fix: say a config detection fails to read

tests/test_install_experience.py

@@ -737,6 +737,43 @@ class ASettingsFileThisUserCannotRead(unittest.TestCase):
                 self.assertIn(f"cannot open {path}", err.getvalue())
 
 
+class AConfigThatFailsToRead(unittest.TestCase):
+    """retroarch.cfg on a failing card raised EIO, and a link that loops
+    ELOOP: both counted as absent without a word, as only a permission
+    refusal was said. Absence itself stays silent."""
+
+    def setUp(self):
+        self.root = Path(tempfile.mkdtemp(dir=REPO_ROOT / "tmp"))
+        self.addCleanup(shutil.rmtree, self.root, True)
+        install._DETECTION_REFUSED.clear()
+        self.addCleanup(install._DETECTION_REFUSED.clear)
+        install._SETUP_SEARCH.active = True
+        self.addCleanup(setattr, install._SETUP_SEARCH, "active", False)
+
+    def test_a_read_error_is_said_with_its_own_words(self):
+        cfg = self.root / "retroarch.cfg"
+        cfg.write_text('system_directory = "/x"\n', encoding="utf-8")
+        failure = OSError(errno.EIO, "Input/output error", str(cfg))
+        with mock.patch.object(Path, "read_text", side_effect=failure), \
+                contextlib.redirect_stderr(io.StringIO()) as err:
+            self.assertIsNone(install._retroarch_cfg_dir(cfg, "system_directory",
+                                                         self.root, "system"))
+        self.assertEqual(install._DETECTION_REFUSED, [(cfg, "Input/output error")])
+        said = " ".join(err.getvalue().split())
+        self.assertIn(f"{cfg} could not be read (Input/output error)", said)
+        self.assertNotIn("Run as its owner", said)
+
+    @unittest.skipIf(os.name == "nt", "a symbolic link needs privileges on Windows")
+    def test_a_link_that_loops_is_said_and_absence_is_not(self):
+        loop = self.root / "retroarch.cfg"
+        loop.symlink_to(loop.name)
+        with contextlib.redirect_stderr(io.StringIO()):
+            self.assertFalse(install._exists(loop))
+            self.assertFalse(install._exists(self.root / "absent.cfg"))
+            self.assertFalse(install._exists(self.root / "absent" / "x.cfg"))
+        self.assertEqual([folder for folder, _why in install._DETECTION_REFUSED], [loop])
+
+
 class TheFrontendsOwnFolderClosed(unittest.TestCase):
     """The Flatpak RetroArch's folder at 000: its retroarch.cfg names
     ~/Games/bios, and the run took ~/.config/retroarch/system, a folder it