we are in the "configure" stage:

testing/test_cacheprovider.py

@@ -56,9 +56,7 @@ def test_cache_writefail_permissions(self, testdir):
             testdir.tmpdir.ensure_dir(".pytest_cache").chmod(mode)
 
     @pytest.mark.skipif(sys.platform.startswith("win"), reason="no chmod on windows")
-    @pytest.mark.filterwarnings(
-        "ignore:could not create cache path:pytest.PytestWarning"
-    )
+    @pytest.mark.filterwarnings("default")
     def test_cache_failure_warns(self, testdir, monkeypatch):
         monkeypatch.setenv("PYTEST_DISABLE_PLUGIN_AUTOLOAD", "1")
         cache_dir = str(testdir.tmpdir.ensure_dir(".pytest_cache"))