Handle setup.py possibly using sys.exit

Since `setup.py` is an arbitrary Python script legacy
scripts may be complex and may use `sys.exit`.
To adhere to PEP 517 handle `SystemExit` exception
by ignoring it with exit code `0` and converting it to
`CalledProcessError(exit_code, 'setup.py')` in case of
non-zero exit code.

fixes #3973

setuptools/build_meta.py

@@ -337,7 +337,15 @@ def run_setup(self, setup_script='setup.py'):
         with _open_setup_script(__file__) as f:
             code = f.read().replace(r'\r\n', r'\n')
 
-        exec(code, locals())
+        try:
+            exec(code, locals())
+        except SystemExit as e:
+            if e.code:
+                # We pretend under PEP 517 that the implementation
+                # called a subprocess, which failed.
+                from subprocess import CalledProcessError
+                raise CalledProcessError(e.code, "setup.py")
+            # We ignore exit code indicating success
 
     def get_requires_for_build_wheel(self, config_settings=None):
         return self._get_build_requires(config_settings, requirements=['wheel'])

setuptools/tests/test_build_meta.py

@@ -9,6 +9,7 @@
 import re
 from zipfile import ZipFile
 from pathlib import Path
+from subprocess import CalledProcessError
 
 import pytest
 from jaraco import path
@@ -217,6 +218,37 @@ def run():
     },
 ]
 
+sys_exit_defns = [
+    {  # simple setup.py script
+        'setup.py': DALS(
+            """
+            __import__('setuptools').setup(
+                name='foo',
+                version='0.0.0',
+                py_modules=['hello'],
+                setup_requires=['six'],
+            )
+            import sys
+            sys.exit(0)
+            """
+        ),
+        'hello.py': DALS(
+            """
+            def run():
+                print('hello')
+            """
+        ),
+    },
+    {  # simple setup.py script
+        'setup.py': DALS(
+            """
+            import sys
+            sys.exit(1)
+            """
+        ),
+    },
+]
+
 
 class TestBuildMetaBackend:
     backend_name = 'setuptools.build_meta'
@@ -260,6 +292,30 @@ def test_build_wheel(self, build_backend):
         modules = [f for f in python_scripts if not f.endswith('setup.py')]
         assert len(modules) == 1
 
+    @pytest.fixture(params=sys_exit_defns)
+    def sys_exit_build_backend(self, tmpdir, request):
+        path.build(request.param, prefix=str(tmpdir))
+        with tmpdir.as_cwd():
+            yield self.get_build_backend()
+
+    def test_get_requires_for_build_wheel_with_sys_exit(self, sys_exit_build_backend):
+        try:
+            actual = sys_exit_build_backend.get_requires_for_build_wheel()
+            expected = ['six', 'wheel']
+            assert sorted(actual) == sorted(expected)
+        except CalledProcessError as e:
+            assert e.returncode == 1
+            assert e.cmd == "setup.py"
+
+    def test_get_requires_for_build_sdist_with_sys_exit(self, sys_exit_build_backend):
+        try:
+            actual = sys_exit_build_backend.get_requires_for_build_sdist()
+            expected = ['six']
+            assert sorted(actual) == sorted(expected)
+        except CalledProcessError as e:
+            assert e.returncode == 1
+            assert e.cmd == "setup.py"
+
     @pytest.mark.parametrize('build_type', ('wheel', 'sdist'))
     def test_build_with_existing_file_present(self, build_type, tmpdir_cwd):
         # Building a sdist/wheel should still succeed if there's