Add docker uid and gid fallback tests

pre_commit/languages/docker.py

@@ -75,14 +75,14 @@ def install_environment(
         os.mkdir(directory)
 
 
-def getuid():
+def getuid():  # pragma: windows no cover
     try:
         return os.getuid()
     except AttributeError:
         return FALLBACK_UID
 
 
-def getgid():
+def getgid():  # pragma: windows no cover
     try:
         return os.getgid()
     except AttributeError:

tests/languages/docker_test.py

@@ -13,3 +13,17 @@ def test_docker_is_running_process_error():
         side_effect=CalledProcessError(*(None,) * 4),
     ):
         assert docker.docker_is_running() is False
+
+
+def test_docker_fallback_uid():
+    def invalid_attribute():
+        raise AttributeError
+    with mock.patch('os.getuid', invalid_attribute):
+        assert docker.getuid() == docker.FALLBACK_UID
+
+
+def test_docker_fallback_gid():
+    def invalid_attribute():
+        raise AttributeError
+    with mock.patch('os.getgid', invalid_attribute):
+        assert docker.getgid() == docker.FALLBACK_GID