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

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

Lib/multiprocessing/popen_spawn_posix.py

@@ -0,0 +1,75 @@
+import fcntl
+import io
+import os
+
+from . import popen
+from . import popen_fork
+from . import reduction
+from . import spawn
+from . import util
+
+from . import current_process
+
+__all__ = ['Popen']
+
+
+#
+# Wrapper for an fd used while launching a process
+#
+
+class _DupFd(object):
+    def __init__(self, fd):
+        self.fd = fd
+    def detach(self):
+        return self.fd
+
+#
+# Start child process using a fresh interpreter
+#
+
+class Popen(popen_fork.Popen):
+    method = 'spawn'
+    DupFd = _DupFd
+
+    def __init__(self, process_obj):
+        self._fds = []
+        super().__init__(process_obj)
+
+    def duplicate_for_child(self, fd):
+        self._fds.append(fd)
+        return fd
+
+    def _launch(self, process_obj):
+        tracker_fd = current_process()._config['semaphore_tracker_fd']
+        self._fds.append(tracker_fd)
+        prep_data = spawn.get_preparation_data(process_obj._name)
+        fp = io.BytesIO()
+        popen.set_spawning_popen(self)
+        try:
+            reduction.dump(prep_data, fp)
+            reduction.dump(process_obj, fp)
+        finally:
+            popen.set_spawning_popen(None)
+
+        parent_r = child_w = child_r = parent_w = None
+        try:
+            parent_r, child_w = util.pipe()
+            child_r, parent_w = util.pipe()
+            cmd = spawn.get_command_line() + [str(child_r)]
+            self._fds.extend([child_r, child_w])
+            self.pid = util.spawnv_passfds(spawn.get_executable(),
+                                           cmd, self._fds)
+            self.sentinel = parent_r
+            with open(parent_w, 'wb', closefd=False) as f:
+                f.write(fp.getbuffer())
+        finally:
+            if parent_r is not None:
+                util.Finalize(self, os.close, (parent_r,))
+            for fd in (child_r, child_w, parent_w):
+                if fd is not None:
+                    os.close(fd)
+
+    @staticmethod
+    def ensure_helpers_running():
+        from . import semaphore_tracker
+        semaphore_tracker.ensure_running()