Issue #18999: Make multiprocessing use context objects.

This allows different parts of a program to use different methods for
starting processes without interfering with each other.

Lib/multiprocessing/popen_spawn_win32.py

@@ -4,8 +4,8 @@ import signal
 import sys
 import _winapi
 
+from . import context
 from . import spawn
-from . import popen
 from . import reduction
 from . import util
 
@@ -60,15 +60,15 @@ class Popen(object):
             util.Finalize(self, _winapi.CloseHandle, (self.sentinel,))
 
             # send information to child
-            popen.set_spawning_popen(self)
+            context.set_spawning_popen(self)
             try:
                 reduction.dump(prep_data, to_child)
                 reduction.dump(process_obj, to_child)
             finally:
-                popen.set_spawning_popen(None)
+                context.set_spawning_popen(None)
 
     def duplicate_for_child(self, handle):
-        assert self is popen.get_spawning_popen()
+        assert self is context.get_spawning_popen()
         return reduction.duplicate(handle, self.sentinel)
 
     def wait(self, timeout=None):
@@ -97,7 +97,3 @@ class Popen(object):
             except OSError:
                 if self.wait(timeout=1.0) is None:
                     raise
-
-    @staticmethod
-    def ensure_helpers_running():
-        pass