Change always_forkserver_on_unix() to `use_multiprocessing_forkserv…

…er_on_linux()` (#4577)

tests/conftest.py

@@ -8,8 +8,8 @@
 import difflib
 import gc
 import multiprocessing
-import os
 import re
+import sys
 import textwrap
 import traceback
 
@@ -25,17 +25,16 @@
 
 
 @pytest.fixture(scope="session", autouse=True)
-def always_forkserver_on_unix():
-    if os.name == "nt":
+def use_multiprocessing_forkserver_on_linux():
+    if sys.platform != "linux":
+        # The default on Windows and macOS is "spawn": If it's not broken, don't fix it.
         return
 
     # Full background: https://github.com/pybind/pybind11/issues/4105#issuecomment-1301004592
     # In a nutshell: fork() after starting threads == flakiness in the form of deadlocks.
     # It is actually a well-known pitfall, unfortunately without guard rails.
     # "forkserver" is more performant than "spawn" (~9s vs ~13s for tests/test_gil_scoped.py,
     # visit the issuecomment link above for details).
-    # Windows does not have fork() and the associated pitfall, therefore it is best left
-    # running with defaults.
     multiprocessing.set_start_method("forkserver")