fix: handle unreadable config files gracefully at startup

aider/main.py

@@ -484,6 +484,10 @@ def main(argv=None, input=None, output=None, force_git_root=None, return_coder=F
             if check_config_files_for_yes(default_config_files):
                 return 1
         raise e
+    except OSError as err:
+        print(f"Unable to read configuration file: {err}")
+        print("Check the file's permissions and that it is a readable file, not a directory.")
+        return 1
 
     if args.verbose:
         print("Config files search order, if no --config:")
@@ -495,13 +499,23 @@ def main(argv=None, input=None, output=None, force_git_root=None, return_coder=F
 
     parser = get_parser(default_config_files, git_root)
 
-    args, unknown = parser.parse_known_args(argv)
+    try:
+        args, unknown = parser.parse_known_args(argv)
+    except OSError as err:
+        print(f"Unable to read configuration file: {err}")
+        print("Check the file's permissions and that it is a readable file, not a directory.")
+        return 1
 
     # Load the .env file specified in the arguments
     loaded_dotenvs = load_dotenv_files(git_root, args.env_file, args.encoding)
 
     # Parse again to include any arguments that might have been defined in .env
-    args = parser.parse_args(argv)
+    try:
+        args = parser.parse_args(argv)
+    except OSError as err:
+        print(f"Unable to read configuration file: {err}")
+        print("Check the file's permissions and that it is a readable file, not a directory.")
+        return 1
 
     if args.shell_completions:
         # Ensure parser.prog is set for shtab, though it should be by default

tests/basic/test_main.py

@@ -1,3 +1,4 @@
+import contextlib
 import json
 import os
 import subprocess
@@ -100,6 +101,18 @@ def test_main_with_git_config_yml(self):
             _, kwargs = MockCoder.call_args
             assert kwargs["auto_commits"] is True
 
+    def test_main_with_unreadable_config_file(self):
+        # A directory named .aider.conf.yml can not be opened as a config file.
+        # aider should print a friendly error and exit non-zero instead of
+        # dumping an uncaught PermissionError/IsADirectoryError traceback.
+        # https://github.com/Aider-AI/aider/issues/5466
+        Path(".aider.conf.yml").mkdir()
+        stdout = StringIO()
+        with contextlib.redirect_stdout(stdout):
+            result = main(["--exit"], input=DummyInput(), output=DummyOutput())
+        self.assertEqual(result, 1)
+        self.assertIn("Unable to read configuration file", stdout.getvalue())
+
     def test_main_with_empty_git_dir_new_subdir_file(self):
         make_repo()
         subdir = Path("subdir")