Add custom build backend to support build args (#328)

This implements a custom build backend, inspired by the
[solution used by Pillow.](https://github.com/python-pillow/Pillow/pull/7171)

install-option is deprecated and was removed with pip 23.1. The
comonly accepted solution seems to be to define a custom build
backend for now
https://github.com/pypa/setuptools/issues/2491#issuecomment-1589764230

This commit changes the usage from `--install-option` to `--config-settings`.

README.md

@@ -38,25 +38,27 @@ cd pycapnp
 pip install .
 ```
 
+By default, the setup script will automatically use the locally installed Cap'n Proto.
+If Cap'n Proto is not installed, it will bundle and build the matching Cap'n Proto library.
+
+To enforce bundling, the Cap'n Proto library:
+
+```bash
+pip install . -C force-bundled-libcapnp=True
+```
+
 If you wish to install using the latest upstream C++ Cap'n Proto:
 
 ```bash
-pip install \
-    --install-option "--libcapnp-url" \
-    --install-option "https://github.com/capnproto/capnproto/archive/master.tar.gz" \
-    --install-option "--force-bundled-libcapnp" .
+pip install . \
+    -C force-bundled-libcapnp=True \
+    -C libcapnp-url="https://github.com/capnproto/capnproto/archive/master.tar.gz"
 ```
 
-To force bundled python:
+To enforce using the installed Cap'n Proto from the system:
 
 ```bash
-pip install --install-option "--force-bundled-libcapnp" .
-```
-
-Slightly more prompt error messages using distutils rather than pip.
-
-```bash
-python setup.py install --force-bundled-libcapnp
+pip install . -C force-system-libcapnp=True
 ```
 
 The bundling system isn't that smart so it might be necessary to clean up the bundled build when changing versions:
@@ -92,19 +94,12 @@ pipenv run pytest
 
 ### Binary Packages
 
-Building a dumb binary distribution:
+Building a Python wheel distributiion
 
