Issue #8713: Support alternative start methods in multiprocessing on Unix.
See http://hg.python.org/sandbox/sbt#spawn
diff --git a/Lib/multiprocessing/popen.py b/Lib/multiprocessing/popen.py
new file mode 100644
index 0000000..b0c80d5
--- /dev/null
+++ b/Lib/multiprocessing/popen.py
@@ -0,0 +1,78 @@
+import sys
+import threading
+
+__all__ = ['Popen', 'get_spawning_popen', 'set_spawning_popen',
+ 'assert_spawning']
+
+#
+# Check that the current thread is spawning a child process
+#
+
+_tls = threading.local()
+
+def get_spawning_popen():
+ return getattr(_tls, 'spawning_popen', None)
+
+def set_spawning_popen(popen):
+ _tls.spawning_popen = popen
+
+def assert_spawning(obj):
+ if get_spawning_popen() is None:
+ raise RuntimeError(
+ '%s objects should only be shared between processes'
+ ' through inheritance' % type(obj).__name__
+ )
+
+#
+#
+#
+
+_Popen = None
+
+def Popen(process_obj):
+ if _Popen is None:
+ set_start_method()
+ return _Popen(process_obj)
+
+def get_start_method():
+ if _Popen is None:
+ set_start_method()
+ return _Popen.method
+
+def set_start_method(meth=None, *, start_helpers=True):
+ global _Popen
+ try:
+ modname = _method_to_module[meth]
+ __import__(modname)
+ except (KeyError, ImportError):
+ raise ValueError('could not use start method %r' % meth)
+ module = sys.modules[modname]
+ if start_helpers:
+ module.Popen.ensure_helpers_running()
+ _Popen = module.Popen
+
+
+if sys.platform == 'win32':
+
+ _method_to_module = {
+ None: 'multiprocessing.popen_spawn_win32',
+ 'spawn': 'multiprocessing.popen_spawn_win32',
+ }
+
+ def get_all_start_methods():
+ return ['spawn']
+
+else:
+ _method_to_module = {
+ None: 'multiprocessing.popen_fork',
+ 'fork': 'multiprocessing.popen_fork',
+ 'spawn': 'multiprocessing.popen_spawn_posix',
+ 'forkserver': 'multiprocessing.popen_forkserver',
+ }
+
+ def get_all_start_methods():
+ from . import reduction
+ if reduction.HAVE_SEND_HANDLE:
+ return ['fork', 'spawn', 'forkserver']
+ else:
+ return ['fork', 'spawn']