[stable-2.14] 📦 Switch sdist build-system to pure setuptools (#80255) (#80261)

* [stable-2.14] 📦 Switch sdist build-system to pure setuptools (#80255)

This patch modifies the in-tree build backend to build sdists that swap
out pointers to it in the `pyproject.toml`'s `[build-system]` section.

The effect of this is that the first build from source (for example,
from a Git checkout) uses our PEP 517 in-tree build backend. But the
produced tarball has `build-backend` set to `setuptools.build_meta`
which is the native build backend of `setuptools`. So any following
builds from that sdist will skip using the in-tree build backend,
calling the setuptools' one.
The good news is that if the first build generated the manpages, they
will be included and won't go anywhere even though, a different build
system is in place.

Combined with #80253, this will make sure not to modify the current
source checkout on that first build.

Co-authored-by: Matt Clay <matt@mystile.com>
(cherry picked from commit 7097df3eed979446830fc579613ffb9b7e7c19bf)

Co-authored-by: Sviatoslav Sydorenko <webknjaz@redhat.com>

* Make PEP 517 mutation tests use pinned old setuptools (#80262)

* Pin setuptools to lowest supported @ PEP 517 test

This allows catching the behavior of builds under old setuptools.

* Stop invoking `setup.py install` in tests

This is not the part we care about since it involves dealing with the
external runtime dependencies rather than building our source
distribution.

(cherry picked from commit eebfd71a6da396b011e664604c9991543205d780)

---------

Co-authored-by: Sviatoslav Sydorenko <webknjaz@redhat.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/packaging/pep517_backend/_backend.py b/packaging/pep517_backend/_backend.py
index 9c253296ff..d4d10f2364 100644
--- a/packaging/pep517_backend/_backend.py
+++ b/packaging/pep517_backend/_backend.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import os
+import re
 import subprocess
 import sys
 import typing as t
@@ -131,6 +132,19 @@ def build_sdist(  # noqa: WPS210, WPS430
                 )
                 rst_in.unlink()
 
+        Path('pyproject.toml').write_text(
+            re.sub(
+                r"""(?x)
+                backend-path\s=\s\[  # value is a list of double-quoted strings
+                    [^]]+
+                ].*\n
+                build-backend\s=\s"[^"]+".*\n  # value is double-quoted
+                """,
+                'build-backend = "setuptools.build_meta"\n',
+                Path('pyproject.toml').read_text(),
+            )
+        )
+
         built_sdist_basename = _setuptools_build_sdist(
             sdist_directory=sdist_directory,
             config_settings=config_settings,