 ```bash
-python setup.py bdist_dumb
+pip wheel .
 ```
 
-Building a Python wheel distributiion:
-
-```bash
-python setup.py bdist_wheel
-```
-
-
 ## Documentation/Example
 
 There is some basic documentation [here](http://capnproto.github.io/pycapnp/).

_custom_build/backend.py

@@ -0,0 +1,31 @@
+import sys
+
+from setuptools.build_meta import *  # noqa: F401, F403
+from setuptools.build_meta import build_wheel
+
+backend_class = build_wheel.__self__.__class__
+
+
+class _CustomBuildMetaBackend(backend_class):
+    def run_setup(self, setup_script="setup.py"):
+        if self.config_settings:
+            flags = []
+            if self.config_settings.get("force-bundled-libcapnp"):
+                flags.append("--force-bundled-libcapnp")
+            if self.config_settings.get("force-system-libcapnp"):
+                flags.append("--force-system-libcapnp")
+            if self.config_settings.get("libcapnp-url"):
+                flags.append("--libcapnp-url")
+                flags.append(self.config_settings["libcapnp-url"])
+            if flags:
+                sys.argv = sys.argv[:1] + ["build_ext"] + flags + sys.argv[1:]
+        return super().run_setup(setup_script)
+
+    def build_wheel(
+        self, wheel_directory, config_settings=None, metadata_directory=None
+    ):
+        self.config_settings = config_settings
+        return super().build_wheel(wheel_directory, config_settings, metadata_directory)
+
+
+build_wheel = _CustomBuildMetaBackend().build_wheel

docs/install.rst

@@ -15,8 +15,8 @@ To force rebuilding the pip package from source (you'll need requirments.txt or
 
 To force bundling libcapnp (or force system libcapnp), just in case setup.py isn't doing the right thing::
 
-    pip install --no-binary :all: --install-option "--force-bundled-libcapnp"
-    pip install --no-binary :all: --install-option "--force-system-libcapnp"
+    pip install --no-binary :all: -C force-bundled-libcapnp=True
+    pip install --no-binary :all: -C force-system-libcapnp=True
 
 If you're using an older Linux distro (e.g. CentOS 6) you many need to set `LDFLAGS="-Wl,--no-as-needed -lrt"`::
 
@@ -24,7 +24,7 @@ If you're using an older Linux distro (e.g. CentOS 6) you many need to set `LDFL
 
 It's also possible to specify the libcapnp url when bundling (this may not work, there be dragons)::
 
-    pip install --no-binary :all: --install-option "--force-bundled-libcapnp" --install-option "--libcapnp-url" --install-option "https://github.com/capnproto/capnproto/archive/master.tar.gz"
+    pip install --no-binary :all: -C force-bundled-libcapnp=True -C libcapnp-url="https://github.com/capnproto/capnproto/archive/master.tar.gz"
 
 From Source
 -----------

pyproject.toml

@@ -1,5 +1,7 @@
 [build-system]
 requires = ["setuptools", "wheel", "pkgconfig", "cython<3"]
+build-backend = "backend"
+backend-path = ["_custom_build"]
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"

setup.cfg

@@ -1,3 +1,3 @@
 [metadata]
-description-file = README.md
+description_file = README.md
 license_files = LICENSE.md

setup.py

@@ -15,10 +15,12 @@ from distutils.command.clean import clean as _clean
 
 from setuptools import setup, Extension
 
+_this_dir = os.path.dirname(__file__)
+sys.path.insert(1, _this_dir)
+
 from buildutils.build import build_libcapnp
 from buildutils.bundle import fetch_libcapnp
 
-_this_dir = os.path.dirname(__file__)
 
 MAJOR = 1
 MINOR = 3
@@ -85,26 +87,6 @@ class clean(_clean):
                 shutil.rmtree(x, ignore_errors=True)
 
 
-# hack to parse commandline arguments
-force_bundled_libcapnp = "--force-bundled-libcapnp" in sys.argv
-if force_bundled_libcapnp:
-    sys.argv.remove("--force-bundled-libcapnp")
-force_system_libcapnp = "--force-system-libcapnp" in sys.argv
-if force_system_libcapnp:
-    sys.argv.remove("--force-system-libcapnp")
-force_cython = "--force-cython" in sys.argv
-if force_cython:
-    sys.argv.remove("--force-cython")
-    # Always use cython, ignoring option
-libcapnp_url = None
-try:
-    libcapnp_url_index = sys.argv.index("--libcapnp-url")
-    libcapnp_url = sys.argv[libcapnp_url_index + 1]
-    sys.argv.remove("--libcapnp-url")
-    sys.argv.remove(libcapnp_url)
-except Exception:
-    pass
-
 from Cython.Distutils import build_ext as build_ext_c  # noqa: E402
 
 
@@ -113,13 +95,29 @@ class build_libcapnp_ext(build_ext_c):
     Build capnproto library
     """
 
+    user_options = build_ext_c.user_options + [
+        ("force-bundled-libcapnp", None, "Bundle capnp library into the installer"),
+        ("force-system-libcapnp", None, "Use system capnp library"),
+        ("libcapnp-url=", "u", "URL to download libcapnp from (only if bundled)"),
+    ]
+
+    def initialize_options(self):
+        build_ext_c.initialize_options(self)
+        self.force_bundled_libcapnp = None
+        self.force_system_libcapnp = None
+        self.libcapnp_url = None
+
+    def finalize_options(self):
+        # print('The custom option for install is ', self.custom_option)
+        build_ext_c.finalize_options(self)
+
     def build_extension(self, ext):
         build_ext_c.build_extension(self, ext)
 
     def run(self):  # noqa: C901
-        if force_bundled_libcapnp:
+        if self.force_bundled_libcapnp:
             need_build = True
-        elif force_system_libcapnp:
+        elif self.force_system_libcapnp:
             need_build = False
         else:
             # Try to use capnp executable to find include and lib path
@@ -167,7 +165,7 @@ class build_libcapnp_ext(build_ext_c):
 
             if not os.path.exists(capnp_bin):
                 # Not built, fetch and build
-                fetch_libcapnp(bundle_dir, libcapnp_url)
+                fetch_libcapnp(bundle_dir, self.libcapnp_url)
                 build_libcapnp(bundle_dir, build_dir)
             else:
                 print("capnproto already built at {}".format(build_dir))

tox.ini

@@ -7,10 +7,11 @@ deps=
     pkgconfig
     Jinja2
     pytest
+    pytest-asyncio
     cython<3
 
 commands =
-    python setup.py install
+    pip install .
     py.test {posargs}
 
 setenv =