Add error_handler and use it.

tests/error_handler_test.py

@@ -0,0 +1,93 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+import io
+import os.path
+import mock
+import pytest
+import re
+
+from pre_commit import error_handler
+from pre_commit.errors import FatalError
+
+
+@pytest.yield_fixture
+def mocked_log_and_exit():
+    with mock.patch.object(error_handler, '_log_and_exit') as log_and_exit:
+        yield log_and_exit
+
+
+def test_error_handler_no_exception(mocked_log_and_exit):
+    with error_handler.error_handler():
+        pass
+    assert mocked_log_and_exit.call_count == 0
+
+
+def test_error_handler_fatal_error(mocked_log_and_exit):
+    exc = FatalError('just a test')
+    with error_handler.error_handler():
+        raise exc
+
+    mocked_log_and_exit.assert_called_once_with(
+        'An error has occurred',
+        exc,
+        # Tested below
+        mock.ANY,
+    )
+
+    assert re.match(
+        'Traceback \(most recent call last\):\n'
+        '  File ".+/pre_commit/error_handler.py", line \d+, in error_handler\n'
+        '    yield\n'
+        '  File ".+/tests/error_handler_test.py", line \d+, '
+        'in test_error_handler_fatal_error\n'
+        '    raise exc\n'
+        '(pre_commit\.errors\.)?FatalError: just a test\n',
+        mocked_log_and_exit.call_args[0][2],
+    )
+
+
+def test_error_handler_uncaught_error(mocked_log_and_exit):
+    exc = ValueError('another test')
+    with error_handler.error_handler():
+        raise exc
+
+    mocked_log_and_exit.assert_called_once_with(
+        'An unexpected error has occurred',
+        exc,
+        # Tested below
+        mock.ANY,
+    )
+    assert re.match(
+        'Traceback \(most recent call last\):\n'
+        '  File ".+/pre_commit/error_handler.py", line \d+, in error_handler\n'
+        '    yield\n'
+        '  File ".+/tests/error_handler_test.py", line \d+, '
+        'in test_error_handler_uncaught_error\n'
+        '    raise exc\n'
+        'ValueError: another test\n',
+        mocked_log_and_exit.call_args[0][2],
+    )
+
+
+def test_log_and_exit(mock_out_store_directory):
+    mocked_print = mock.Mock()
+    with pytest.raises(error_handler.PreCommitSystemExit):
+        error_handler._log_and_exit(
+            'msg', FatalError('hai'), "I'm a stacktrace",
+            print_fn=mocked_print,
+        )
+
+    printed = '\n'.join(call[0][0] for call in mocked_print.call_args_list)
+    assert printed == (
+        'msg: FatalError: hai\n'
+        'Check the log at ~/.pre-commit/pre-commit.log'
+    )
+
+    log_file = os.path.join(mock_out_store_directory, 'pre-commit.log')
+    assert os.path.exists(log_file)
+    contents = io.open(log_file).read()
+    assert contents == (
+        'msg: FatalError: hai\n'
+        "I'm a stacktrace\n"
+    )