Add virtualenv seeder plugin to install build

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.seeder: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

@@ -88,10 +88,19 @@ 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)],
-            network_isolation=False,
+            [
+                sys.executable,
+                "-m",
+                "virtualenv",
+                "--never-download",
+                "--no-periodic-update",
+                "--seeder=fromager",
+                str(self.path),
+            ],
+            network_isolation=self._ctx.network_isolation,
         )
         logger.info("created build environment in %s", self.path)
 

src/fromager/virtualenv/seeder.py

@@ -0,0 +1,68 @@
+"""Virtualenv seeder plugin for Fromager
+
+The plugin installs ``build`` and its dependencies ``packaging`` +
+``pyproject_hooks`` into a new virtual environment. The wheel files for
+the three packages are bundled and shipped with Fromager. This solves
+problems with bootstrapping.
+
+Either Fromager would have to implement a special bootstrapping mode to
+build ``build``, ``packaging``, ``pyproject_hooks``, and ``flit-core``
+wheels first, then switch its wheel build command to ``build``.
+
+Or Fromager would have to download three pre-built wheels from PyPI.
+
+2. Fromager can create a build environment with the ``build`` command without
+   downloading any content from PyPI. Instead it uses the same bundle approach
+   as ``virtualenv`` and ``ensurepip``.
+
+
+"""
+
+import pathlib
+import typing
+
+from packaging.utils import parse_wheel_filename
+
+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
+from virtualenv.seed.wheels.embed import BUNDLE_SUPPORT
+
+BUNDLED_DIR = pathlib.Path(__file__).parent.resolve() / "bundled"
+# `build`` depends on packaging and pyproject_hooks
+BUNDLED_PACKAGES = ["build", "packaging", "pyproject_hooks"]
+
+
+class FromagerSeeder(FromAppData):
+    """Custom virtualenv seeder to install build command"""
+
+    def __init__(self, options: VirtualEnvOptions) -> None:
+        # register our packages
+        for whl, name, version in self.list_extra_packages():
+            # add option defaults
+            setattr(self, f"no_{name}", False)
+            setattr(self, f"{name}_version", version)
+            # register wheel files with virtualenv's bundle support
+            for py_version in BUNDLE_SUPPORT:
+                BUNDLE_SUPPORT[py_version][name] = str(whl)
+
+        # virtualenv no longer installs setuptool and wheels for
+        # Python >= 3.12, force installation.
+        for opt in ("setuptools", "wheel"):
+            if getattr(options, opt) == "none":
+                setattr(options, opt, Version.bundle)
+
+        super().__init__(options)
+
+    @classmethod
+    def list_extra_packages(cls) -> typing.Iterable[tuple[pathlib.Path, str, str]]:
+        for whl in sorted(BUNDLED_DIR.glob("*.whl")):
+            name, version, _, _ = parse_wheel_filename(whl.name)
+            yield whl, name, str(version)
+
+    @classmethod
+    def distributions(cls) -> dict[str, str]:
+        dist = super().distributions()
+        for _, name, version in cls.list_extra_packages():
+            dist[name] = version
+        return dist

src/fromager/virtualenv/update.py

@@ -0,0 +1,16 @@
+"""Update bundled wheels"""
+
+import subprocess
+
+from .seeder import BUNDLED_DIR, BUNDLED_PACKAGES
+
+
+def main():
+    for whl in BUNDLED_DIR.glob("*.whl"):
+        whl.unlink()
+    cmd = ["pip", "download", "-d", str(BUNDLED_DIR), *BUNDLED_PACKAGES]
+    subprocess.check_call(cmd)
+
+
+if __name__ == "__main__":
+    main()