pyx.build: Allow to use custom build_ext

setuptools_dso hardcodes to use its own build_ext in its setup.
We amend that build_ext and inject our own version carefully while
setup() is run - see 7ae8c4f3 (pyx.build: Allow to combine C and C++
sources in one extension). However, currently, if user uses cmdclass =
{'build_ext': ...} in his own setup, this will be ignored and
overwritten by golang.pyx.build.build_ext . One example where this
breaks things is wendelin.core which hooks into build_ext to first
configure/build CCAN before any extension:

https://lab.nexedi.com/nexedi/wendelin.core/blob/b26ba558/setup.py#L147-153

-> Fix it by taking into account what user could put into cmdclass.

golang/pyx/build.py

@@ -111,10 +111,10 @@ class build_ext(_dso_build_ext):
 # setup should be used instead of setuptools.setup
 def setup(**kw):
     # setuptools_dso.setup hardcodes setuptools_dso.build_ext to be used.
-    # temporarily inject our code there.
+    # temporarily inject what user specified in cmdclass, or our code there.
     _ = setuptools_dso.build_ext
     try:
-        setuptools_dso.build_ext = build_ext
+        setuptools_dso.build_ext = kw.get('cmdclass', {}).get('build_ext', build_ext)
         setuptools_dso.setup(**kw)
     finally:
         setuptools_dso.build_ext = _

golang/pyx/build_test.py

@@ -23,9 +23,11 @@ from __future__ import print_function, absolute_import
 from golang.golang_test import pyrun, pyout
 from os.path import dirname
 
+testprog = dirname(__file__) + "/testprog"
+
 # verify that we can build/run external package that uses pygolang in pyx mode.
 def test_pyx_build():
-    pyxuser = dirname(__file__) + "/testprog/golang_pyx_user"
+    pyxuser = testprog + "/golang_pyx_user"
     pyrun(["setup.py", "build_ext", "-i"], cwd=pyxuser)
 
     # run built test.
@@ -37,3 +39,9 @@ def test_pyx_build():
         "import golang;" +
         "from pyxuser import test; test.main()"], cwd=pyxuser)
     assert _ == b"test.pyx: OK\n"
+
+
+# verify that custom classes can be used via cmdclass
+def test_pyx_build_cmdclass():
+    _ = pyout(["cmdclass_custom.py", "build_ext"], cwd=testprog)
+    assert b"pyx.build:RUN_BUILD_EXT" in _

golang/pyx/testprog/cmdclass_custom.py

+# Copyright (C) 2019  Nexedi SA and Contributors.
+#                     Kirill Smelkov <kirr@nexedi.com>
+#
+# This program is free software: you can Use, Study, Modify and Redistribute
+# it under the terms of the GNU General Public License version 3, or (at your
+# option) any later version, as published by the Free Software Foundation.
+#
+# You can also Link and Combine this program with other software covered by
+# the terms of any of the Free Software licenses or any of the Open Source
+# Initiative approved licenses and Convey the resulting work. Corresponding
+# source of such a combination shall include the source code for all other
+# software used.
+#
+# This program is distributed WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+#
+# See COPYING file for full licensing terms.
+# See https://www.nexedi.com/licensing for rationale and options.
+"""cmdclass_custom.py helps tests to verify that e.g. custom build_ext can be used"""
+
+from __future__ import print_function, absolute_import
+
+from golang.pyx.build import setup, build_ext
+
+class mybuild_ext(build_ext):
+    def run(self):
+        print('pyx.build:RUN_BUILD_EXT')
+        # just print - _not_ recursing into build_ext.run
+
+setup(
+    cmdclass    = {'build_ext': mybuild_ext},
+)