inspection: normalise legacy build backend handling

This change ensures that when inspection packages for metadata, poetry
correctly handles cases where the legacy build backend is used.

In addition to improving handling of PEP 517 metadata builds, error
handling when reading setup files have also been improved.

poetry/inspection/info.py

@@ -12,7 +12,8 @@
 
 import pkginfo
 
-from pep517.meta import build
+from pep517.build import compat_system as pep517_compat_system
+from pep517.meta import build as pep517_metadata_build
 from poetry.core.factory import Factory
 from poetry.core.packages import Package
 from poetry.core.packages import ProjectPackage
@@ -255,19 +256,22 @@ def _from_sdist_file(cls, path):  # type: (Path) -> PackageInfo
         return info.update(new_info)
 
     @classmethod
-    def from_setup_py(cls, path):  # type: (Union[str, Path]) -> PackageInfo
+    def from_setup_files(cls, path):  # type: (Path) -> PackageInfo
         """
-        Mechanism to parse package information from a `setup.py` file. This uses the implentation
+        Mechanism to parse package information from a `setup.[py|cfg]` file. This uses the implementation
         at `poetry.utils.setup_reader.SetupReader` in order to parse the file. This is not reliable for
         complex setup files and should only attempted as a fallback.
 
         :param path: Path to `setup.py` file
-        :return:
         """
-        if not (Path(path) / "setup.py").exists():
+        if not any((path / f).exists() for f in SetupReader.FILES):
+            raise PackageInfoError(path)
+
+        try:
+            result = SetupReader.read_from_directory(path)
+        except Exception:
             raise PackageInfoError(path)
 
-        result = SetupReader.read_from_directory(Path(path))
         python_requires = result["python_requires"]
         if python_requires is None:
             python_requires = "*"
@@ -289,14 +293,20 @@ def from_setup_py(cls, path):  # type: (Union[str, Path]) -> PackageInfo
 
         requirements = parse_requires(requires)
 
-        return cls(
+        info = cls(
             name=result.get("name"),
             version=result.get("version"),
             summary=result.get("description", ""),
             requires_dist=requirements or None,
             requires_python=python_requires,
         )
 
+        if not (info.name and info.version) and not info.requires_dist:
+            # there is nothing useful here
+            raise PackageInfoError(path)
+
+        return info
+
     @staticmethod
     def _find_dist_info(path):  # type: (Path) -> Iterator[Path]
         """
@@ -309,22 +319,20 @@ def _find_dist_info(path):  # type: (Path) -> Iterator[Path]
             # Sometimes pathlib will fail on recursive symbolic links, so we need to workaround it
             # and use the glob module instead. Note that this does not happen with pathlib2
             # so it's safe to use it for Python < 3.4.
-            directories = glob.iglob(Path(path, pattern).as_posix(), recursive=True)
+            directories = glob.iglob(path.joinpath(pattern).as_posix(), recursive=True)
         else:
             directories = path.glob(pattern)
 
         for d in directories:
             yield Path(d)
 
     @classmethod
-    def from_metadata(cls, path):  # type: (Union[str, Path]) -> Optional[PackageInfo]
+    def from_metadata(cls, path):  # type: (Path) -> Optional[PackageInfo]
         """
         Helper method to parse package information from an unpacked metadata directory.
 
         :param path: The metadata directory to parse information from.
         """
-        path = Path(path)
-
         if path.suffix in {".dist-info", ".egg-info"}:
             directories = [path]
         else:
@@ -400,36 +408,37 @@ def _pep517_metadata(cls, path):  # type (Path) -> PackageInfo
 
         :param path: Path to package source to build and read metadata for.
         """
-        pyproject_toml = path.joinpath("pyproject.toml")
-        setup_py = path.joinpath("setup.py")
-
-        system = None
-        if not pyproject_toml.exists() and setup_py.exists():
-            # use default PEP-517 backend configuration
-            system = {
-                "requires": ["setuptools", "wheel"],
-                "build-backend": "setuptools.build_meta",
-            }
+        info = None
+        try:
+            info = cls.from_setup_files(path)
+            if info.requires_dist is not None:
+                return info
+        except PackageInfoError:
+            pass
 
         try:
             with temporary_directory() as tmp_dir:
-                build(source_dir=path.as_posix(), dest=tmp_dir, system=system)
-                return cls.from_metadata(tmp_dir)
+                pep517_metadata_build(
+                    source_dir=path.as_posix(),
+                    dest=tmp_dir,
+                    system=pep517_compat_system(source_dir=path.as_posix()),
+                )
+                return cls.from_metadata(Path(tmp_dir))
         except subprocess.CalledProcessError:
             cls._log("PEP 517 metadata build failed for {}".format(path), "debug")
-            if setup_py.exists():
+            if info:
                 cls._log(
-                    "Falling back to parsing setup.py file for {}".format(path), "debug"
+                    "Falling back to parsed setup.py file for {}".format(path), "debug"
                 )
-                return cls.from_setup_py(setup_py)
+                return info
 
         # if we reach here, everything has failed and all hope is lost
         raise PackageInfoError(path)
 
     @classmethod
     def from_directory(
         cls, path, allow_build=False
-    ):  # type: (Union[str, Path], bool) -> PackageInfo
+    ):  # type: (Path, bool) -> PackageInfo
         """
         Generate package information from a package source directory. When `allow_build` is enabled and
         introspection of all available metadata fails, the package is attempted to be build in an isolated
@@ -438,7 +447,6 @@ def from_directory(
         :param path: Path to generate package information from.
         :param allow_build: If enabled, as a fallback, build the project to gather metadata.
         """
-        path = Path(path)
         info = cls.from_metadata(path)
 
         if info and info.requires_dist is not None:
@@ -451,7 +459,7 @@ def from_directory(
 
         try:
             if not allow_build:
-                return cls.from_setup_py(path)
+                return cls.from_setup_files(path)
             return cls._pep517_metadata(path)
         except PackageInfoError as e:
             if info:
@@ -460,7 +468,7 @@ def from_directory(
             raise e
 
