Add virtualenv seeder plugin to install build

Implements a seeder plugin that extends the seed function of virtualenv.
The plugin allows us to seed the `build` package into a new virtual env
and work around missing `setuptools` and `wheel` commands in Python
3.12+ virtual envs.

Related: python-wheel-build#126
Signed-off-by: Christian Heimes <[email protected]>

Author: tiran

pyproject.toml

@@ -54,6 +54,9 @@ resolve_source = "fromager.sources:default_resolve_source"
 build_sdist = "fromager.sources:default_build_sdist"
 build_wheel = "fromager.wheels:default_build_wheel"
 
+[project.entry-points."virtualenv.seed"]
+fromager = "fromager.virtualenv:FromagerSeeder"
+
 [tool.coverage.run]
 branch = true
 parallel = true
@@ -130,5 +133,5 @@ exclude = [
 
 [[tool.mypy.overrides]]
 # packages without typing annotations and stubs
-module = ["pyproject_hooks", "requests_mock", "resolver", "stevedore"]
+module = ["pyproject_hooks", "requests_mock", "resolver", "stevedore", "virtualenv.*"]
 ignore_missing_imports = true

src/fromager/build_environment.py

@@ -88,9 +88,17 @@ def _createenv(self) -> None:
             logger.info("reusing build environment in %s", self.path)
             return
 
+        # use our seeder plugin to install `build` command
         logger.debug("creating build environment in %s", self.path)
         external_commands.run(
-            [sys.executable, "-m", "virtualenv", str(self.path)],
+            [
+                sys.executable,
+                "-m",
+                "virtualenv",
+                "--download",
+                "--seeder=fromager",
+                str(self.path),
+            ],
             network_isolation=False,
         )
         logger.info("created build environment in %s", self.path)

src/fromager/virtualenv.py

@@ -0,0 +1,42 @@
+"""Virtual seed plugin"""
+
+import argparse
+import typing
+
+from virtualenv.config.cli.parser import VirtualEnvOptions
+from virtualenv.seed.embed.via_app_data.via_app_data import FromAppData
+from virtualenv.seed.wheels import Version
+
+
+class FromagerSeeder(FromAppData):
+    """Custom virtualenv seeder to install build command"""
+
+    extra_packages: typing.ClassVar[dict[str, Version]] = {"build": Version.bundle}
+
+    def __init__(self, options: VirtualEnvOptions) -> None:
+        for distribution, default in self.extra_packages.items():
+            setattr(self, f"no_{distribution}", False)
+            setattr(self, f"{distribution}_version", default)
+
+        super().__init__(options)
+        if not self.download:
+            raise argparse.ArgumentError(
+                None, "--download is required to install extra packages"
+            )
+
+    @classmethod
+    def distributions(cls) -> dict[str, Version]:
+        dist = super().distributions()
+        dist.update(cls.extra_packages)
+        return dist
+
+    @classmethod
+    def add_parser_arguments(
+        cls, parser: argparse.ArgumentParser, interpreter, app_data
+    ) -> None:
+        super().add_parser_arguments(parser, interpreter, app_data)
+        # virtualenv no longer installs setuptool and wheels for
+        # Python >= 3.12. force installation
+        for action in parser._actions:
+            if action.dest in {"setuptools", "wheel"}:
+                action.default = Version.bundle