feat: add get_build_system_dependencies global hook point

src/fromager/dependencies.py

@@ -18,6 +18,7 @@
 from . import (
     build_environment,
     external_commands,
+    hooks,
     overrides,
     packagesettings,
     requirements_file,
@@ -65,6 +66,13 @@ def get_build_system_dependencies(
         sdist_root_dir=sdist_root_dir,
         build_dir=pbi.build_dir(sdist_root_dir),
     )
+    orig_deps = hooks.run_get_build_system_dependencies_hooks(
+        ctx=ctx,
+        req=req,
+        sdist_root_dir=sdist_root_dir,
+        build_dir=pbi.build_dir(sdist_root_dir),
+        requirements=list(orig_deps),
+    )
     deps = _filter_requirements(req, orig_deps)
 
     _write_requirements_file(

src/fromager/hooks.py

@@ -18,6 +18,7 @@
 
 # Event callbacks that run for every package (e.g., after build, after bootstrap).
 GLOBAL_HOOK_NAMES: tuple[str, ...] = (
+    "get_build_system_dependencies",
     "post_bootstrap",
     "post_build",
     "prebuilt_wheel",
@@ -71,6 +72,31 @@ def _die_on_plugin_load_failure(
     raise RuntimeError(f"failed to load overrides for {ep.name}") from err
 
 
+def run_get_build_system_dependencies_hooks(
+    ctx: context.WorkContext,
+    req: Requirement,
+    sdist_root_dir: pathlib.Path,
+    build_dir: pathlib.Path,
+    requirements: list[str],
+) -> list[str]:
+    """Run global hooks that post-process build-system dependencies.
+
+    Each hook receives the current requirements list and returns a
+    (possibly modified) list.  Hooks are chained: the output of one
+    becomes the input of the next.
+    """
+    hook_mgr = _get_hooks("get_build_system_dependencies")
+    for ext in hook_mgr:
+        requirements = ext.plugin(
+            ctx=ctx,
+            req=req,
+            sdist_root_dir=sdist_root_dir,
+            build_dir=build_dir,
+            requirements=requirements,
+        )
+    return requirements
+
+
 def run_post_build_hooks(
     ctx: context.WorkContext,
     req: Requirement,

tests/test_dependencies.py

@@ -394,6 +394,46 @@ def test_validate_dist_name_version(
             validate()
 
 
+@patch("fromager.dependencies._write_requirements_file")
+@patch("fromager.hooks._get_hooks")
+def test_get_build_system_dependencies_runs_global_hooks(
+    mock_get_hooks: Mock,
+    _: Mock,
+    tmp_context: context.WorkContext,
+    tmp_path: pathlib.Path,
+) -> None:
+    pyproject_content = textwrap.dedent("""\
+        [build-system]
+        requires = ["setuptools>=40.0"]
+        build-backend = "setuptools.build_meta"
+    """)
+    (tmp_path / "pyproject.toml").write_text(pyproject_content)
+
+    called_with: dict[str, typing.Any] = {}
+
+    def fake_hook(**kwargs: typing.Any) -> list[str]:
+        called_with.update(kwargs)
+        return kwargs["requirements"] + ["setuptools<82"]
+
+    from unittest.mock import MagicMock
+
+    fake_mgr = MagicMock()
+    fake_mgr.names.return_value = ["fake_hook"]
+    fake_mgr.__iter__ = lambda self: iter([MagicMock(plugin=fake_hook)])
+    mock_get_hooks.return_value = fake_mgr
+
+    results = dependencies.get_build_system_dependencies(
+        ctx=tmp_context,
+        req=Requirement("testpkg"),
+        version=Version("1.0.0"),
+        sdist_root_dir=tmp_path,
+    )
+
+    names = set(r.name for r in results)
+    assert "setuptools" in names
+    assert called_with["requirements"] == ["setuptools>=40.0"]
+
+
 def test_get_install_dependencies_of_wheel(tmp_path: pathlib.Path) -> None:
     """Test extracting install dependencies from a wheel file built with real tools."""
     # Arrange: Build a real wheel with dependencies

tests/test_hooks.py

@@ -221,6 +221,101 @@ def fake_plugin(**kwargs: typing.Any) -> None:
     assert "sdist_filename" not in called_with
 
