fix(setuptools): only inject logic if cmake_* keywords present (#768)

This is a sort of follow up to #413 and #414. In sage-on-gentoo we
discovered that the presence of scikit-build-core breaks the building
sagemath (cschwan/sage-on-gentoo#783).
scikit-build-core also happens to be an indirect dependency for building
sagemath's documentation.
On the basis that the presence of setuptolls_rust does not break
sagemath's build, I studied the difference with scikit-build-core. I
noticed that the elements `bundled in the single
"finalize_distribution_options" entrypoint in scikit is split between
two different setuptools entrypoints.
This PR reproduce that setup and does a bit of clean up. The bundling of
the old _prepare_extension_detection and _prepare_build_cmake_command
into finalize_distribution_options being redundant after attributing
them directly to separate entrypoints. I also renamed the functions to
better characterise them.

Using this PR, sagemath builds without problems and pyzmq, which uses
scikit-build-core also builds properly.

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

src/scikit_build_core/setuptools/build_cmake.py

@@ -19,7 +19,12 @@
 
     from .._compat.typing import Literal
 
-__all__ = ["BuildCMake", "cmake_source_dir", "finalize_distribution_options"]
+__all__ = [
+    "BuildCMake",
+    "cmake_args",
+    "cmake_source_dir",
+    "finalize_distribution_options",
+]
 
 
 def __dir__() -> list[str]:
@@ -176,9 +181,25 @@ def _has_cmake(dist: Distribution) -> bool:
     )
 
 
-def _prepare_extension_detection(dist: Distribution) -> None:
-    # Setuptools needs to know that it has extensions modules
+def finalize_distribution_options(dist: Distribution) -> None:
+    # Prepare new build_cmake command and make sure build calls it
+    build = dist.get_command_class("build")
+    assert build is not None
+    if "build_cmake" not in {x for x, _ in build.sub_commands}:
+        build.sub_commands.append(
+            ("build_cmake", lambda cmd: _has_cmake(cmd.distribution))
+        )
 
+
+def _cmake_extension(dist: Distribution) -> None:
+    # Every keyword argument needs to call this
+    # Run this only once
+    if getattr(dist, "_has_cmake_extensions", False):
+        return
+    # pylint: disable-next=protected-access
+    dist._has_cmake_extensions = True  # type: ignore[attr-defined]
+
+    # Setuptools needs to know that it has extensions modules
     orig_has_ext_modules = dist.has_ext_modules
     dist.has_ext_modules = lambda: orig_has_ext_modules() or _has_cmake(dist)  # type: ignore[method-assign]
 
@@ -192,34 +213,21 @@ def __len__(self) -> int:
         dist.ext_modules = getattr(dist, "ext_modules", []) or EvilList()
 
 
-def _prepare_build_cmake_command(dist: Distribution) -> None:
-    # Prepare new build_cmake command and make sure build calls it
-    build = dist.get_command_class("build")
-    assert build is not None
-    if "build_cmake" not in {x for x, _ in build.sub_commands}:
-        build.sub_commands.append(
-            ("build_cmake", lambda cmd: _has_cmake(cmd.distribution))
-        )
-
-
 def cmake_args(
-    _dist: Distribution, attr: Literal["cmake_args"], value: list[str]
+    dist: Distribution, attr: Literal["cmake_args"], value: list[str]
 ) -> None:
     assert attr == "cmake_args"
+    _cmake_extension(dist)
     if not isinstance(value, list):
         msg = "cmake_args must be a list"
         raise setuptools.errors.SetupError(msg)
 
 
 def cmake_source_dir(
-    _dist: Distribution, attr: Literal["cmake_source_dir"], value: str
+    dist: Distribution, attr: Literal["cmake_source_dir"], value: str
 ) -> None:
     assert attr == "cmake_source_dir"
+    _cmake_extension(dist)
     if not Path(value).is_dir():
         msg = "cmake_source_dir must be an existing directory"
         raise setuptools.errors.SetupError(msg)
-
-
-def finalize_distribution_options(dist: Distribution) -> None:
-    _prepare_extension_detection(dist)
-    _prepare_build_cmake_command(dist)