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

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

Lib/multiprocessing/popen_fork.py

@@ -0,0 +1,87 @@
+import os
+import sys
+import signal
+import errno
+
+from . import util
+
+__all__ = ['Popen']
+
+#
+# Start child process using fork
+#
+
+class Popen(object):
+    method = 'fork'
+
+    def __init__(self, process_obj):
+        sys.stdout.flush()
+        sys.stderr.flush()
+        self.returncode = None
+        self._launch(process_obj)
+
+    def duplicate_for_child(self, fd):
+        return fd
+
+    def poll(self, flag=os.WNOHANG):
+        if self.returncode is None:
+            while True:
+                try:
+                    pid, sts = os.waitpid(self.pid, flag)
+                except OSError as e:
+                    if e.errno == errno.EINTR:
+                        continue
+                    # Child process not yet created. See #1731717
+                    # e.errno == errno.ECHILD == 10
+                    return None
+                else:
+                    break
+            if pid == self.pid:
+                if os.WIFSIGNALED(sts):
+                    self.returncode = -os.WTERMSIG(sts)
+                else:
+                    assert os.WIFEXITED(sts)
+                    self.returncode = os.WEXITSTATUS(sts)
+        return self.returncode
+
+    def wait(self, timeout=None):
+        if self.returncode is None:
+            if timeout is not None:
+                from .connection import wait
+                if not wait([self.sentinel], timeout):
+                    return None
+            # This shouldn't block if wait() returned successfully.
+            return self.poll(os.WNOHANG if timeout == 0.0 else 0)
+        return self.returncode
+
+    def terminate(self):
+        if self.returncode is None:
+            try:
+                os.kill(self.pid, signal.SIGTERM)
+            except ProcessLookupError:
+                pass
+            except OSError:
+                if self.wait(timeout=0.1) is None:
+                    raise
+
+    def _launch(self, process_obj):
+        code = 1
+        parent_r, child_w = util.pipe()
+        self.pid = os.fork()
+        if self.pid == 0:
+            try:
+                os.close(parent_r)
+                if 'random' in sys.modules:
+                    import random
+                    random.seed()
+                code = process_obj._bootstrap()
+            finally:
+                os._exit(code)
+        else:
+            os.close(child_w)
+            util.Finalize(self, os.close, (parent_r,))
+            self.sentinel = parent_r
+
+    @staticmethod
+    def ensure_helpers_running():
+        pass