 
+@patch("fromager.hooks._get_hooks")
+def test_run_get_build_system_dependencies_hooks_calls_plugin(
+    mock_get: Mock,
+) -> None:
+    called_with: dict[str, typing.Any] = {}
+
+    def fake_plugin(**kwargs: typing.Any) -> list[str]:
+        called_with.update(kwargs)
+        return kwargs["requirements"]
+
+    mock_get.return_value = _make_fake_mgr([fake_plugin])
+
+    ctx = Mock()
+    req = Requirement("numpy>=1.0")
+    sdist = pathlib.Path("/tmp/numpy-1.0")
+    build = pathlib.Path("/tmp/numpy-1.0/build")
+    reqs = ["setuptools>=40.0", "wheel"]
+
+    result = hooks.run_get_build_system_dependencies_hooks(
+        ctx=ctx,
+        req=req,
+        sdist_root_dir=sdist,
+        build_dir=build,
+        requirements=reqs,
+    )
+
+    mock_get.assert_called_once_with("get_build_system_dependencies")
+    assert called_with["ctx"] is ctx
+    assert called_with["req"] is req
+    assert called_with["sdist_root_dir"] is sdist
+    assert called_with["build_dir"] is build
+    assert called_with["requirements"] == reqs
+    assert result == reqs
+
+
+@patch("fromager.hooks._get_hooks")
+def test_run_get_build_system_dependencies_hooks_chains(
+    mock_get: Mock,
+) -> None:
+    def hook_a(**kwargs: typing.Any) -> list[str]:
+        return kwargs["requirements"] + ["extra-a"]
+
+    def hook_b(**kwargs: typing.Any) -> list[str]:
+        return kwargs["requirements"] + ["extra-b"]
+
+    mock_get.return_value = _make_fake_mgr([hook_a, hook_b])
+
+    result = hooks.run_get_build_system_dependencies_hooks(
+        ctx=Mock(),
+        req=Requirement("pkg"),
+        sdist_root_dir=pathlib.Path("/tmp/pkg"),
+        build_dir=pathlib.Path("/tmp/pkg/build"),
+        requirements=["setuptools"],
+    )
+
+    assert result == ["setuptools", "extra-a", "extra-b"]
+
+
+@patch("fromager.hooks._get_hooks")
+def test_run_get_build_system_dependencies_hooks_no_hooks(
+    mock_get: Mock,
+) -> None:
+    mock_get.return_value = _make_fake_mgr([])
+
+    reqs = ["setuptools>=40.0", "wheel"]
+    result = hooks.run_get_build_system_dependencies_hooks(
+        ctx=Mock(),
+        req=Requirement("pkg"),
+        sdist_root_dir=pathlib.Path("/tmp/pkg"),
+        build_dir=pathlib.Path("/tmp/pkg/build"),
+        requirements=reqs,
+    )
+
+    assert result == reqs
+
+
+@patch("fromager.hooks._get_hooks")
+def test_run_get_build_system_dependencies_hooks_exception_propagates(
+    mock_get: Mock,
+) -> None:
+    def bad_plugin(**kwargs: typing.Any) -> list[str]:
+        raise ValueError("hook failed")
+
+    mock_get.return_value = _make_fake_mgr([bad_plugin])
+
+    with pytest.raises(ValueError, match="hook failed"):
+        hooks.run_get_build_system_dependencies_hooks(
+            ctx=Mock(),
+            req=Requirement("pkg"),
+            sdist_root_dir=pathlib.Path("/tmp/pkg"),
+            build_dir=pathlib.Path("/tmp/pkg/build"),
+            requirements=["setuptools"],
+        )
+
+
 @patch("fromager.hooks.overrides._get_dist_info", return_value=("mypkg", "1.0.0"))
 @patch("fromager.hooks.extension.ExtensionManager")
 def test_log_hooks_logs_each_extension(