bpo-38092: Reduce overhead when using multiprocessing in a Windows virtual environment (GH-16098)

https://bugs.python.org/issue38092
(cherry picked from commit f2b7556ef8)

Co-authored-by: Steve Dower <steve.dower@python.org>

Lib/multiprocessing/popen_spawn_win32.py

@@ -72,7 +72,7 @@ class Popen(object):
             try:
                 hp, ht, pid, tid = _winapi.CreateProcess(
                     python_exe, cmd,
-                    env, None, False, 0, None, None, None)
+                    None, None, False, 0, env, None, None)
                 _winapi.CloseHandle(ht)
             except:
                 _winapi.CloseHandle(rhandle)

Lib/multiprocessing/spawn.py

@@ -36,7 +36,7 @@ else:
 if WINSERVICE:
     _python_exe = os.path.join(sys.exec_prefix, 'python.exe')
 else:
-    _python_exe = sys.executable
+    _python_exe = sys._base_executable
 
 def set_executable(exe):
     global _python_exe

Misc/NEWS.d/next/Windows/2019-09-13-14-11-42.bpo-38092.x31ehI.rst

@@ -0,0 +1 @@
+Reduce overhead when using multiprocessing in a Windows virtual environment.