From d0e233604989fad7162775b75ba5ec0741e4932c Mon Sep 17 00:00:00 2001
From: Scott K Logan <logans@cottsay.net>
Date: Fri, 20 Sep 2024 13:07:52 -0500
Subject: [PATCH] Add function for reading and caching PEP 518 spec

---
 colcon_core/python_project/__init__.py |  0
 colcon_core/python_project/spec.py     | 49 ++++++++++++++++++++++++
 setup.cfg                              |  1 +
 test/spell_check.words                 |  5 +++
 test/test_pyproject_spec.py            | 53 ++++++++++++++++++++++++++
 5 files changed, 108 insertions(+)
 create mode 100644 colcon_core/python_project/__init__.py
 create mode 100644 colcon_core/python_project/spec.py
 create mode 100644 test/test_pyproject_spec.py

diff --git a/colcon_core/python_project/__init__.py b/colcon_core/python_project/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/colcon_core/python_project/spec.py b/colcon_core/python_project/spec.py
new file mode 100644
index 000000000..2fcf8303d
--- /dev/null
+++ b/colcon_core/python_project/spec.py
@@ -0,0 +1,49 @@
+# Copyright 2022 Open Source Robotics Foundation, Inc.
+# Licensed under the Apache License, Version 2.0
+
+try:
+    # Python 3.11+
+    from tomllib import load as toml_load
+except ImportError:
+    from tomli import load as toml_load
+
+
+SPEC_NAME = 'pyproject.toml'
+
+_DEFAULT_BUILD_SYSTEM = {
+    'build-backend': 'setuptools.build_meta:__legacy__',
+    'requires': ['setuptools >= 40.8.0', 'wheel'],
+}
+
+
+def load_spec(project_path):
+    """
+    Load build system specifications for a Python project.
+
+    :param project_path: Path to the root directory of the project
+    """
+    spec_file = project_path / SPEC_NAME
+    try:
+        with spec_file.open('rb') as f:
+            spec = toml_load(f)
+    except FileNotFoundError:
+        spec = {}
+
+    spec.setdefault('build-system', _DEFAULT_BUILD_SYSTEM)
+
+    return spec
+
+
+def load_and_cache_spec(desc):
+    """
+    Get the cached spec for a package descriptor.
+
+    If the spec has not been loaded yet, load and cache it.
+
+    :param desc: The package descriptor
+    """
+    spec = desc.metadata.get('python_project_spec')
+    if spec is None:
+        spec = load_spec(desc.path)
+        desc.metadata['python_project_spec'] = spec
+    return spec
diff --git a/setup.cfg b/setup.cfg
index 3928d02e7..e20b5115a 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -39,6 +39,7 @@ install_requires =
   pytest-repeat
   pytest-rerunfailures
   setuptools>=30.3.0
+  tomli>=1.0.0;python_version < "3.11"
 packages = find:
 zip_safe = false
 
diff --git a/test/spell_check.words b/test/spell_check.words
index 18c2677bf..9bde54cf7 100644
--- a/test/spell_check.words
+++ b/test/spell_check.words
@@ -4,6 +4,7 @@ apache
 argparse
 asyncio
 autouse
+backend
 backported
 basepath
 bazqux
@@ -85,6 +86,7 @@ prepending
 proactor
 purelib
 pydocstyle
+pyproject
 pytest
 pytests
 pythondontwritebytecode
@@ -136,6 +138,9 @@ testsuite
 thomas
 tmpdir
 todo
+toml
+tomli
+tomllib
 traceback
 tryfirst
 tuples
diff --git a/test/test_pyproject_spec.py b/test/test_pyproject_spec.py
new file mode 100644
index 000000000..072eb0a9d
--- /dev/null
+++ b/test/test_pyproject_spec.py
@@ -0,0 +1,53 @@
+# Copyright 2024 Open Source Robotics Foundation, Inc.
+# Licensed under the Apache License, Version 2.0
+
+from colcon_core.package_descriptor import PackageDescriptor
+from colcon_core.python_project.spec import load_and_cache_spec
+
+
+def test_pyproject_missing(tmp_path):
+    desc = PackageDescriptor(tmp_path)
+
+    spec = load_and_cache_spec(desc)
+    assert spec.get('build-system') == {
+        'build-backend': 'setuptools.build_meta:__legacy__',
+        'requires': ['setuptools >= 40.8.0', 'wheel'],
+    }
+
+
+def test_pyproject_empty(tmp_path):
+    desc = PackageDescriptor(tmp_path)
+
+    (tmp_path / 'pyproject.toml').write_text('')
+
+    spec = load_and_cache_spec(desc)
+    assert spec.get('build-system') == {
+        'build-backend': 'setuptools.build_meta:__legacy__',
+        'requires': ['setuptools >= 40.8.0', 'wheel'],
+    }
+
+
+def test_specified(tmp_path):
+    desc = PackageDescriptor(tmp_path)
+
+    (tmp_path / 'pyproject.toml').write_text('\n'.join((
+        '[build-system]',
+        'build-backend = "my_build_backend.meta"',
+        'requires = ["my-build-backend"]',
+    )))
+
+    spec = load_and_cache_spec(desc)
+    assert spec.get('build-system') == {
+        'build-backend': 'my_build_backend.meta',
+        'requires': ['my-build-backend'],
+    }
+
+    # truncate the pyproject.toml and call again
+    # this verifies that the spec is cached
+    (tmp_path / 'pyproject.toml').write_text('')
+
+    spec = load_and_cache_spec(desc)
+    assert spec.get('build-system') == {
+        'build-backend': 'my_build_backend.meta',
+        'requires': ['my-build-backend'],
+    }