Fix auth config path on Windows

The Engine client looks *only* at the USERPROFILE environment variable
on Windows, so we should do that too.
Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

docker/auth.py

@@ -7,6 +7,7 @@ import dockerpycreds
 import six
 
 from . import errors
+from .constants import IS_WINDOWS_PLATFORM
 
 INDEX_NAME = 'docker.io'
 INDEX_URL = 'https://{0}/v1/'.format(INDEX_NAME)
@@ -210,19 +211,12 @@ def parse_auth(entries, raise_on_error=False):
 
 
 def find_config_file(config_path=None):
-    environment_path = os.path.join(
-        os.environ.get('DOCKER_CONFIG'),
-        os.path.basename(DOCKER_CONFIG_FILENAME)
-    ) if os.environ.get('DOCKER_CONFIG') else None
-
-    paths = filter(None, [
+    paths = list(filter(None, [
         config_path,  # 1
-        environment_path,  # 2
-        os.path.join(os.path.expanduser('~'), DOCKER_CONFIG_FILENAME),  # 3
-        os.path.join(
-            os.path.expanduser('~'), LEGACY_DOCKER_CONFIG_FILENAME
-        )  # 4
-    ])
+        config_path_from_environment(),  # 2
+        os.path.join(home_dir(), DOCKER_CONFIG_FILENAME),  # 3
+        os.path.join(home_dir(), LEGACY_DOCKER_CONFIG_FILENAME),  # 4
+    ]))
 
     log.debug("Trying paths: {0}".format(repr(paths)))
 
@@ -236,6 +230,24 @@ def find_config_file(config_path=None):
     return None
 
 
+def config_path_from_environment():
+    config_dir = os.environ.get('DOCKER_CONFIG')
+    if not config_dir:
+        return None
+    return os.path.join(config_dir, os.path.basename(DOCKER_CONFIG_FILENAME))
+
+
+def home_dir():
+    """
+    Get the user's home directory, using the same logic as the Docker Engine
+    client - use %USERPROFILE% on Windows, $HOME/getuid on POSIX.
+    """
+    if IS_WINDOWS_PLATFORM:
+        return os.environ.get('USERPROFILE', '')
+    else:
+        return os.path.expanduser('~')
+
+
 def load_config(config_path=None):
     """
     Loads authentication data from a Docker configuration file in the given

tests/unit/auth_test.py

@@ -9,6 +9,9 @@ import shutil
 import tempfile
 import unittest
 
+from py.test import ensuretemp
+from pytest import mark
+
 from docker import auth, errors
 
 try:
@@ -269,6 +272,56 @@ class ResolveAuthTest(unittest.TestCase):
         )
 
 
+class FindConfigFileTest(unittest.TestCase):
+    def tmpdir(self, name):
+        tmpdir = ensuretemp(name)
+        self.addCleanup(tmpdir.remove)
+        return tmpdir
+
+    def test_find_config_fallback(self):
+        tmpdir = self.tmpdir('test_find_config_fallback')
+
+        with mock.patch.dict(os.environ, {'HOME': str(tmpdir)}):
+            assert auth.find_config_file() is None
+
+    def test_find_config_from_explicit_path(self):
+        tmpdir = self.tmpdir('test_find_config_from_explicit_path')
+        config_path = tmpdir.ensure('my-config-file.json')
+
+        assert auth.find_config_file(str(config_path)) == str(config_path)
+
+    def test_find_config_from_environment(self):
+        tmpdir = self.tmpdir('test_find_config_from_environment')
+        config_path = tmpdir.ensure('config.json')
+
+        with mock.patch.dict(os.environ, {'DOCKER_CONFIG': str(tmpdir)}):
+            assert auth.find_config_file() == str(config_path)
+
+    @mark.skipif("sys.platform == 'win32'")
+    def test_find_config_from_home_posix(self):
+        tmpdir = self.tmpdir('test_find_config_from_home_posix')
+        config_path = tmpdir.ensure('.docker', 'config.json')
+
+        with mock.patch.dict(os.environ, {'HOME': str(tmpdir)}):
+            assert auth.find_config_file() == str(config_path)
+
+    @mark.skipif("sys.platform == 'win32'")
+    def test_find_config_from_home_legacy_name(self):
+        tmpdir = self.tmpdir('test_find_config_from_home_legacy_name')
+        config_path = tmpdir.ensure('.dockercfg')
+
+        with mock.patch.dict(os.environ, {'HOME': str(tmpdir)}):
+            assert auth.find_config_file() == str(config_path)
+
+    @mark.skipif("sys.platform != 'win32'")
+    def test_find_config_from_home_windows(self):
+        tmpdir = self.tmpdir('test_find_config_from_home_windows')
+        config_path = tmpdir.ensure('.docker', 'config.json')
+
+        with mock.patch.dict(os.environ, {'USERPROFILE': str(tmpdir)}):
+            assert auth.find_config_file() == str(config_path)
+
+
 class LoadConfigTest(unittest.TestCase):
     def test_load_config_no_file(self):
         folder = tempfile.mkdtemp()