From 93d5b398b69496190da06740e3b29d5f90817ae7 Mon Sep 17 00:00:00 2001
From: Paul Ganssle <paul@ganssle.io>
Date: Sun, 27 Jan 2019 12:31:28 -0500
Subject: [PATCH] Add build_meta_legacy backend

This is part of the solution to GH #1642, it is a
backwards-compatibility backend that can be used as a default PEP 517
backend for projects that use setuptools but haven't opted in to
build_meta.
---
 setuptools/build_meta_legacy.py     | 44 +++++++++++++++++++++++++++++
 setuptools/tests/test_build_meta.py |  1 -
 2 files changed, 44 insertions(+), 1 deletion(-)
 create mode 100644 setuptools/build_meta_legacy.py

diff --git a/setuptools/build_meta_legacy.py b/setuptools/build_meta_legacy.py
new file mode 100644
index 00000000000..7eed41f111f
--- /dev/null
+++ b/setuptools/build_meta_legacy.py
@@ -0,0 +1,44 @@
+"""Compatibility backend for setuptools
+
+This is a version of setuptools.build_meta that endeavors to maintain backwards
+compatibility with pre-PEP 517 modes of invocation. It exists as a temporary
+bridge between the old packaging mechanism and the new packaging mechanism,
+and will eventually be removed.
+"""
+
+import sys
+
+from setuptools.build_meta import _BuildMetaBackend
+from setuptools.build_meta import SetupRequirementsError
+
+
+__all__ = ['get_requires_for_build_sdist',
+           'get_requires_for_build_wheel',
+           'prepare_metadata_for_build_wheel',
+           'build_wheel',
+           'build_sdist',
+           'SetupRequirementsError']
+
+
+class _BuildMetaLegacyBackend(_BuildMetaBackend):
+    def run_setup(self, setup_script='setup.py'):
+        # In order to maintain compatibility with scripts assuming that
+        # the setup.py script is in a directory on the PYTHONPATH, inject
+        # '' into sys.path. (pypa/setuptools#1642)
+        sys_path = list(sys.path)       # Save the old path
+        if '' not in sys.path:
+            sys.path.insert(0, '')
+
+        super(_BuildMetaLegacyBackend,
+              self).run_setup(setup_script=setup_script)
+
+        sys.path = sys_path             # Restore the old path
+
+
+_BACKEND = _BuildMetaLegacyBackend()
+
+get_requires_for_build_wheel = _BACKEND.get_requires_for_build_wheel
+get_requires_for_build_sdist = _BACKEND.get_requires_for_build_sdist
+prepare_metadata_for_build_wheel = _BACKEND.prepare_metadata_for_build_wheel
+build_wheel = _BACKEND.build_wheel
+build_sdist = _BACKEND.build_sdist
diff --git a/setuptools/tests/test_build_meta.py b/setuptools/tests/test_build_meta.py
index 5d97ebcec26..b29d6f29e52 100644
--- a/setuptools/tests/test_build_meta.py
+++ b/setuptools/tests/test_build_meta.py
@@ -258,7 +258,6 @@ def test_build_sdist_relative_path_import(self, tmpdir_cwd):
             build_backend.build_sdist("temp")
 
 
-@pytest.mark.xfail
 class TestBuildMetaLegacyBackend(TestBuildMetaBackend):
     backend_name = 'setuptools.build_meta_legacy'