Make __file__ an absolute path in setuptools.build_meta (#3795)

PR MESSAGE:

This is a difference between pip's legacy wheel build (or direct
invocations of setup.py) and the PEP 517 build.

While setup.py scripts that rely on this are fragile, it was quite
painful to debug! Since Python 3.4, __file__ is usually an absolute
path, so this change might result in fewer surprises.

changelog.d/3795.change.rst

@@ -0,0 +1,2 @@
+Ensured that ``__file__`` is an absolute path when executing ``setup.py`` as
+part of ``setuptools.build_meta``.

setuptools/build_meta.py

@@ -326,7 +326,7 @@ def _get_build_requires(self, config_settings, requirements):
     def run_setup(self, setup_script='setup.py'):
         # Note that we can reuse our build directory between calls
         # Correctness comes first, then optimization later
-        __file__ = setup_script
+        __file__ = os.path.abspath(setup_script)
         __name__ = '__main__'
 
         with _open_setup_script(__file__) as f:

setuptools/tests/test_build_meta.py

@@ -820,6 +820,24 @@ def test_sys_argv_passthrough(self, tmpdir_cwd):
         with pytest.raises(AssertionError):
             build_backend.build_sdist("temp")
 
+    _setup_py_file_abspath = {
+        'setup.py': DALS("""
+            import os
+            assert os.path.isabs(__file__)
+            __import__('setuptools').setup(
+                name='foo',
+                version='0.0.0',
+                py_modules=['hello'],
+                setup_requires=['six'],
+            )
+            """)
+    }
+
+    def test_setup_py_file_abspath(self, tmpdir_cwd):
+        path.build(self._setup_py_file_abspath)
+        build_backend = self.get_build_backend()
+        build_backend.build_sdist("temp")
+
     @pytest.mark.parametrize('build_hook', ('build_sdist', 'build_wheel'))
     def test_build_with_empty_setuppy(self, build_backend, build_hook):
         files = {'setup.py': ''}