libvirtaio: add .drain() coroutine

The intended use is to ensure that the implementation is empty, which is
one way to ensure that all connections were properly closed and file
descriptors reclaimed.

Signed-off-by: Wojtek Porczyk <woju@invisiblethingslab.com>

libvirtaio.py

@@ -269,10 +269,27 @@ class virEventAsyncIOImpl(object):
         self.descriptors = DescriptorDict(self)
         self.log = logging.getLogger(self.__class__.__name__)
 
+        # NOTE invariant: _finished.is_set() iff _pending == 0
+        self._pending = 0
+        self._finished = asyncio.Event(loop=loop)
+        self._finished.set()
+
     def __repr__(self):
         return '<{} callbacks={} descriptors={}>'.format(
             type(self).__name__, self.callbacks, self.descriptors)
 
+    def _pending_inc(self):
+        '''Increase the count of pending affairs. Do not use directly.'''
+        self._pending += 1
+        self._finished.clear()
+
+    def _pending_dec(self):
+        '''Decrease the count of pending affairs. Do not use directly.'''
+        assert self._pending > 0
+        self._pending -= 1
+        if self._pending == 0:
+            self._finished.set()
+
     def register(self):
         '''Register this instance as event loop implementation'''
         # pylint: disable=bad-whitespace
@@ -293,7 +310,20 @@ class virEventAsyncIOImpl(object):
         This is a coroutine.
         '''
         self.log.debug('ff_callback(iden=%d, opaque=...)', iden)
-        return libvirt.virEventInvokeFreeCallback(opaque)
+        ret = libvirt.virEventInvokeFreeCallback(opaque)
+        self._pending_dec()
+        return ret
+
+    @asyncio.coroutine
+    def drain(self):
+        '''Wait for the implementation to become idle.
+
+        This is a coroutine.
+        '''
+        self.log.debug('drain()')
+        if self._pending:
+            yield from self._finished.wait()
+        self.log.debug('drain ended')
 
     def is_idle(self):
         '''Returns False if there are leftovers from a connection
@@ -301,7 +331,7 @@ class virEventAsyncIOImpl(object):
         Those may happen if there are sematical problems while closing
         a connection. For example, not deregistered events before .close().
         '''
-        return not self.callbacks
+        return not self.callbacks and not self._pending
 
     def _add_handle(self, fd, event, cb, opaque):
         '''Register a callback for monitoring file handle events
@@ -324,6 +354,7 @@ class virEventAsyncIOImpl(object):
                 fd, event, callback.iden)
         self.callbacks[callback.iden] = callback
         self.descriptors[fd].add_handle(callback)
+        self._pending_inc()
         return callback.iden
 
     def _update_handle(self, watch, event):
@@ -378,6 +409,7 @@ class virEventAsyncIOImpl(object):
                 timeout, callback.iden)
         self.callbacks[callback.iden] = callback
         callback.update(timeout=timeout)
+        self._pending_inc()
         return callback.iden
 
     def _update_timeout(self, timer, timeout):