glib_selector: Only ever iterate the mainloop once

As it is right now, any python code that runs may modify the timeout
that would be passed into the select() call. As such, we can only run a
single mainloop iteration before the select() call needs to be
restarted.

Also, we cannot use g_source_set_ready_time, because GLib will always
do a full mainloop iteration afterwards to ensure that all sources had a
chance to dispatch.
This is easy to work around though as we can use the prepare callback to
pass the required timeout from python into the GLib main loop code.

Note that with this we end up iterating the main context but we never
actually run a GLib mainloop.

Fixes: jhenstridge#3

asyncio_glib/glib_selector.py

@@ -7,27 +7,17 @@
     'GLibSelector',
 )
 
-
-# The override for GLib.MainLoop.run installs a signal wakeup fd,
-# which interferes with asyncio signal handlers.  Try to get the
-# direct version.
-try:
-    g_main_loop_run = super(GLib.MainLoop, GLib.MainLoop).run
-except AttributeError:
-    g_main_loop_run = GLib.MainLoop.run
-
-
 class _SelectorSource(GLib.Source):
     """A GLib source that gathers selector """
 
-    def __init__(self, main_loop):
+    def __init__(self, selector):
         super().__init__()
         self._fd_to_tag = {}
         self._fd_to_events = {}
-        self._main_loop = main_loop
+        self._iteration_timeout = 0
 
     def prepare(self):
-        return False, -1
+        return False, self._iteration_timeout
 
     def check(self):
         return False
@@ -41,7 +31,6 @@ def dispatch(self, callback, args):
             if condition & GLib.IOCondition.OUT:
                 events |= selectors.EVENT_WRITE
             self._fd_to_events[fd] = events
-        self._main_loop.quit()
         return GLib.SOURCE_CONTINUE
 
     def register(self, fd, events):
@@ -70,8 +59,7 @@ class GLibSelector(selectors._BaseSelectorImpl):
     def __init__(self, context):
         super().__init__()
         self._context = context
-        self._main_loop = GLib.MainLoop.new(self._context, False)
-        self._source = _SelectorSource(self._main_loop)
+        self._source = _SelectorSource(self)
         self._source.attach(self._context)
 
     def close(self):
@@ -89,20 +77,15 @@ def unregister(self, fileobj):
         return key
 
     def select(self, timeout=None):
-        may_block = True
-        self._source.set_ready_time(-1)
+        # Calling .set_ready_time() always causes a mainloop iteration to finish.
         if timeout is not None:
-            if timeout > 0:
-                self._source.set_ready_time(
-                    GLib.get_monotonic_time() + int(timeout * 1000000))
-            else:
-                may_block = False
+            # Negative timeout implies an immediate dispatch
+            self._source._iteration_timeout = int(max(0, timeout) * 1000)
+        else:
+            self._source._iteration_timeout = -1
 
         self._source.clear()
-        if may_block:
-            g_main_loop_run(self._main_loop)
-        else:
-            self._context.iteration(False)
+        self._context.iteration(True)
 
         ready = []
         for key in self.get_map().values():