Disable multiprocessing when using stdin

Fixes #165

flake8/engine.py

@@ -6,7 +6,7 @@ import pep8
 
 from flake8 import __version__
 from flake8.reporter import multiprocessing, BaseQReport, QueueReport
-from flake8.util import OrderedSet, is_windows
+from flake8.util import OrderedSet, is_windows, is_using_stdin
 
 _flake8_noqa = re.compile(r'flake8[:=]\s*noqa', re.I).search
 
@@ -57,7 +57,7 @@ def get_parser():
         parser.config_options.append('jobs')
         parser.add_option('-j', '--jobs', type='string', default='auto',
                           help="number of jobs to run simultaneously, "
-                          "or 'auto'")
+                          "or 'auto'. This is ignored on Windows.")
 
     parser.add_option('--exit-zero', action='store_true',
                       help="exit with code 0 even if there are errors")
@@ -95,7 +95,9 @@ def get_style_guide(**kwargs):
     if options.diff:
         options.jobs = None
 
-    if multiprocessing and options.jobs and not is_windows():
+    force_disable_jobs = is_windows() or is_using_stdin(styleguide.paths)
+
+    if multiprocessing and options.jobs and not force_disable_jobs:
         if options.jobs.isdigit():
             n_jobs = int(options.jobs)
         else:

flake8/tests/test_engine.py

@@ -6,7 +6,7 @@ try:
 except ImportError:
     import mock  # < PY33
 
-from flake8 import engine, util, __version__
+from flake8 import engine, util, __version__, reporter
 
 
 class TestEngine(unittest.TestCase):
@@ -82,5 +82,17 @@ class TestEngine(unittest.TestCase):
         # Also we can never be sure (without reconstructing the string
         # ourselves) what system we may be testing on.
 
+    def test_windows_disables_jobs(self):
+        with mock.patch('flake8.engine.is_windows') as is_windows:
+            is_windows.return_value = True
+            guide = engine.get_style_guide()
+            assert isinstance(guide, reporter.BaseQReport) is False
+
+    def test_stdin_disables_jobs(self):
+        with mock.patch('flake8.engine.is_using_stdin') as is_using_stdin:
+            is_using_stdin.return_value = True
+            guide = engine.get_style_guide()
+            assert isinstance(guide, reporter.BaseQReport) is False
+
 if __name__ == '__main__':
     unittest.main()

flake8/util.py

@@ -49,6 +49,11 @@ def is_windows():
     return os.name == 'nt'
 
 
+def is_using_stdin(paths):
+    """Determine if we're running checks on stdin."""
+    return '-' in paths
+
+
 def flag_on(val):
     """Return true if flag is on"""
     return str(val).upper() in ('1', 'T', 'TRUE', 'ON')