gh-100228: Warn from os.fork() if other threads exist. (#100229)

Not comprehensive, best effort warning. There are cases when threads exist on some platforms that this code cannot detect. macOS when API permissions allow and Linux with a readable /proc procfs present are the currently supported cases where a warning should show up reliably.

Starting with a DeprecationWarning for now, it is less disruptive than something like RuntimeWarning and most likely to only be seen in people's CI tests - a good place to start with this messaging.

Lib/test/fork_wait.py

@@ -13,6 +13,7 @@ import os, sys, time, unittest
 import threading
 from test import support
 from test.support import threading_helper
+import warnings
 
 
 LONGSLEEP = 2
@@ -63,19 +64,17 @@ class ForkWait(unittest.TestCase):
 
         prefork_lives = self.alive.copy()
 
-        if sys.platform in ['unixware7']:
-            cpid = os.fork1()
-        else:
-            cpid = os.fork()
-
-        if cpid == 0:
-            # Child
-            time.sleep(LONGSLEEP)
-            n = 0
-            for key in self.alive:
-                if self.alive[key] != prefork_lives[key]:
-                    n += 1
-            os._exit(n)
-        else:
-            # Parent
-            self.wait_impl(cpid, exitcode=0)
+        # Ignore the warning about fork with threads.
+        with warnings.catch_warnings(category=DeprecationWarning,
+                                     action="ignore"):
+            if (cpid := os.fork()) == 0:
+                # Child
+                time.sleep(LONGSLEEP)
+                n = 0
+                for key in self.alive:
+                    if self.alive[key] != prefork_lives[key]:
+                        n += 1
+                os._exit(n)
+            else:
+                # Parent
+                self.wait_impl(cpid, exitcode=0)