config: fallback confcutdir to rootpath if inipath is not set

Currently, if `--confcutdir` is not set, `inipath.parent` is used, and
if `initpath` is not set, then `confcutdir` is None, which means there
is no cutoff.

Having no cutoff is not great, it means we potentially start probing
stuff all the way up to the filesystem root directory. So let's add
another fallback, to `rootpath`, which is always something reasonable.

changelog/11043.improvement.rst

@@ -0,0 +1,3 @@
+When `--confcutdir` is not specified, and there is no config file present, the conftest cutoff directory (`--confcutdir`) is now set to the :ref:`rootdir`.
+Previously in such cases, `conftest.py` files would be probed all the way to the root directory of the filesystem.
+If you are badly affected by this change, consider adding an empty config file to your desired cutoff directory, or explicitly set `--confcutdir`.

src/_pytest/config/__init__.py

@@ -1261,8 +1261,11 @@ class Config:
                 _pytest.deprecated.STRICT_OPTION, stacklevel=2
             )
 
-        if self.known_args_namespace.confcutdir is None and self.inipath is not None:
+        if self.known_args_namespace.confcutdir is None:
+            if self.inipath is not None:
                 confcutdir = str(self.inipath.parent)
+            else:
+                confcutdir = str(self.rootpath)
             self.known_args_namespace.confcutdir = confcutdir
         try:
             self.hook.pytest_load_initial_conftests(

testing/test_config.py

@@ -179,6 +179,23 @@ class TestParseIni:
         assert result.ret != 0
         result.stderr.fnmatch_lines("ERROR: *pyproject.toml: Invalid statement*")
 
+    def test_confcutdir_default_without_configfile(self, pytester: Pytester) -> None:
+        # If --confcutdir is not specified, and there is no configfile, default
+        # to the roothpath.
+        sub = pytester.mkdir("sub")
+        os.chdir(sub)
+        config = pytester.parseconfigure()
+        assert config.pluginmanager._confcutdir == sub
+
+    def test_confcutdir_default_with_configfile(self, pytester: Pytester) -> None:
+        # If --confcutdir is not specified, and there is a configfile, default
+        # to the configfile's directory.
+        pytester.makeini("[pytest]")
+        sub = pytester.mkdir("sub")
+        os.chdir(sub)
+        config = pytester.parseconfigure()
+        assert config.pluginmanager._confcutdir == pytester.path
+
     @pytest.mark.xfail(reason="probably not needed")
     def test_confcutdir(self, pytester: Pytester) -> None:
         sub = pytester.mkdir("sub")

testing/test_conftest.py

@@ -594,7 +594,13 @@ class TestConftestVisibility:
         print("pytestarg        : %s" % testarg)
         print("expected pass    : %s" % expect_ntests_passed)
         os.chdir(dirs[chdir])
-        reprec = pytester.inline_run(testarg, "-q", "--traceconfig")
+        reprec = pytester.inline_run(
+            testarg,
+            "-q",
+            "--traceconfig",
+            "--confcutdir",
+            pytester.path,
+        )
         reprec.assertoutcome(passed=expect_ntests_passed)