[feat] Enable build of pyproject.toml projects

Enable zc.buildout.easy_install.build, which builds a project manually
from an unpacked sdist archive and is used by zc.recipe.egg:custom, to
build a project which only has a pyproject.toml but no setup.py.

src/zc/buildout/easy_install.py

@@ -945,28 +945,31 @@ class Installer(object):
             try:
                 setuptools.archive_util.unpack_archive(dist.location,
                                                        build_tmp)
-                if os.path.exists(os.path.join(build_tmp, 'setup.py')):
-                    base = build_tmp
+
+                # Find pyproject.toml or setup.py
+                for filename in ('pyproject.toml', 'setup.py'):
+                    if os.path.exists(os.path.join(build_tmp, filename)):
+                        base = build_tmp
+                        break
                 else:
-                    setups = glob.glob(
-                        os.path.join(build_tmp, '*', 'setup.py'))
-                    if not setups:
-                        raise distutils.errors.DistutilsError(
-                            "Couldn't find a setup script in %s"
-                            % os.path.basename(dist.location)
-                            )
-                    if len(setups) > 1:
+                    for filename in ('pyproject.toml', 'setup.py'):
+                        setups = glob.glob(
+                            os.path.join(build_tmp, '*', filename))
+                        if setups:
+                            if len(setups) > 1:
+                                raise distutils.errors.DistutilsError(
+                                    "Multiple %s in %s" % (
+                                      filename, os.path.basename(dist.location)))
+                            base = os.path.dirname(setups[0])
+                            break
+                    else:
                         raise distutils.errors.DistutilsError(
-                            "Multiple setup scripts in %s"
+                            "Couldn't find a pyproject.toml nor setup.py in %s"
                             % os.path.basename(dist.location)
                             )
-                    base = os.path.dirname(setups[0])
 
-                setup_cfg_dict = {'build_ext':build_ext}
+                # Apply patches.
                 if patch_dict:
-                    setup_cfg_dict.update(
-                      {'egg_info':{'tag_build':'+%s%03d' % (PATCH_MARKER,
-                                                            patch_dict['patch_revision'])}})
                     download = zc.buildout.download.Download(
                         cache=tmp, hash_name=True)
                     patch_dict = patch_dict[requirement.key]
@@ -981,12 +984,44 @@ class Installer(object):
                         if popen.returncode:
                             raise subprocess.CalledProcessError(
                                 popen.returncode, ' '.join(args))
-                setup_cfg = os.path.join(base, 'setup.cfg')
-                if not os.path.exists(setup_cfg):
-                    f = open(setup_cfg, 'w')
-                    f.close()
-                setuptools.command.setopt.edit_config(
-                    setup_cfg, setup_cfg_dict)
+
+                # Attempt to determine whether setuptools is used.
+                def uses_setuptools():
+                    if filename == 'setup.py':
+                        return True
+                    for setup in ('setup.py', 'setup.cfg'):
+                        if os.path.exists(os.path.join(base, setup)):
+                            return True
+                    with open(os.path.join(base, 'pyproject.toml')) as f:
+                        config = f.read()
+                    pattern = "^build-backend\s*=\s*['\"]+setuptools"
+                    if re.search(pattern, config, re.MULTILINE):
+                        return True
+                    return False
+
+                # Edit setup.cfg if required; warn if setuptools is not used.
+                if uses_setuptools():
+                    setup_cfg_dict = {'build_ext': build_ext}
+                    if patch_dict:
+                        # TODO: find alternative for without setuptools.
+                        setup_cfg_dict.update(
+                          {'egg_info':{'tag_build':'+%s%03d' % (
+                              PATCH_MARKER,
+                              patch_dict['patch_revision'])}})
+                    setup_cfg = os.path.join(base, 'setup.cfg')
+                    if not os.path.exists(setup_cfg):
+                        with open(setup_cfg, 'w'):
+                            pass
+                    setuptools.command.setopt.edit_config(
+                        setup_cfg, setup_cfg_dict)
+                else:
+                    if build_ext or patch_dict:
+                        logger.warning(
+                            "Build options call for editing setup.cfg "
+                            "but project %s does not use setuptools. "
+                            "Proceeding without."
+                            % dist
+                            )
 
                 dists = self._call_pip_wheel(base, self._dest, dist)