Fix #6163: Default to setuptools.build_meta:__legacy__

The main setuptools PEP 517 backend is intended for
explicit usage in `pyproject.toml`, when the project
authors can ensure that their `setup.py` runs without
that directory being implicitly on `sys.path`.

For implicit usage, setuptools now offers a separate
legacy backend that more closely mimics direct
execution of the `setup.py` script.

.gitignore

@@ -27,6 +27,7 @@ nosetests.xml
 coverage.xml
 *.cover
 tests/data/common_wheels/
+pip-wheel-metadata
 
 # Misc
 *~

docs/html/reference/pip.rst

@@ -145,26 +145,28 @@ explicitly manage the build environment. For such workflows, build isolation
 can be problematic. If this is the case, pip provides a
 ``--no-build-isolation`` flag to disable build isolation. Users supplying this
 flag are responsible for ensuring the build environment is managed
-appropriately.
+appropriately (including ensuring that all required build dependencies are
+installed).
 
-By default, pip will continue to use the legacy (``setuptools`` based) build
-processing for projects that do not have a ``pyproject.toml`` file. Projects
-with a ``pyproject.toml`` file will use a :pep:`517` backend. Projects with
-a ``pyproject.toml`` file, but which don't have a ``build-system`` section,
+By default, pip will continue to use the legacy (direct ``setup.py`` execution
+based) build processing for projects that do not have a ``pyproject.toml`` file.
+Projects with a ``pyproject.toml`` file will use a :pep:`517` backend. Projects
+with a ``pyproject.toml`` file, but which don't have a ``build-system`` section,
 will be assumed to have the following backend settings::
 
     [build-system]
-    requires = ["setuptools>=40.2.0", "wheel"]
-    build-backend = "setuptools.build_meta"
+    requires = ["setuptools>=40.8.0", "wheel"]
+    build-backend = "setuptools.build_meta:__legacy__"
 
 .. note::
 
-    ``setuptools`` 40.2.0 is the first version of setuptools with full
-    :pep:`517` support.
-    
-If a project has ``[build-system]``, but no ``build-backend``, pip will use
-``setuptools.build_meta``, but will assume the project requirements include
-``setuptools>=40.2.0`` and ``wheel`` (and will report an error if not).
+    ``setuptools`` 40.8.0 is the first version of setuptools that offers a
+    :pep:`517` backend that closely mimics directly executing ``setup.py``.
+
+If a project has ``[build-system]``, but no ``build-backend``, pip will also use
+``setuptools.build_meta:__legacy__``, but will expect the project requirements
+to include ``setuptools`` and ``wheel`` (and will report an error if the
+installed version of ``setuptools`` is not recent enough).
 
 If a user wants to explicitly request :pep:`517` handling even though a project
 doesn't have a ``pyproject.toml`` file, this can be done using the

news/6163.bugfix

@@ -0,0 +1,5 @@
+The implicit default backend used for projects that provide a ``pyproject.toml``
+file without explicitly specifying ``build-backend`` now behaves more like direct
+execution of ``setup.py``, and hence should restore compatibility with projects
+that were unable to be installed with ``pip`` 19.0. This raised the minimum
+required version of ``setuptools`` for such builds to 40.8.0.

src/pip/_internal/pyproject.py

@@ -111,11 +111,13 @@ def load_pyproject_toml(
         # section, or the user has no pyproject.toml, but has opted in
         # explicitly via --use-pep517.
         # In the absence of any explicit backend specification, we
-        # assume the setuptools backend, and require wheel and a version
-        # of setuptools that supports that backend.
+        # assume the setuptools backend that most closely emulates the
+        # traditional direct setup.py execution, and require wheel and
+        # a version of setuptools that supports that backend.
+
         build_system = {
-            "requires": ["setuptools>=40.2.0", "wheel"],
-            "build-backend": "setuptools.build_meta",
+            "requires": ["setuptools>=40.8.0", "wheel"],
+            "build-backend": "setuptools.build_meta:__legacy__",
         }
 
