Apply AsyncioSafeProcess to Stream's multiprocess

Note this stops explictly using the get_context object from
multiprocessing which allows for fork to be used in a process
where spawn is the default. This obviously breaks the linux CI
targets for FORCE SPAWN.

src/buildstream/_multiprocessing.py

diff --git a/src/buildstream/_scheduler/_multiprocessing.py b/src/buildstream/_multiprocessing.py
similarity index 100%
rename from src/buildstream/_scheduler/_multiprocessing.py
rename to src/buildstream/_multiprocessing.py

src/buildstream/_scheduler/jobs/job.py

diff --git a/src/buildstream/_scheduler/jobs/job.py b/src/buildstream/_scheduler/jobs/job.py
index 4cb80b8..8e90997 100644
--- a/src/buildstream/_scheduler/jobs/job.py
+++ b/src/buildstream/_scheduler/jobs/job.py
@@ -32,8 +32,7 @@
 from ..._exceptions import ImplError, BstError, SkipJob
 from ..._message import Message, MessageType, unconditional_messages
 from ...types import FastEnum
-from ... import _signals, utils
-from .. import _multiprocessing
+from ... import _signals, utils, _multiprocessing
 
 from .jobpickler import pickle_child_job, do_pickled_child_job
 

src/buildstream/_stream.py

diff --git a/src/buildstream/_stream.py b/src/buildstream/_stream.py
index 653f150..e62c255 100644
--- a/src/buildstream/_stream.py
+++ b/src/buildstream/_stream.py
@@ -63,7 +63,7 @@
 from ._state import State
 from .types import _KeyStrength, _SchedulerErrorAction
 from .plugin import Plugin
-from . import utils, _yaml, _site, _signals
+from . import utils, _yaml, _site, _signals, _multiprocessing
 from . import Scope, Consistency
 
 
@@ -151,7 +151,7 @@
     def run_in_subprocess(self, func, *args, **kwargs):
         assert not self._subprocess
 
-        mp_context = mp.get_context(method="fork")
+        # mp_context = _multiprocessing.get_context(method='fork')
         process_name = "stream-{}".format(func.__name__)
 
         self._notify_front_queue = mp.Queue()
@@ -164,7 +164,7 @@
         args.insert(0, self._notify_front_queue)
         args.insert(0, func)
 
-        self._subprocess = mp_context.Process(
+        self._subprocess = _multiprocessing.AsyncioSafeProcess(
             target=Stream._subprocess_main, args=args, kwargs=kwargs, name=process_name
         )