Add tool to help with "bootstraping" build

MANIFEST.in

@@ -18,3 +18,5 @@ include pytest.ini
 include tox.ini
 include setuptools/tests/config/setupcfg_examples.txt
 global-exclude *.py[cod] __pycache__
+prune dist
+prune build

docs/development/bootstrap.rst

@@ -0,0 +1,57 @@
+----------------------------
+Bootstrapping ``setuptools``
+----------------------------
+
+If you need to *build* ``setuptools`` without the help of any third party tool
+(like :pypi:`build` or :pypi:`pip`), you can use the following procedure:
+
+1. Obtain ``setuptools``'s source code and change to the project root directory.
+   For example::
+
+        $ git clone https://github.com/pypa/setuptools
+        $ cd setuptools
+
+2. Run the bootstrap utility with the version of Python you intend to use
+   ``setuptools`` with::
+
+        $ python3 -m setuptools._bootstrap
+
+   This will create a :term:`setuptools-*.whl <PyPUG:Wheel>` file in the ``./dist`` directory.
+
+Furthermore, if you also need to bootstrap the *installation* of ``setuptools``,
+you can follow the additional steps:
+
+3. Find out the directory where Python expects packages to be installed.
+   The following command can help with that::
+
+       $ python3 -m sysconfig
+
+   Since ``setuptools`` is a pure-Python distribution,
+   usually you will only need the path referring to ``purelib``.
+
+4. Extract the created ``.whl`` file into the relevant directory.
+   For example::
+
+      $ python3 -m zipfile -e ./dist/setuptools-*.whl $TARGET_DIR
+
+
+Notes
+~~~~~
+
+This procedure assumes that you have access to a fully functional Python
+installation, including the standard library.
+
+The ``setuptools._bootstrap`` tool is a modest bare-bones implementation
+that follows the :pep:`PyPA's build-system spec <517>`,
+simplified and stripped down to only support the ``setuptools`` package.
+
+This procedure is not intended for other packages, it will not
+provide the same guarantees as a proper Python package installer
+or build-frontend tool, and it is still experimental.
+
+The naming intentionally uses a ``_`` character to discourage
+regular users, as the tool is only provided for developers (or downstream packaging
+consumers) that need to deploy ``setuptools`` from scratch.
+
+This is a CLI-only implementation, with no API provided.
+Users interested in API usage are invited to follow :pep:`PyPA's build-system spec <517>`.

docs/development/index.rst

@@ -32,3 +32,4 @@ setuptools changes.  You have been warned.
 
    developer-guide
    releases
+   bootstrap

pyproject.toml

@@ -167,6 +167,7 @@ exclude = [
 	"newsfragments*",
 	"docs",
 	"docs.*",
+	"build*",
 ]
 namespaces = true
 

setuptools/_bootstrap.py

@@ -0,0 +1,65 @@
+from __future__ import annotations
+
+import argparse
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+__all__: list[str] = []  # No public function, only CLI is provided.
+
+
+def _build(output_dir: Path) -> None:
+    """Emulate as close as possible the way a build frontend would work."""
+    # Call build_wheel hook from CWD
+    _hook("build_sdist", Path.cwd(), output_dir)
+    sdist = _find_or_halt(output_dir, "setuptools*.tar.gz", "Error building sdist")
+    print(f"**** sdist created in `{sdist}` ****")
+
+    # Call build_wheel hook from the sdist
+    with tempfile.TemporaryDirectory() as tmp:
+        subprocess.run([sys.executable, "-m", "tarfile", "-e", str(sdist), tmp])
+
+        root = _find_or_halt(Path(tmp), "setuptools-*", "Error finding sdist root")
+        _hook("build_wheel", root, output_dir)
+
+    wheel = _find_or_halt(output_dir, "setuptools*.whl", "Error building wheel")
+    print(f"**** wheel created in `{wheel}` ****")
+
+
+def _find_or_halt(parent: Path, pattern: str, error: str) -> Path:
+    if file := next(parent.glob(pattern), None):
+        return file
+    raise SystemExit(f"{error}. Cannot find `{parent / pattern}`")
+
+
+def _hook(name: str, source_dir: Path, output_dir: Path) -> None:
+    # Call each hook in a fresh subprocess as required by PEP 517
+    out = str(output_dir.absolute())
+    script = f"from setuptools.build_meta import {name}; {name}({out!r})"
+    subprocess.run([sys.executable, "-c", script], cwd=source_dir)
+
+
+def _cli() -> None:
+    parser = argparse.ArgumentParser(
+        description="**EXPERIMENTAL** bootstrapping script for setuptools. "
+        "Note that this script will perform a **simplified** procedure and may not "
+        "provide all the guarantees of full-blown Python build-frontend.\n"
+        "To install the created wheel, please extract it into the relevant directory.",
+        formatter_class=argparse.ArgumentDefaultsHelpFormatter,
+    )
+    parser.add_argument(
+        "--output-dir",
+        type=Path,
+        default="./dist",
+        help="Where to store the build artifacts",
+    )
+    params = parser.parse_args()
+    if params.output_dir.exists() and len(list(params.output_dir.iterdir())) > 0:
+        # Let's avoid accidents by preventing multiple wheels in the directory
+        raise SystemExit(f'--output-dir="{params.output_dir}" must be empty.')
+    _build(params.output_dir)
+
+
+if __name__ == "__main__":
+    _cli()

setuptools/tests/test_bootstrap.py

@@ -0,0 +1,51 @@
+import os
+import shutil
+
+import pytest
+from setuptools.archive_util import unpack_archive
+
+
+CMD = ["python", "-m", "setuptools._bootstrap"]
+
+
+@pytest.fixture
+def setuptools_sourcetree(tmp_path, setuptools_sdist, request):
+    """
+    Recreate the setuptools source tree.
+    We use sdist in a temporary directory to avoid race conditions with build/dist dirs.
+    """
+    unpack_archive(setuptools_sdist, tmp_path)
+    root = next(tmp_path.glob("setuptools-*"))
+    # Remove sdist's metadata/cache/artifacts to simulate fresh repo
+    shutil.rmtree(root / "setuptools.egg-info", ignore_errors=True)
+    (root / "PKG-INFO").unlink()
+    # We need the bootstrap folder (not included in the sdist)
+    shutil.copytree(
+        os.path.join(request.config.rootdir, "bootstrap.egg-info"),
+        os.path.join(root, "bootstrap.egg-info"),
+    )
+    return root
+
+
+def test_bootstrap_sourcetree(tmp_path, bare_venv, setuptools_sourcetree):
+    bare_venv.run(CMD, cwd=str(setuptools_sourcetree))
+    wheel = next((setuptools_sourcetree / "dist").glob("*.whl"))
+    assert wheel.name.startswith("setuptools")
+
+    target = tmp_path / "target"
+    target.mkdir()
+    bare_venv.run(["python", "-m", "zipfile", "-e", str(wheel), str(target)])
+
+    # Included in wheel:
+    assert (target / "distutils-precedence.pth").is_file()
+    assert (target / "setuptools/__init__.py").is_file()
+    assert (target / "pkg_resources/__init__.py").is_file()
+    # Excluded from wheel:
+    assert not (target / "setuptools/tests").is_dir()
+    assert not (target / "pkg_resources/tests").is_dir()
+
+    # Avoid errors on Windows by copying env before modifying
+    # https://stackoverflow.com/questions/58997105
+    env = {**os.environ, "PYTHONPATH": str(target)}
+    test = ["python", "-c", "print(__import__('setuptools').__version__)"]
+    bare_venv.run(test, env=env)