Correctly handle longpath prefix in process_dockerfile when joining paths

Signed-off-by: Joffrey F <joffrey@docker.com>

docker/api/build.py

@@ -339,7 +339,14 @@ def process_dockerfile(dockerfile, path):
     abs_dockerfile = dockerfile
     if not os.path.isabs(dockerfile):
         abs_dockerfile = os.path.join(path, dockerfile)
-
+        if constants.IS_WINDOWS_PLATFORM and path.startswith(
+                constants.WINDOWS_LONGPATH_PREFIX):
+            abs_dockerfile = '{}{}'.format(
+                constants.WINDOWS_LONGPATH_PREFIX,
+                os.path.normpath(
+                    abs_dockerfile[len(constants.WINDOWS_LONGPATH_PREFIX):]
+                )
+            )
     if (os.path.splitdrive(path)[0] != os.path.splitdrive(abs_dockerfile)[0] or
             os.path.relpath(abs_dockerfile, path).startswith('..')):
         # Dockerfile not in context - read data to insert into tar later

docker/constants.py

@@ -14,6 +14,7 @@ INSECURE_REGISTRY_DEPRECATION_WARNING = \
     'is deprecated and non-functional. Please remove it.'
 
 IS_WINDOWS_PLATFORM = (sys.platform == 'win32')
+WINDOWS_LONGPATH_PREFIX = '\\\\?\\'
 
 DEFAULT_USER_AGENT = "docker-sdk-python/{0}".format(version)
 DEFAULT_NUM_POOLS = 25

tests/unit/api_build_test.py

 import gzip
 import io
+import shutil
 
 import docker
 from docker import auth
+from docker.api.build import process_dockerfile
 
-from .api_test import BaseAPIClientTest, fake_request, url_prefix
 import pytest
 
+from ..helpers import make_tree
+from .api_test import BaseAPIClientTest, fake_request, url_prefix
+
 
 class BuildTest(BaseAPIClientTest):
     def test_build_container(self):
@@ -161,3 +165,61 @@ class BuildTest(BaseAPIClientTest):
 
         self.client._set_auth_headers(headers)
         assert headers == expected_headers
+
+    @pytest.mark.skipif(
+        not docker.constants.IS_WINDOWS_PLATFORM,
+        reason='Windows-specific syntax')
+    def test_process_dockerfile_win_longpath_prefix(self):
+        dirs = [
+            'foo', 'foo/bar', 'baz',
+        ]
+
+        files = [
+            'Dockerfile', 'foo/Dockerfile.foo', 'foo/bar/Dockerfile.bar',
+            'baz/Dockerfile.baz',
+        ]
+
+        base = make_tree(dirs, files)
+        self.addCleanup(shutil.rmtree, base)
+
+        def pre(path):
+            return docker.constants.WINDOWS_LONGPATH_PREFIX + path
+
+        assert process_dockerfile(None, pre(base)) == (None, None)
+        assert process_dockerfile('Dockerfile', pre(base)) == (
+            'Dockerfile', None
+        )
+        assert process_dockerfile('foo/Dockerfile.foo', pre(base)) == (
+            'foo/Dockerfile.foo', None
+        )
+        assert process_dockerfile(
+            '../Dockerfile', pre(base + '\\foo')
+        )[1] is not None
+        assert process_dockerfile(
+            '../baz/Dockerfile.baz', pre(base + '/baz')
+        ) == ('../baz/Dockerfile.baz', None)
+
+    def test_process_dockerfile(self):
+        dirs = [
+            'foo', 'foo/bar', 'baz',
+        ]
+
+        files = [
+            'Dockerfile', 'foo/Dockerfile.foo', 'foo/bar/Dockerfile.bar',
+            'baz/Dockerfile.baz',
+        ]
+
+        base = make_tree(dirs, files)
+        self.addCleanup(shutil.rmtree, base)
+
+        assert process_dockerfile(None, base) == (None, None)
+        assert process_dockerfile('Dockerfile', base) == ('Dockerfile', None)
+        assert process_dockerfile('foo/Dockerfile.foo', base) == (
+            'foo/Dockerfile.foo', None
+        )
+        assert process_dockerfile(
+            '../Dockerfile', base + '/foo'
+        )[1] is not None
+        assert process_dockerfile('../baz/Dockerfile.baz', base + '/baz') == (
+            '../baz/Dockerfile.baz', None
+        )