Issue #8713: Support alternative start methods in multiprocessing on Unix.

See http://hg.python.org/sandbox/sbt#spawn

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']