     # If we're using PEP 517, we have build system information (either
@@ -154,7 +156,7 @@ def load_pyproject_toml(
         # If the user didn't specify a backend, we assume they want to use
         # the setuptools backend. But we can't be sure they have included
         # a version of setuptools which supplies the backend, or wheel
-        # (which is neede by the backend) in their requirements. So we
+        # (which is needed by the backend) in their requirements. So we
         # make a note to check that those requirements are present once
         # we have set up the environment.
         # This is quite a lot of work to check for a very specific case. But
@@ -163,7 +165,7 @@ def load_pyproject_toml(
         # execute setup.py, but never considered needing to mention the build
         # tools themselves. The original PEP 518 code had a similar check (but
         # implemented in a different way).
-        backend = "setuptools.build_meta"
-        check = ["setuptools>=40.2.0", "wheel"]
+        backend = "setuptools.build_meta:__legacy__"
+        check = ["setuptools>=40.8.0", "wheel"]
 
     return (requires, backend, check)

tests/functional/test_install.py

@@ -62,7 +62,7 @@ def test_pep518_refuses_conflicting_requires(script, data):
         result.returncode != 0 and
         ('Some build dependencies for %s conflict with PEP 517/518 supported '
          'requirements: setuptools==1.0 is incompatible with '
-         'setuptools>=40.2.0.' % path_to_url(project_dir)) in result.stderr
+         'setuptools>=40.8.0.' % path_to_url(project_dir)) in result.stderr
     ), str(result)
 
 

tests/functional/test_pep517.py

@@ -123,3 +123,78 @@ def test_pep517_install_with_no_cache_dir(script, tmpdir, data):
         project_dir,
     )
     result.assert_installed('project', editable=False)
+
+
+def make_pyproject_with_setup(tmpdir, build_system=True, set_backend=True):
+    project_dir = (tmpdir / 'project').mkdir()
+    setup_script = (
+        'from setuptools import setup\n'
+    )
+    expect_script_dir_on_path = True
+    if build_system:
+        buildsys = {
+            'requires': ['setuptools', 'wheel'],
+        }
+        if set_backend:
+            buildsys['build-backend'] = 'setuptools.build_meta'
+            expect_script_dir_on_path = False
+        project_data = pytoml.dumps({'build-system': buildsys})
+    else:
+        project_data = ''
+
+    if expect_script_dir_on_path:
+        setup_script += (
+            'from pep517_test import __version__\n'
+        )
+    else:
+        setup_script += (
+            'try:\n'
+            '    import pep517_test\n'
+            'except ImportError:\n'
+            '    pass\n'
+            'else:\n'
+            '    raise RuntimeError("Source dir incorrectly on sys.path")\n'
+        )
+
+    setup_script += (
+        'setup(name="pep517_test", version="0.1", packages=["pep517_test"])'
+    )
+
+    project_dir.join('pyproject.toml').write(project_data)
+    project_dir.join('setup.py').write(setup_script)
+    package_dir = (project_dir / "pep517_test").mkdir()
+    package_dir.join('__init__.py').write('__version__ = "0.1"')
+    return project_dir, "pep517_test"
+
+
+def test_no_build_system_section(script, tmpdir, data, common_wheels):
+    """Check builds with setup.py, pyproject.toml, but no build-system section.
+    """
+    project_dir, name = make_pyproject_with_setup(tmpdir, build_system=False)
+    result = script.pip(
+        'install', '--no-cache-dir', '--no-index', '-f', common_wheels,
+        project_dir,
+    )
+    result.assert_installed(name, editable=False)
+
+
+def test_no_build_backend_entry(script, tmpdir, data, common_wheels):
+    """Check builds with setup.py, pyproject.toml, but no build-backend entry.
+    """
+    project_dir, name = make_pyproject_with_setup(tmpdir, set_backend=False)
+    result = script.pip(
+        'install', '--no-cache-dir', '--no-index', '-f', common_wheels,
+        project_dir,
+    )
+    result.assert_installed(name, editable=False)
+
+
+def test_explicit_setuptools_backend(script, tmpdir, data, common_wheels):
+    """Check builds with setup.py, pyproject.toml, and a build-backend entry.
+    """
+    project_dir, name = make_pyproject_with_setup(tmpdir)
+    result = script.pip(
+        'install', '--no-cache-dir', '--no-index', '-f', common_wheels,
+        project_dir,
+    )
+    result.assert_installed(name, editable=False)

tools/tests-common_wheels-requirements.txt

@@ -1,2 +1,9 @@
-setuptools
+# Create local setuptools wheel files for testing by:
+# 1. Cloning setuptools and checking out the branch of interest
+# 2. Running `python3 bootstrap.py` in that directory
+# 3. Running `python3 -m pip wheel --no-cache -w /tmp/setuptools_build_meta_legacy/ .`
+# 4. Replacing the `setuptools` entry below with a `file:///...` URL
+# (Adjust artifact directory used based on preference and operating system)
+
+setuptools >= 40.8.0
 wheel