Don't crash on un-stringable exceptions

pre_commit/error_handler.py

@@ -14,14 +14,24 @@ class FatalError(RuntimeError):
     pass
 
 
+def _exception_to_bytes(exc: BaseException) -> bytes:
+    with contextlib.suppress(TypeError):
+        return bytes(exc)  # type: ignore
+    with contextlib.suppress(Exception):
+        return str(exc).encode()
+    return f'<unprintable {type(exc).__name__} object>'.encode()
+
+
 def _log_and_exit(msg: str, exc: BaseException, formatted: str) -> None:
-    error_msg = f'{msg}: {type(exc).__name__}: {exc}'
-    output.write_line(error_msg)
+    error_msg = f'{msg}: {type(exc).__name__}: '.encode()
+    error_msg += _exception_to_bytes(exc)
+    output.write_line_b(error_msg)
     log_path = os.path.join(Store().directory, 'pre-commit.log')
     output.write_line(f'Check the log at {log_path}')
 
     with open(log_path, 'wb') as log:
         _log_line = functools.partial(output.write_line, stream=log)
+        _log_line_b = functools.partial(output.write_line_b, stream=log)
 
         _log_line('### version information')
         _log_line()
@@ -39,7 +49,7 @@ def _log_and_exit(msg: str, exc: BaseException, formatted: str) -> None:
         _log_line('### error information')
         _log_line()
         _log_line('```')
-        _log_line(error_msg)
+        _log_line_b(error_msg)
         _log_line('```')
         _log_line()
         _log_line('```')

tests/error_handler_test.py

@@ -6,6 +6,7 @@ from unittest import mock
 import pytest
 
 from pre_commit import error_handler
+from pre_commit.util import CalledProcessError
 from testing.util import cmd_output_mocked_pre_commit_home
 
 
@@ -135,6 +136,22 @@ def test_error_handler_non_ascii_exception(mock_store_dir):
             raise ValueError('☃')
 
 
+def test_error_handler_non_utf8_exception(mock_store_dir):
+    with pytest.raises(SystemExit):
+        with error_handler.error_handler():
+            raise CalledProcessError(1, ('exe',), 0, b'error: \xa0\xe1', b'')
+
+
+def test_error_handler_non_stringable_exception(mock_store_dir):
+    class C(Exception):
+        def __str__(self):
+            raise RuntimeError('not today!')
+
+    with pytest.raises(SystemExit):
+        with error_handler.error_handler():
+            raise C()
+
+
 def test_error_handler_no_tty(tempdir_factory):
     pre_commit_home = tempdir_factory.get()
     ret, out, _ = cmd_output_mocked_pre_commit_home(