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

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

Lib/multiprocessing/heap.py

@@ -8,15 +8,17 @@
 #
 
 import bisect
+import itertools
 import mmap
 import os
 import sys
+import tempfile
 import threading
-import itertools
-
 import _multiprocessing
-from multiprocessing.util import Finalize, info
-from multiprocessing.forking import assert_spawning
+
+from . import popen
+from . import reduction
+from . import util
 
 __all__ = ['BufferWrapper']
 
@@ -30,17 +32,25 @@ if sys.platform == 'win32':
 
     class Arena(object):
 
-        _counter = itertools.count()
+        _rand = tempfile._RandomNameSequence()
 
         def __init__(self, size):
             self.size = size
-            self.name = 'pym-%d-%d' % (os.getpid(), next(Arena._counter))
-            self.buffer = mmap.mmap(-1, self.size, tagname=self.name)
-            assert _winapi.GetLastError() == 0, 'tagname already in use'
+            for i in range(100):
+                name = 'pym-%d-%s' % (os.getpid(), next(self._rand))
+                buf = mmap.mmap(-1, size, tagname=name)
+                if _winapi.GetLastError() == 0:
+                    break
+                # We have reopened a preexisting mmap.
+                buf.close()
+            else:
+                raise FileExistsError('Cannot find name for new mmap')
+            self.name = name
+            self.buffer = buf
             self._state = (self.size, self.name)
 
         def __getstate__(self):
-            assert_spawning(self)
+            popen.assert_spawning(self)
             return self._state
 
         def __setstate__(self, state):
@@ -52,10 +62,28 @@ else:
 
     class Arena(object):
 
-        def __init__(self, size):
-            self.buffer = mmap.mmap(-1, size)
+        def __init__(self, size, fd=-1):
             self.size = size
-            self.name = None
+            self.fd = fd
+            if fd == -1:
+                self.fd, name = tempfile.mkstemp(
+                     prefix='pym-%d-'%os.getpid(), dir=util.get_temp_dir())
+                os.unlink(name)
+                util.Finalize(self, os.close, (self.fd,))
+                with open(self.fd, 'wb', closefd=False) as f:
+                    f.write(b'\0'*size)
+            self.buffer = mmap.mmap(self.fd, self.size)
+
+    def reduce_arena(a):
+        if a.fd == -1:
+            raise ValueError('Arena is unpicklable because '
+                             'forking was enabled when it was created')
+        return rebuild_arena, (a.size, reduction.DupFd(a.fd))
+
+    def rebuild_arena(size, dupfd):
+        return Arena(size, dupfd.detach())
+
+    reduction.register(Arena, reduce_arena)
 
 #
 # Class allowing allocation of chunks of memory from arenas
@@ -90,7 +118,7 @@ class Heap(object):
         if i == len(self._lengths):
             length = self._roundup(max(self._size, size), mmap.PAGESIZE)
             self._size *= 2
-            info('allocating a new mmap of length %d', length)
+            util.info('allocating a new mmap of length %d', length)
             arena = Arena(length)
             self._arenas.append(arena)
             return (arena, 0, length)
@@ -216,7 +244,7 @@ class BufferWrapper(object):
         assert 0 <= size < sys.maxsize
         block = BufferWrapper._heap.malloc(size)
         self._state = (block, size)
-        Finalize(self, BufferWrapper._heap.free, args=(block,))
+        util.Finalize(self, BufferWrapper._heap.free, args=(block,))
 
     def create_memoryview(self):
         (arena, start, stop), size = self._state