add support for readonly remotes

Currently only the web special remote is readonly, but it'd be possible to
also have readonly drives, or other remotes. These are handled in the
assistant by only downloading from them, and never trying to upload to
them.

Assistant/DaemonStatus.hs

@@ -14,6 +14,7 @@ import Utility.ThreadScheduler
 import Utility.TempFile
 import Utility.NotificationBroadcaster
 import Logs.Transfer
+import Logs.Trust
 import qualified Remote
 
 import Control.Concurrent.STM
@@ -81,8 +82,13 @@ modifyDaemonStatus dstatus a = do
 	sendNotification $ changeNotifier s
 	return b
 
+{- Remotes ordered by cost, with dead ones thrown out. -}
 calcKnownRemotes :: Annex [Remote]
-calcKnownRemotes = concat . Remote.byCost <$> Remote.enabledRemoteList
+calcKnownRemotes = do
+	rs <- concat . Remote.byCost <$> Remote.enabledRemoteList
+	alive <- snd <$> trustPartition DeadTrusted (map Remote.uuid rs)
+	let good r = Remote.uuid r `elem` alive
+	return $ filter good rs
 
 {- Updates the cached ordered list of remotes from the list in Annex
  - state. -}