skip local remotes that are not available (ie, not mounted)

With --fast, unavailable local remotes are filtered out of the fast set.
This way, if there are local remotes, --fast always acts only on them,
and if none are mounted, acts on nothing. This consistency is better
than --fast acting on different remotes depending on what's mounted.

Command/Sync.hs

@@ -57,12 +57,13 @@ syncRemotes rs = do
 		then nub <$> pickfast
 		else wanted
 	where
-		pickfast = (++) <$> listed <*> (fastest <$> available)
+		pickfast = (++) <$> listed <*> (good =<< fastest <$> available)
 		wanted
-			| null rs = available
+			| null rs = good =<< available
 			| otherwise = listed
 		listed = mapM Remote.byName rs
 		available = filter nonspecial <$> Remote.enabledRemoteList
+		good = filterM $ Remote.Git.repoAvail . Types.Remote.repo
 		nonspecial r = Types.Remote.remotetype r == Remote.Git.remote
 		fastest = fromMaybe [] . headMaybe .
 			map snd . sort . M.toList . costmap

Git/Config.hs

@@ -29,7 +29,8 @@ read repo@(Repo { location = Dir d }) = do
 	bracket_ (changeWorkingDirectory d) (changeWorkingDirectory cwd) $
 		pOpen ReadFromPipe "git" ["config", "--null", "--list"] $
 			hRead repo
-read r = assertLocal r $ error "internal"
+read r = assertLocal r $
+	error $ "internal error; trying to read config of " ++ show r
 
 {- Reads git config from a handle and populates a repo with it. -}
 hRead :: Repo -> Handle -> IO Repo

Remote/Git.hs

@@ -5,7 +5,7 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-module Remote.Git (remote) where
+module Remote.Git (remote, repoAvail) where
 
 import Control.Exception.Extensible
 import qualified Data.Map as M
@@ -164,6 +164,13 @@ inAnnex r key
 				dispatch (Right Nothing) = unknown
 		unknown = Left $ "unable to check " ++ Git.repoDescribe r
 
+{- Checks inexpensively if a repository is available for use. -}
+repoAvail :: Git.Repo -> Annex Bool
+repoAvail r 
+	| Git.repoIsHttp r = return True
+	| Git.repoIsUrl r = return True
+	| otherwise = liftIO $ catchBoolIO $ onLocal r $ return True
+
 {- Runs an action on a local repository inexpensively, by making an annex
  - monad using that repository. -}
 onLocal :: Git.Repo -> Annex a -> IO a