Use spawn instead of forkserver

This avoids forkserver errors of the form:

OSError: AF_UNIX path too long

Bug: https://bugs.gentoo.org/941956
Signed-off-by: Zac Medico <[email protected]>

Author: zmedico

Diff for: .github/workflows/ci.yml

@@ -96,4 +96,7 @@ jobs:
           export PYTEST_ADDOPTS="-vv -ra -l -o console_output_style=count -n $(nproc) --dist=worksteal"
           # Use pytest-rerunfailures to workaround pytest-xdist worker crashes with spawn start-method (bug 924416).
           [[ "${{ matrix.start-method }}" == "spawn" ]] && PYTEST_ADDOPTS+=" --reruns 5 --only-rerun 'worker .* crashed while running'"
+          [[ "${{ matrix.start-method }}" == "fork" ]] &&
+              [[ "${{ matrix.python-version }}" == "3.14-dev" ]] &&
+              PYTEST_ADDOPTS+=" --reruns 5 --only-rerun 'node down: Not properly terminated'"
           meson test -C /tmp/build --verbose

Diff for: lib/portage/process.py

@@ -19,6 +19,9 @@
 import os as _os
 import warnings
 
+if multiprocessing.get_start_method() == "forkserver":
+    multiprocessing.set_start_method("spawn", force=True)
+
 from dataclasses import dataclass
 from functools import lru_cache, partial
 from typing import Any, Optional, Callable, Union