     @classmethod
-    def from_sdist(cls, path):  # type: (Union[Path, pkginfo.SDist]) -> PackageInfo
+    def from_sdist(cls, path):  # type: (Path) -> PackageInfo
         """
         Gather package information from an sdist file, packed or unpacked.
 

tests/inspection/test_info.py

@@ -27,6 +27,39 @@ def demo_wheel():  # type: () -> Path
     return FIXTURE_DIR_BASE / "distributions" / "demo-0.1.0-py2.py3-none-any.whl"
 
 
+@pytest.fixture
+def demo_setup(tmp_path):  # type: (Path) -> Path
+    setup_py = tmp_path / "setup.py"
+    setup_py.write_text(
+        decode(
+            "from setuptools import setup; "
+            'setup(name="demo", '
+            'version="0.1.0", '
+            'install_requires=["package"])'
+        )
+    )
+    yield tmp_path
+
+
+@pytest.fixture
+def demo_setup_cfg(tmp_path):  # type: (Path) -> Path
+    setup_cfg = tmp_path / "setup.cfg"
+    setup_cfg.write_text(
+        decode(
+            "\n".join(
+                [
+                    "[metadata]",
+                    "name = demo",
+                    "version = 0.1.0",
+                    "[options]",
+                    "install_requires = package",
+                ]
+            )
+        )
+    )
+    yield tmp_path
+
+
 @pytest.fixture
 def demo_setup_complex(tmp_path):  # type: (Path) -> Path
     setup_py = tmp_path / "setup.py"
@@ -41,6 +74,15 @@ def demo_setup_complex(tmp_path):  # type: (Path) -> Path
     yield tmp_path
 
 
+@pytest.fixture
+def demo_setup_complex_pep517_legacy(demo_setup_complex):  # type: (Path) -> Path
+    pyproject_toml = demo_setup_complex / "pyproject.toml"
+    pyproject_toml.write_text(
+        decode("[build-system]\n" 'requires = ["setuptools", "wheel"]')
+    )
+    yield demo_setup_complex
+
+
 def demo_check_info(info, requires_dist=None):  # type: (PackageInfo, Set[str]) -> None
     assert info.name == "demo"
     assert info.version == "0.1.0"
@@ -82,9 +124,15 @@ def test_info_from_requires_txt():
 
 
 @pytest.mark.skipif(not PY35, reason="Parsing of setup.py is skipped for Python < 3.5")
-def test_info_from_setup_py():
-    info = PackageInfo.from_setup_py(FIXTURE_DIR_INSPECTIONS / "demo_only_setup")
-    demo_check_info(info)
+def test_info_from_setup_py(demo_setup):
+    info = PackageInfo.from_setup_files(demo_setup)
+    demo_check_info(info, requires_dist={"package"})
+
+
+@pytest.mark.skipif(not PY35, reason="Parsing of setup.cfg is skipped for Python < 3.5")
+def test_info_from_setup_cfg(demo_setup_cfg):
+    info = PackageInfo.from_setup_files(demo_setup_cfg)
+    demo_check_info(info, requires_dist={"package"})
 
 
 def test_info_no_setup_pkg_info_no_deps():
@@ -96,14 +144,38 @@ def test_info_no_setup_pkg_info_no_deps():
     assert info.requires_dist is None
 
 
+def test_info_setup_simple(mocker, demo_setup):
+    pep517_metadata_build = mocker.patch("pep517.meta.build")
+    info = PackageInfo.from_directory(demo_setup, allow_build=True)
+    pep517_metadata_build.assert_not_called()
+    demo_check_info(info, requires_dist={"package"})
+
+
+@pytest.mark.skipif(not PY35, reason="Parsing of setup.cfg is skipped for Python < 3.5")
+def test_info_setup_cfg(mocker, demo_setup_cfg):
+    pep517_metadata_build = mocker.patch("pep517.meta.build")
+    info = PackageInfo.from_directory(demo_setup_cfg, allow_build=True)
+    pep517_metadata_build.assert_not_called()
+    demo_check_info(info, requires_dist={"package"})
+
+
 def test_info_setup_complex(demo_setup_complex):
     info = PackageInfo.from_directory(demo_setup_complex, allow_build=True)
     demo_check_info(info, requires_dist={"package"})
 
 
+def test_info_setup_complex_pep517_legacy(demo_setup_complex_pep517_legacy):
+    info = PackageInfo.from_directory(
+        demo_setup_complex_pep517_legacy, allow_build=True
+    )
+    demo_check_info(info, requires_dist={"package"})
+
+
 @pytest.mark.skipif(not PY35, reason="Parsing of setup.py is skipped for Python < 3.5")
-def test_info_setup_complex_disable_build(demo_setup_complex):
+def test_info_setup_complex_disable_build(mocker, demo_setup_complex):
+    pep517_metadata_build = mocker.patch("pep517.meta.build")
     info = PackageInfo.from_directory(demo_setup_complex, allow_build=False)
+    pep517_metadata_build.assert_not_called()
     assert info.name == "demo"
     assert info.version == "0.1.0"
     assert info.requires_dist is None