Switch to Tornado's event loop

This provides us with several additional features the builtin
select() loop does not, such as:

- Chainable async actions via coroutines
- asyncio bridge

This will keep the code simpler and read more like a synchronous
application.

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>

requirements.txt

@@ -5,3 +5,4 @@ nose-cov
 nose
 flake8
 python3-parted
+tornado

subiquity/async.py

@@ -17,30 +17,27 @@
 Provides async operations for various api calls and other non-blocking
 work.
 
-The way this works is you create your IO/CPU bound thread:
+The way this works is you create your non-blocking thread:
 
 .. code::
 
     def my_async_method(self):
         pool.submit(func, *args)
 
-    # In your controller you would then call
+    # In your controller you would then call using coroutines
 
-    my_async_method_f = my_async_method()
-    my_async_method_f.add_done_callback(self.handle_async_method)
-
-    def handle_async_method(self, future):
+    @coroutine
+    def do_async_action(self):
         try:
-            result = future.result()
-        except Exception as e:
-            raise Exception("Program in thread {}".format(e))
-
+            yield my_async_method()
+            # Move to next coroutine or return if finish
+            self.do_second_async_action()
+        except Exeception as e:
+            log.exception("Failed in our non-blocking code.")
 """
 
-from multiprocessing import cpu_count
-from concurrent.futures import ThreadPoolExecutor, ProcessPoolExecutor
+from concurrent.futures import ThreadPoolExecutor
 
 
 class Async:
-    pool = ThreadPoolExecutor(10)
-    ppool = ProcessPoolExecutor(cpu_count())
+    pool = ThreadPoolExecutor(4)

subiquity/core.py

@@ -16,6 +16,7 @@
 import logging
 import urwid
 import urwid.curses_display
+from tornado.ioloop import IOLoop
 from subiquity.signals import Signal
 from subiquity.palette import STYLES, STYLES_MONO
 
@@ -102,8 +103,10 @@ class Controller:
                 additional_opts['screen'].set_terminal_properties(colors=256)
                 additional_opts['screen'].reset_default_terminal_palette()
 
+            evl = urwid.TornadoEventLoop(IOLoop())
             self.loop = urwid.MainLoop(
-                self.ui, palette, **additional_opts)
+                self.ui, palette, event_loop=evl, **additional_opts)
+            log.debug("Running event loop: {}".format(self.loop.event_loop))
 
         try:
             self.set_alarm_in(0.05, self.welcome)