Abstract mp Queue usage, prep to spawn processes

Pave the way to supporting starting processes by the 'spawn' method, by
abstracting our usage of `multiprocessing.Queue`. This means we can
easily switch to using a multiprocessing.Manager() and associated queues
instead when necessary.

4 files changed, 89 insertions, 25 deletions

diff --git a/src/buildstream/_platform/multiprocessing.py b/src/buildstream/_platform/multiprocessing.py
new file mode 100644
index 000000000..1f7be8338
--- /dev/null
+++ b/src/buildstream/_platform/multiprocessing.py
@@ -0,0 +1,65 @@
+#
+#  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())
+
+
+# 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()
diff --git a/src/buildstream/_platform/platform.py b/src/buildstream/_platform/platform.py
index d6e0755f2..18e26a9a1 100644
--- a/src/buildstream/_platform/platform.py
+++ b/src/buildstream/_platform/platform.py
@@ -27,6 +27,8 @@ import psutil
 from .._exceptions import PlatformError, ImplError, SandboxError
 from .. import utils
 
+from .multiprocessing import QueueManager
+
 
 class Platform():
     # Platform()
@@ -172,6 +174,9 @@ class Platform():
         uname_machine = platform.uname().machine
         return Platform.canonicalize_arch(uname_machine)
 
+    def make_queue_manager(self):
+        return QueueManager()
+
     ##################################################################
     #                        Sandbox functions                       #
     ##################################################################
diff --git a/src/buildstream/_scheduler/jobs/job.py b/src/buildstream/_scheduler/jobs/job.py
index 7cbde2d39..4104ddc73 100644
--- a/src/buildstream/_scheduler/jobs/job.py
+++ b/src/buildstream/_scheduler/jobs/job.py
@@ -136,7 +136,7 @@ class Job():
         # Private members
         #
         self._scheduler = scheduler            # The scheduler
-        self._queue = None                     # A message passing queue
+        self._queue_wrapper = None             # A wrapper of a message passing queue
         self._process = None                   # The Process object
         self._watcher = None                   # Child process watcher
         self._listening = False                # Whether the parent is currently listening
@@ -162,8 +162,7 @@ class Job():
     # Starts the job.
     #
     def start(self):
-
-        self._queue = multiprocessing.Queue()
+        self._queue_wrapper = self._scheduler.ipc_queue_manager.make_queue_wrapper()
 
         self._tries += 1
         self._parent_start_listening()
@@ -478,7 +477,7 @@ class Job():
         self._scheduler.job_completed(self, status)
 
         # Force the deletion of the queue and process objects to try and clean up FDs
-        self._queue = self._process = None
+        self._queue_wrapper = self._process = None
 
     # _parent_process_envelope()
     #
@@ -523,8 +522,8 @@ class Job():
     # in the parent process.
     #
     def _parent_process_queue(self):
-        while not self._queue.empty():
-            envelope = self._queue.get_nowait()
+        while not self._queue_wrapper.queue.empty():
+            envelope = self._queue_wrapper.queue.get_nowait()
             self._parent_process_envelope(envelope)
 
     # _parent_recv()
@@ -540,20 +539,9 @@ class Job():
     # Starts listening on the message queue
     #
     def _parent_start_listening(self):
-        # 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
-        #
         if not self._listening:
-            self._scheduler.loop.add_reader(
-                self._queue._reader.fileno(), self._parent_recv)
+            self._queue_wrapper.set_potential_callback_on_queue_event(
+                self._scheduler.loop, self._parent_recv)
             self._listening = True
 
     # _parent_stop_listening()
@@ -562,7 +550,8 @@ class Job():
     #
     def _parent_stop_listening(self):
         if self._listening:
-            self._scheduler.loop.remove_reader(self._queue._reader.fileno())
+            self._queue_wrapper.clear_potential_callback_on_queue_event(
+                self._scheduler.loop)
             self._listening = False
 
 
@@ -605,7 +594,7 @@ class ChildJob():
         self._message_element_name = message_element_name
         self._message_element_key = message_element_key
 
-        self._queue = None
+        self._queue_wrapper = None
 
     # message():
     #
@@ -692,7 +681,7 @@ class ChildJob():
     # Args:
     #    queue (multiprocessing.Queue): The message queue for IPC
     #
-    def child_action(self, queue):
+    def child_action(self, queue_wrapper):
 
         # This avoids some SIGTSTP signals from grandchildren
         # getting propagated up to the master process
@@ -710,7 +699,7 @@ class ChildJob():
         #
         # Set the global message handler in this child
         # process to forward messages to the parent process
-        self._queue = queue
+        self._queue_wrapper = queue_wrapper
         self._messenger.set_message_handler(self._child_message_handler)
 
         starttime = datetime.datetime.now()
@@ -808,7 +797,7 @@ class ChildJob():
     #                        instances). This is sent to the parent Job.
     #
     def _send_message(self, message_type, message_data):
-        self._queue.put(_Envelope(message_type, message_data))
+        self._queue_wrapper.queue.put(_Envelope(message_type, message_data))
 
     # _child_send_error()
     #
@@ -854,7 +843,7 @@ class ChildJob():
     #    exit_code (_ReturnCode): The exit code to exit with
     #
     def _child_shutdown(self, exit_code):
-        self._queue.close()
+        self._queue_wrapper.close()
         assert isinstance(exit_code, _ReturnCode)
         sys.exit(exit_code.value)
 
diff --git a/src/buildstream/_scheduler/scheduler.py b/src/buildstream/_scheduler/scheduler.py
index 9d7cf5d09..398e52e74 100644
--- a/src/buildstream/_scheduler/scheduler.py
+++ b/src/buildstream/_scheduler/scheduler.py
@@ -80,6 +80,11 @@ class Scheduler():
         self.terminated = False     # Whether the scheduler was asked to terminate or has terminated
         self.suspended = False      # Whether the scheduler is currently suspended
 
+        # A manager for creating and monitoring IPC queues, note that this
+        # can't be part of the platform or context as it is not always
+        # picklable.
+        self.ipc_queue_manager = self.context.platform.make_queue_manager()
+
         # These are shared with the Job, but should probably be removed or made private in some way.
         self.loop = None            # Shared for Job access to observe the message queue
         self.internal_stops = 0     # Amount of SIGSTP signals we've introduced, this is shared with job.py