Handle CPU detection errors and running on Travis

pre_commit/languages/helpers.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 import multiprocessing
+import os
 import shlex
 
 from pre_commit.util import cmd_output
@@ -52,5 +53,11 @@ def target_concurrency(hook):
     if hook['require_serial']:
         return 1
     else:
-        # TODO: something smart!
+        # Travis appears to have a bunch of CPUs, but we can't use them all.
-        return multiprocessing.cpu_count()
+        if 'TRAVIS' in os.environ:
+            return 2
+        else:
+            try:
+                return multiprocessing.cpu_count()
+            except NotImplementedError:
+                return 1

tests/languages/helpers_test.py

@@ -1,8 +1,11 @@
 from __future__ import absolute_import
 from __future__ import unicode_literals
 
+import multiprocessing
+import os
 import sys
 
+import mock
 import pytest
 
 from pre_commit.languages import helpers
@@ -28,3 +31,25 @@ def test_failed_setup_command_does_not_unicode_error():
     # an assertion that this does not raise `UnicodeError`
     with pytest.raises(CalledProcessError):
         helpers.run_setup_cmd(Prefix('.'), (sys.executable, '-c', script))
+
+
+def test_target_concurrency_normal():
+    with mock.patch.object(multiprocessing, 'cpu_count', return_value=123):
+        with mock.patch.dict(os.environ, {}, clear=True):
+            assert helpers.target_concurrency({'require_serial': False}) == 123
+
+
+def test_target_concurrency_cpu_count_require_serial_true():
+    assert helpers.target_concurrency({'require_serial': True}) == 1
+
+
+def test_target_concurrency_on_travis():
+    with mock.patch.dict(os.environ, {'TRAVIS': '1'}, clear=True):
+        assert helpers.target_concurrency({'require_serial': False}) == 2
+
+
+def test_target_concurrency_cpu_count_not_implemented():
+    with mock.patch.object(
+            multiprocessing, 'cpu_count', side_effect=NotImplementedError,
+    ):
+        assert helpers.target_concurrency({'require_serial': False}) == 1