Add unit tests around serial retries

tests/unit/test_checker_manager.py

@@ -0,0 +1,27 @@
+"""Tests for the Manager object for FileCheckers."""
+import errno
+import mock
+
+import pytest
+
+from flake8 import checker
+
+
+def test_oserrors_cause_serial_fall_back():
+    """Verify that OSErrors will cause the Manager to fallback to serial."""
+    err = OSError(errno.ENOSPC, 'Ominous message about spaceeeeee')
+    style_guide = mock.Mock()
+    style_guide.options = mock.Mock(diff=False, jobs='4')
+    with mock.patch('multiprocessing.Queue', side_effect=err):
+        manager = checker.Manager(style_guide, [], [])
+    assert manager.using_multiprocessing is False
+
+
+def test_oserrors_are_reraised():
+    """Verify that OSErrors will cause the Manager to fallback to serial."""
+    err = OSError(errno.EAGAIN, 'Ominous message')
+    style_guide = mock.Mock()
+    style_guide.options = mock.Mock(diff=False, jobs='4')
+    with mock.patch('multiprocessing.Queue', side_effect=err):
+        with pytest.raises(OSError):
+            checker.Manager(style_guide, [], [])