avoid pairlistener crash

I noticed this while offline (so that lack of solar power is good for something).

Apparently it tries to bind multicast to lo, and that fails.
If this happens, catch it, and retry until a real network interface becomes
available.

It may be that this should tie into the NetWatcher, and rebind whenever
an interface comes up. Needs testing..

Assistant/Threads/PairListener.hs

@@ -17,6 +17,7 @@ import Assistant.DaemonStatus
 import Assistant.WebApp
 import Assistant.WebApp.Types
 import Assistant.Alert
+import Utility.ThreadScheduler
 
 import Network.Multicast
 import Network.Socket
@@ -27,12 +28,17 @@ thisThread :: ThreadName
 thisThread = "PairListener"
 
 pairListenerThread :: ThreadState -> DaemonStatusHandle -> ScanRemoteMap -> UrlRenderer -> NamedThread
-pairListenerThread st dstatus scanremotes urlrenderer = thread $ withSocketsDo $ do
+pairListenerThread st dstatus scanremotes urlrenderer = thread $ withSocketsDo $
-	sock <- multicastReceiver (multicastAddress $ IPv4Addr undefined) pairingPort
+	runEvery (Seconds 1) $ void $ tryIO $ do
-	go sock [] []
+		sock <- getsock
+		go sock [] []
 	where
 		thread = NamedThread thisThread
 		
+		{- Note this can crash if there's no network interface,
+		 - or only one like lo that doesn't support multicast. -}
+		getsock = multicastReceiver (multicastAddress $ IPv4Addr undefined) pairingPort
+		
 		go sock reqs cache = getmsg sock [] >>= \msg -> case readish msg of
 			Nothing -> go sock reqs cache
 			Just m -> do