Remove uneccesary _platform.multiprocessingaevri/nomp

It turns out we don't need to use multiprocessing.Manager() queues when
using the 'spawn' method - the regular multiprocessing queues are also
picklable, if passed as parameters to the new process.

Thanks to @BenjaminSchubert for pointing this out.

2 files changed, 0 insertions, 116 deletions

diff --git a/src/buildstream/_platform/multiprocessing.py b/src/buildstream/_platform/multiprocessing.py
deleted file mode 100644
index c036651e7..000000000
--- a/src/buildstream/_platform/multiprocessing.py
+++ /dev/null
@@ -1,108 +0,0 @@
-#
-#  Copyright (C) 2019 Bloomberg Finance LP
-#
-#  This program is free software; you can redistribute it and/or
-#  modify it under the terms of the GNU Lesser General Public
-#  License as published by the Free Software Foundation; either
-#  version 2 of the License, or (at your option) any later version.
-#
-#  This library is distributed in the hope that it will be useful,
-#  but WITHOUT ANY WARRANTY; without even the implied warranty of
-#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
-#  Lesser General Public License for more details.
-#
-#  You should have received a copy of the GNU Lesser General Public
-#  License along with this library. If not, see <http://www.gnu.org/licenses/>.
-#
-
-import multiprocessing
-
-
-# QueueManager()
-#
-# This abstracts our choice of creating picklable or non-picklable Queues.
-#
-# Note that when choosing the 'spawn' or 'forkserver' methods of starting
-# processes with the `multiprocessing` standard library module, we must use
-# only picklable type as parameters to jobs.
-#
-class QueueManager:
-    def make_queue_wrapper(self):
-        return _PlainQueueWrapper(multiprocessing.Queue())
-
-
-# PicklableQueueManager()
-#
-# A QueueManager that creates pickable types.
-#
-# Note that the requirement of being picklable adds extra runtime burden, as we
-# must create and maintain a `SyncManager` process that will create and manage
-# the real objects.
-#
-class PicklableQueueManager(QueueManager):
-    def __init__(self):
-        super().__init__()
-        self._manager = None
-
-    def make_queue_wrapper(self):
-        # Only SyncManager Queues are picklable, so we must make those. Avoid
-        # creating multiple expensive SyncManagers, by keeping this one around.
-        if self._manager is None:
-            self._manager = multiprocessing.Manager()
-        return _SyncManagerQueueWrapper(self._manager.Queue())
-
-
-# QueueWrapper()
-#
-# This abstracts our choice of using picklable or non-picklable Queues.
-#
-class QueueWrapper:
-    pass
-
-
-class _PlainQueueWrapper(QueueWrapper):
-    def __init__(self, queue):
-        super().__init__()
-        self.queue = queue
-
-    def set_potential_callback_on_queue_event(self, event_loop, callback):
-        # Warning: Platform specific code up ahead
-        #
-        #   The multiprocessing.Queue object does not tell us how
-        #   to receive io events in the receiving process, so we
-        #   need to sneak in and get its file descriptor.
-        #
-        #   The _reader member of the Queue is currently private
-        #   but well known, perhaps it will become public:
-        #
-        #      http://bugs.python.org/issue3831
-        #
-        event_loop.add_reader(self.queue._reader.fileno(), callback)
-
-    def clear_potential_callback_on_queue_event(self, event_loop):
-        event_loop.remove_reader(self.queue._reader.fileno())
-
-    def close(self):
-        self.queue.close()
-
-
-class _SyncManagerQueueWrapper(QueueWrapper):
-    def __init__(self, queue):
-        super().__init__()
-        self.queue = queue
-
-    def set_potential_callback_on_queue_event(self, event_loop, callback):
-        # We can't easily support these callbacks for Queues managed by a
-        # SyncManager, so don't support them for now. In later work we should
-        # be able to support them with threading.
-        pass
-
-    def clear_potential_callback_on_queue_event(self, event_loop):
-        pass
-
-    def close(self):
-        # SyncManager queue proxies do not have a `close()` method, they rely
-        # on a callback on garbage collection to release resources. For our
-        # purposes the queue is invalid after closing, so it's ok to release it
-        # here.
-        self.queue = None
diff --git a/src/buildstream/_platform/platform.py b/src/buildstream/_platform/platform.py
index faf3d3c52..11c9217be 100644
--- a/src/buildstream/_platform/platform.py
+++ b/src/buildstream/_platform/platform.py
@@ -28,8 +28,6 @@ import psutil
 from .._exceptions import PlatformError, ImplError, SandboxError
 from .. import utils
 
-from .multiprocessing import QueueManager, PicklableQueueManager
-
 
 class Platform():
     # Platform()
@@ -175,12 +173,6 @@ class Platform():
         uname_machine = platform.uname().machine
         return Platform.canonicalize_arch(uname_machine)
 
-    def make_queue_manager(self):
-        if self.does_multiprocessing_start_require_pickling():
-            return PicklableQueueManager()
-        else:
-            return QueueManager()
-
     # does_multiprocessing_start_require_pickling():
     #
     # Returns True if the multiprocessing start method will pickle arguments