chore: change setup.py to make use of optional build_ext option

This allows skipping the compilation of the c file if it fails for some reason,
like the compiler is missing.
The build will still fail if cython can't generate the c file from the py/pyx file
but that should be ok.

setup.py

@@ -3,65 +3,28 @@
 import os
 from os import path
 import re
-import sys
+import platform
 
-from setuptools import Extension, setup
+from setuptools import setup
+
+try:
+    from Cython.Build import build_ext as _cy_build_ext
+    from Cython.Distutils.extension import Extension as _cy_Extension
+
+    HAS_CYTHON = True
+except ImportError:
+    _cy_build_ext = _cy_Extension = None
+    HAS_CYTHON = False
+
+DISABLE_EXTENSION = bool(os.environ.get('FALCON_DISABLE_CYTHON'))
+IS_CPYTHON = platform.python_implementation() == "CPython"
 
 MYDIR = path.abspath(os.path.dirname(__file__))
 
-try:
-    sys.pypy_version_info
-    PYPY = True
-except AttributeError:
-    PYPY = False
 
-if PYPY:
-    CYTHON = False
-else:
-    try:
-        from Cython.Distutils import build_ext
-
-        CYTHON = True
-    except ImportError:
-        CYTHON = False
-
-
-class BuildFailed(Exception):
-    pass
-
-
-def get_cython_options():
-    # from sqlalchemy setup.py
-    from distutils.errors import (
-        CCompilerError,
-        DistutilsExecError,
-        DistutilsPlatformError,
-    )
-
-    ext_errors = (CCompilerError, DistutilsExecError, DistutilsPlatformError)
-    if sys.platform == 'win32':
-        # Work around issue https://github.com/pypa/setuptools/issues/1902
-        ext_errors += (IOError, TypeError)
-
-    class ve_build_ext(build_ext):
-        # This class allows Cython building to fail.
-
-        def run(self):
-            try:
-                super().run()
-            except DistutilsPlatformError:
-                raise BuildFailed()
-
-        def build_extension(self, ext):
-            try:
-                super().build_extension(ext)
-            except ext_errors as e:
-                raise BuildFailed() from e
-            except ValueError as e:
-                # this can happen on Windows 64 bit, see Python issue 7511
-                if "'path'" in str(e):
-                    raise BuildFailed() from e
-                raise
+if HAS_CYTHON and IS_CPYTHON and not DISABLE_EXTENSION:
+    assert _cy_Extension is not None
+    assert _cy_build_ext is not None
 
     def list_modules(dirname, pattern):
         filenames = glob.glob(path.join(dirname, pattern))
@@ -103,15 +66,17 @@ def list_modules(dirname, pattern):
         'falcon.util.sync',
     ]
 
-    cython_package_names = frozenset(
-        [
-            'falcon.cyutil',
-        ]
-    )
+    cython_package_names = ('falcon.cyutil',)
+    # NOTE(vytas): Now that all our codebase is Python 3.7+, specify the
+    #   Python 3 language level for Cython as well to avoid any surprises.
+    cython_directives = {'language_level': '3', 'annotation_typing': False}
 
     ext_modules = [
-        Extension(
-            package + '.' + module, [path.join(*(package.split('.') + [module + ext]))]
+        _cy_Extension(
+            package + '.' + module,
+            sources=[path.join(*(package.split('.') + [module + ext]))],
+            cython_directives=cython_directives,
+            optional=True,
         )
         for package in package_names
         for module, ext in list_modules(
@@ -121,13 +86,10 @@ def list_modules(dirname, pattern):
         if (package + '.' + module) not in modules_to_exclude
     ]
 
-    # NOTE(vytas): Now that all our codebase is Python 3.7+, specify the
-    #   Python 3 language level for Cython as well to avoid any surprises.
-    for ext_mod in ext_modules:
-        ext_mod.cython_directives = {'language_level': '3', 'annotation_typing': False}
-
-    cmdclass = {'build_ext': ve_build_ext}
-    return cmdclass, ext_modules
+    cmdclass = {'build_ext': _cy_build_ext}
+else:
+    ext_modules = []
+    cmdclass = {}
 
 
 def load_description():
@@ -164,47 +126,12 @@ def load_description():
     return ''.join(description_lines)
 
 
-def run_setup(CYTHON):
-    if CYTHON:
-        cmdclass, ext_modules = get_cython_options()
-    else:
-        cmdclass, ext_modules = {}, []
-
-    setup(
-        long_description=load_description(),
-        cmdclass=cmdclass,
-        ext_modules=ext_modules,
-    )
-
-
 def status_msgs(*msgs):
     print('*' * 75, *msgs, '*' * 75, sep='\n')
 
 
-if not CYTHON:
-    run_setup(False)
-    if not PYPY:
-        status_msgs('Cython compilation not supported in this environment')
-elif os.environ.get('FALCON_DISABLE_CYTHON'):
-    run_setup(False)
-    status_msgs(
-        'FALCON_DISABLE_CYTHON is set, skipping cython compilation.',
-        'Pure-Python build succeeded.',
-    )
-else:
-    try:
-        run_setup(True)
-    except BuildFailed as exc:
-        status_msgs(
-            exc.__cause__,
-            'Cython compilation could not be completed, speedups are not enabled.',
-            'Failure information, if any, is above.',
-            'Retrying the build without the C extension now.',
-        )
-
-        run_setup(False)
-
-        status_msgs(
-            'Cython compilation could not be completed, speedups are not enabled.',
-            'Pure-Python build succeeded.',
-        )
+setup(
+    long_description=load_description(),
+    cmdclass=cmdclass,
+    ext_modules=ext_modules,
+)