fix: tombstone build needs setuptools installed explicitly

.github/workflows/publish_tombstone.yml

@@ -54,7 +54,10 @@ jobs:
 
       - name: Build the tombstone sdists
         run: |
-          python3 -m pip install --upgrade build twine==6.0.1
+          # setuptools is explicit: the sdist builds with --no-isolation, and
+          # Python 3.12+ runners no longer ship setuptools, so the backend
+          # would otherwise be missing (BackendUnavailable, run 32309423898).
+          python3 -m pip install --upgrade build twine==6.0.1 setuptools
           python3 -m autohands.tombstone --out dist-tombstone
 
       - name: Check metadata

autohands/tombstone.py

@@ -67,6 +67,7 @@
 from __future__ import annotations
 
 import argparse
+import importlib.util
 import shutil
 import subprocess
 import sys
@@ -251,6 +252,27 @@ def write_project(
     return project_dir
 
 
+def require_setuptools() -> None:
+    """Fail early, and legibly, when the build backend is missing.
+
+    The sdist is built with `--no-isolation`, so setuptools has to be present in
+    this interpreter — and Python 3.12 dropped it from the default environment.
+    Without this check the failure surfaces as `BackendUnavailable: Cannot
+    import 'setuptools.build_meta'` from inside pyproject_hooks, which names
+    neither the cause nor the fix. A developer machine that happens to have
+    setuptools installed will not reproduce it, so the check has to be explicit
+    rather than left to whatever the environment happens to carry.
+    """
+    if importlib.util.find_spec("setuptools") is None:
+        raise RuntimeError(
+            "setuptools is not installed in this interpreter, so the sdist cannot "
+            "be built. The build deliberately runs with --no-isolation so it "
+            "cannot silently reach the network for a backend; Python 3.12 and "
+            "later no longer ship setuptools by default. Install it first: "
+            "python3 -m pip install setuptools"
+        )
+
+
 def build_sdist(
     project_dir: Path,
     out_dir: Path,
@@ -268,6 +290,7 @@ def build_sdist(
     so "the newest tarball here" would happily hand back a sibling package's
     sdist and verify *its* metadata instead.
     """
+    require_setuptools()
     out_dir.mkdir(parents=True, exist_ok=True)
     subprocess.run(
         [

tests/test_tombstone.py

@@ -163,6 +163,19 @@ def _build_available():
 )
 
 
+def test_missing_setuptools_is_reported_before_the_build(tmp_path, monkeypatch):
+    """`--no-isolation` means the backend must already be installed, and Python
+    3.12 dropped setuptools from the default environment. Left unchecked the
+    failure arrives as `BackendUnavailable: Cannot import
+    'setuptools.build_meta'` from inside pyproject_hooks, naming neither the
+    cause nor the fix — and a machine that happens to have setuptools will
+    never reproduce it (PyAutoHands run 32309423898)."""
+    monkeypatch.setattr(tombstone.importlib.util, "find_spec", lambda name: None)
+
+    with pytest.raises(RuntimeError, match="pip install setuptools"):
+        tombstone.build_sdist(tmp_path / "project", tmp_path / "dist", "autolens")
+
+
 @requires_build
 def test_built_sdist_declares_sub_floor_requires_python(tmp_path):
     project = tombstone.write_project("autolens", tmp_path)