Use os.sched_getaffinity for cpu counts when available

pre_commit/xargs.py

@@ -24,6 +24,14 @@ TRet = TypeVar('TRet')
 
 
 def cpu_count() -> int:
+    try:
+        # On systems that support it, this will return a more accurate count of
+        # usable CPUs for the current process, which will take into account
+        # cgroup limits
+        return len(os.sched_getaffinity(0))
+    except AttributeError:
+        pass
+
     try:
         return multiprocessing.cpu_count()
     except NotImplementedError:

tests/lang_base_test.py

@@ -30,6 +30,19 @@ def homedir_mck():
         yield
 
 
+@pytest.fixture
+def no_sched_getaffinity():
+    # Simulates an OS without os.sched_getaffinity available (mac/windows)
+    # https://docs.python.org/3/library/os.html#interface-to-the-scheduler
+    with mock.patch.object(
+        os,
+        'sched_getaffinity',
+        create=True,
+        side_effect=AttributeError,
+    ):
+        yield
+
+
 def test_exe_exists_does_not_exist(find_exe_mck, homedir_mck):
     find_exe_mck.return_value = None
     assert lang_base.exe_exists('ruby') is False
@@ -116,7 +129,17 @@ def test_no_env_noop(tmp_path):
     assert before == inside == after
 
 
-def test_target_concurrency_normal():
+def test_target_concurrency_sched_getaffinity(no_sched_getaffinity):
+    with mock.patch.object(
+        os,
+        'sched_getaffinity',
+        return_value=set(range(345)),
+    ):
+        with mock.patch.dict(os.environ, clear=True):
+            assert lang_base.target_concurrency() == 345
+
+
+def test_target_concurrency_without_sched_getaffinity(no_sched_getaffinity):
     with mock.patch.object(multiprocessing, 'cpu_count', return_value=123):
         with mock.patch.dict(os.environ, {}, clear=True):
             assert lang_base.target_concurrency() == 123
@@ -134,7 +157,7 @@ def test_target_concurrency_on_travis():
         assert lang_base.target_concurrency() == 2
 
 
-def test_target_concurrency_cpu_count_not_implemented():
+def test_target_concurrency_cpu_count_not_implemented(no_sched_getaffinity):
     with mock.patch.object(
             multiprocessing, 'cpu_count', side_effect=NotImplementedError,
     ):