bitbake: process: Ensure we call select() to know which fds to read

There is an interesting bug in the current code where a sync command
is not seen until the current async command completes, by which time
the UI may have shut down.

The reason is that if there are idle commands, we may not end up sleeping
in the select call at all, partiularly under heavy load like parsing.

Fix this by calling select with a zero timeout so that we see active
fds and know to read from them. This fixes various problems toaster was
having with the recent server changes.

[YOCTO #11898]

(Bitbake rev: bbcce58e824b2793abf50efa52db158ae16e23e4)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/server/process.py

@@ -312,7 +312,7 @@ class ProcessServer(multiprocessing.Process):
                 # Ignore EINTR
                 return []
         else:
-            return []
+            return select.select(fds,[],[],0)[0]
 
 
 class ServerCommunicator():