Asynchronous IPC: Initial implementation

This commit adds a mixin which sets up a modal operator which:
* Starts a subprocess
* Polls for messages from that subprocesses

pkg_ops.py

@@ -1,21 +1,72 @@
 import bpy
-from multiprocessing import Process
+import logging
+from multiprocessing import Process, Pipe
 from . import blenderpack
 
-class PACKAGE_OT_fetch_lists(bpy.types.Operator):
-    bl_idname = "package.fetch_lists"
-    bl_label = "Update package list(s)"
+class SubprocessOperatorMixin:
+    timer = None
+
+    # run once in invoke
+    def setup(self):
+        pass
+    # run on receipt of data from subprocess
+    def handle_response(self, resp):
+        pass
+
+    def __init__(self):
+        self.pipe = Pipe()
+        self.subprocess = None
+
+    def execute(self, context):
+        return self.invoke(context, None)
+
+    def invoke(self, context, event):
+        self.subprocess.start()
+        # we have to explicitly close the end of the pipe we are NOT using,
+        # otherwise no exception will be generated when the other process closes its end.
+        self.pipe[1].close()
+
+        wm = context.window_manager
+        wm.modal_handler_add(self)
+        self.timer = wm.event_timer_add(.01, context.window)
+
+        self.setup()
+
+        return {'RUNNING_MODAL'}
+
+    def modal(self, context, event):
+        print("polling")
+        try:
+            if self.pipe[0].poll():
+                newdata = self.pipe[0].recv()
+            else:
+                newdata = None
+        except EOFError:
+            return {'FINISHED'}
+
+        if newdata is not None:
+            self.handle_response(newdata)
+        return {'PASS_THROUGH'}
+
+class PACKAGE_OT_fetch(SubprocessOperatorMixin, bpy.types.Operator):
+    bl_idname = "package.fetch"
+    bl_label = "Update package list(s)"
+
+    def __init__(self):
+        SubprocessOperatorMixin.__init__(self)
+        settings = bpy.context.window_manager.package_manager_settings
+        self.subprocess = Process(target=blenderpack.fetch, args=(settings.url, self.pipe))
+
+    def handle_response(self, resp):
+        print("your response:", resp)
 
     def execute(self, context):
-        settings = context.window_manager.PackageManagerSettings
-        proc = Process(target=lambda: blenderpack.fetch(settings.url))
-        proc.start()
         return {'FINISHED'}
 
 
 def register():
-    bpy.utils.register_class(PACKAGE_OT_fetch_lists)
+    bpy.utils.register_class(PACKAGE_OT_fetch)
 
 def unregister():
-    bpy.utils.unregister_class(PACKAGE_OT_fetch_lists)
+    bpy.utils.unregister_class(PACKAGE_OT_fetch)