pyproject: only pass metadata_directory when it's not empty

Passing empty `metadata_directory` to `build_wheel` or `build_editable` can
cause some PEP 517 backends to create a wheel without metadata.

Fix #2880

Author: masenf

docs/changelog/2880.bugfix.rst

@@ -0,0 +1,10 @@
+When building a ``wheel`` or ``editable`` package with a PEP 517 backend, no
+longer pass an empty ``metadata_directory`` to the backend ``build_wheel`` or
+``build_editable`` endpoint.
+
+Some backends, such as PDM and poetry, will not generate package metadata in
+the presence of a ``metadata_directory``, even if it is empty.
+
+Prior to this change, attempting to install a wheel created by tox using PDM or
+poetry would return an error like ``There is no item named
+'my-package.0.1.dist-info/WHEEL' in the archive`` - by :user:`masenf`.

src/tox/tox_env/python/virtual_env/package/pyproject.py

@@ -137,6 +137,14 @@ def meta_folder(self) -> Path:
         meta_folder.mkdir(exist_ok=True)
         return meta_folder
 
+    @property
+    def meta_folder_if_populated(self) -> Path | None:
+        """Return the metadata directory if it contains any files, otherwise None."""
+        meta_folder = self.meta_folder
+        if meta_folder.exists() and tuple(meta_folder.iterdir()):
+            return meta_folder
+        return None
+
     def register_run_env(self, run_env: RunToxEnv) -> Generator[tuple[str, str], PackageToxEnv, None]:
         yield from super().register_run_env(run_env)
         build_type = run_env.conf["package"]
@@ -210,7 +218,7 @@ def perform_packaging(self, for_env: EnvConfigSet) -> list[Package]:
                 with self._pkg_lock:
                     wheel = getattr(self._frontend, method)(
                         wheel_directory=self.pkg_dir,
-                        metadata_directory=self.meta_folder,
+                        metadata_directory=self.meta_folder_if_populated,
                         config_settings=self._wheel_config_settings,
                     ).wheel
                     wheel = create_session_view(wheel, self._package_temp_path)