Use spawn instead of forkserver

This avoids forserver errors of the form:

OSError: AF_UNIX path too long

Bug: https://bugs.gentoo.org/941956
Signed-off-by: Zac Medico <zmedico@gentoo.org>

lib/portage/locks.py

@@ -27,6 +27,9 @@
 import typing
 import warnings
 
+if multiprocessing.get_start_method() == "forkserver":
+    multiprocessing = multiprocessing.get_context("spawn")
+
 import portage
 from portage import os, _encodings, _unicode_decode
 from portage.exception import (

lib/portage/util/_async/ForkProcess.py

@@ -2,7 +2,7 @@
 # Distributed under the terms of the GNU General Public License v2
 
 import fcntl
-import multiprocessing
+import multiprocessing as _multiprocessing
 import warnings
 import signal
 import sys
@@ -17,6 +17,11 @@
 
 _registered_run_exitfuncs = None
 
+if _multiprocessing.get_start_method() == "forkserver":
+    multiprocessing = _multiprocessing.get_context("spawn")
+else:
+    multiprocessing = _multiprocessing
+
 
 class ForkProcess(SpawnProcess):
     # NOTE: This class overrides the meaning of the SpawnProcess 'args'
@@ -33,7 +38,7 @@ class ForkProcess(SpawnProcess):
     _file_names = ("connection", "slave_fd")
     _files_dict = slot_dict_class(_file_names, prefix="")
 
-    _HAVE_SEND_HANDLE = getattr(multiprocessing.reduction, "HAVE_SEND_HANDLE", False)
+    _HAVE_SEND_HANDLE = getattr(_multiprocessing.reduction, "HAVE_SEND_HANDLE", False)
 
     def _start(self):
         if multiprocessing.get_start_method() == "fork":
@@ -157,7 +162,7 @@ def _send_fd_pipes(self):
                 (self._fd_pipes, fd_list),
             )
             for fd in fd_list:
-                multiprocessing.reduction.send_handle(
+                _multiprocessing.reduction.send_handle(
                     self._files.connection,
                     fd,
                     self.pid,
@@ -297,7 +302,7 @@ def _bootstrap(child_connection, have_send_handle, fd_pipes, target, args, kwarg
                 fd_pipes, fd_list = child_connection.recv()
                 fd_pipes_map = {}
                 for fd in fd_list:
-                    fd_pipes_map[fd] = multiprocessing.reduction.recv_handle(
+                    fd_pipes_map[fd] = _multiprocessing.reduction.recv_handle(
                         child_connection
                     )
                 child_connection.close()