Support PEP 517 list based backend-path

The PEP 517 shows an example backend-path like this:

    [build-system]
    # Defined by PEP 518:
    requires = ["flit"]
    # Defined by this PEP:
    build-backend = "local_backend"
    backend-path = ["backend"]

https://www.python.org/dev/peps/pep-0517/#source-trees

See that backend-path is a list. Our code previously only supported string path.

Obviously a string path is wrong, but we keep it to support projects that have
made the mistake, such as flit-core.

Add a small integration test for both cases.
Note that the new spec files deliberately don't do much, to save CI time.

pyproject-rpm-macros.spec

@@ -6,7 +6,7 @@ License:        MIT
 
 # Keep the version at zero and increment only release
 Version:        0
-Release:        30%{?dist}
+Release:        31%{?dist}
 
 # Macro files
 Source001:      macros.pyproject
@@ -94,6 +94,9 @@ export HOSTNAME="rpmbuild"  # to speedup tox in network-less mock, see rhbz#1856
 %license LICENSE
 
 %changelog
+* Mon Oct 05 2020 Miro Hrončok <mhroncok@redhat.com> - 0-31
+- Support PEP 517 list based backend-path
+
 * Tue Sep 29 2020 Lumír Balhar <lbalhar@redhat.com> - 0-30
 - Process RECORD files in %%pyproject_install and remove them
 - Support the extras configuration option of tox in %%pyproject_buildrequires -t

pyproject_buildrequires.py

@@ -185,7 +185,10 @@ def get_backend(requirements):
 
     backend_path = buildsystem_data.get('backend-path')
     if backend_path:
-        sys.path.insert(0, backend_path)
+        # PEP 517 example shows the path as a list, but some projects don't follow that
+        if isinstance(backend_path, str):
+            backend_path = [backend_path]
+        sys.path = backend_path + sys.path
 
     module_name, _, object_name = backend_name.partition(":")
     backend_module = importlib.import_module(module_name)

tests/python-flit-core.spec

@@ -0,0 +1,43 @@
+Name:           python-flit-core
+Version:        3.0.0
+Release:        0%{?dist}
+Summary:        Distribution-building parts of Flit
+
+License:        BSD
+URL:            https://pypi.org/project/flit-core/
+Source0:        %{pypi_source flit_core}
+
+BuildArch:      noarch
+BuildRequires:  python3-devel
+BuildRequires:  pyproject-rpm-macros
+
+%description
+Test a build with pyproject.toml backend-path = .
+flit-core builds with flit-core.
+
+
+%package -n python3-flit-core
+Summary:        %{summary}
+
+%description -n python3-flit-core
+...
+
+
+%prep
+%autosetup -p1 -n flit_core-%{version}
+
+
+%generate_buildrequires
+%pyproject_buildrequires
+
+
+%build
+%pyproject_wheel
+
+
+%install
+%pyproject_install
+%pyproject_save_files flit_core
+
+
+%files -n python3-flit-core -f %{pyproject_files}

tests/python-poetry-core.spec

@@ -0,0 +1,45 @@
+Name:           python-poetry-core
+Version:        1.0.0
+Release:        0%{?dist}
+Summary:        Poetry PEP 517 Build Backend
+
+License:        MIT
+URL:            https://pypi.org/project/poetry-core/
+Source0:        %{pypi_source poetry-core}
+
+BuildArch:      noarch
+BuildRequires:  python3-devel
+BuildRequires:  pyproject-rpm-macros
+
+%description
+Test a build with pyproject.toml backend-path = [.]
+poetry-core builds with poetry-core.
+
+
+%package -n python3-poetry-core
+Summary:        %{summary}
+
+%description -n python3-poetry-core
+...
+
+
+%prep
+%autosetup -p1 -n poetry-core-%{version}
+
+
+%generate_buildrequires
+%pyproject_buildrequires
+
+
+%build
+%pyproject_wheel
+
+
+%install
+%pyproject_install
+%pyproject_save_files poetry
+
+
+%files -n python3-poetry-core -f %{pyproject_files}
+%doc README.md
+%license LICENSE

tests/tests.yml

@@ -67,6 +67,12 @@
     - dns_lexicon:
         dir: .
         run: ./mocktest.sh python-dns-lexicon
+    - flit_core:
+        dir: .
+        run: ./mocktest.sh python-flit-core
+    - poetry_core:
+        dir: .
+        run: ./mocktest.sh python-poetry-core
     required_packages:
     - mock
     - rpmdevtools