gh-117378: Fix multiprocessing forkserver preload sys.path inheritance.

`sys.path` was not properly being sent from the parent process when launching
the multiprocessing forkserver process to preload imports.  This bug has been
there since the forkserver start method was introduced in Python ~3.4.  It was
always _supposed_ to inherit `sys.path` the same way the spawn method does.

Observable behavior change: A `''` value in `sys.path` will now be replaced in
the forkserver's `sys.path` with an absolute pathname
`os.path.abspath(os.getcwd())` saved at the time that `multiprocessing` was
imported in the parent process as it already was when using the spawn start
method.

A workaround for the bug this fixes was to set PYTHONPATH in the environment
before the forkserver process was started.

Author: gpshead

Lib/multiprocessing/forkserver.py

@@ -174,6 +174,8 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None):
                 spawn.import_main_path(main_path)
             finally:
                 del process.current_process()._inheriting
+        if sys_path is not None:
+            sys.path[:] = sys_path
         for modname in preload:
             try:
                 __import__(modname)

Lib/test/test_multiprocessing_forkserver/test_forkserver_main.py

@@ -0,0 +1,57 @@
+import os
+import sys
+import unittest
+from unittest import mock
+
+from multiprocessing import forkserver
+
+
+class TestForkserverMain(unittest.TestCase):
+
+    def setUp(self):
+        self._orig_sys_path = list(sys.path)
+
+    def tearDown(self):
+        sys.path[:] = self._orig_sys_path
+
+    @mock.patch("multiprocessing.process.current_process")
+    @mock.patch("multiprocessing.spawn.import_main_path")
+    @mock.patch("multiprocessing.util._close_stdin")
+    def test_preload_kwargs(
+        self,
+        mock_close_stdin,
+        mock_import_main_path,
+        mock_current_process,
+    ):
+        # Very much a whitebox test of the first stanza of main before
+        # we start diddling with file descriptors and pipes.
+        mock_close_stdin.side_effect = RuntimeError("stop test")
+        self.assertNotIn(
+            "colorsys",
+            sys.modules.keys(),
+            msg="Thie test requires a module that has not yet been imported.",
+        )
+
+        with self.assertRaisesRegex(RuntimeError, "stop test"):
+            forkserver.main(None, None, ["sys", "colorsys"])
+        mock_current_process.assert_not_called()
+        mock_import_main_path.assert_not_called()
+        self.assertIn("colorsys", sys.modules.keys())
+        self.assertEqual(sys.path, self._orig_sys_path)  # unmodified
+
+        del sys.modules["colorsys"]  # unimport
+        fake_path = os.path.dirname(__file__)
+        with self.assertRaisesRegex(RuntimeError, "stop test"):
+            forkserver.main(None, None, ["sys", "colorsys"], sys_path=[fake_path])
+        self.assertEqual(
+            sys.path, [fake_path], msg="sys.path should have been overridden"
+        )
+        self.assertNotIn(
+            "colorsys",
+            sys.modules.keys(),
+            msg="import of colorsys should have failed with unusual sys.path",
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()