make git-remote-daemon ssh transport robust

* Remote system might be available, and connection get lost. Should
  reconnect, but needs to avoid bad behavior (ie, constant reconnect
  attempts.) Use exponential backoff.

* Detect if old system had a too old git-annex-shell, and show the user
  a nice message in the webapp. Required parsing error messages, so perhaps
  this code shoudl be removed once enough time has passed..

* Switch the protocol to using remote URI's, rather than remote names.
  Names change. Also avoids issues with serialization of names containing
  whitespace.

This is nearly ready for merge into master now. I'd still like to make the ssh
transport smarter about reusing ssh connection caching during git pull.

This commit was sponsored by Jim Paris.

RemoteDaemon/Transport/Ssh.hs

@@ -13,60 +13,103 @@ import RemoteDaemon.Common
 import Remote.Helper.Ssh
 import qualified RemoteDaemon.Transport.Ssh.Types as SshRemote
 import Utility.SimpleProtocol
+import qualified Git
 import Git.Command
+import Utility.ThreadScheduler
 
 import Control.Concurrent.Chan
 import Control.Concurrent.Async
-import System.Process (std_in, std_out)
+import System.Process (std_in, std_out, std_err)
 
 transport :: Transport
-transport r remotename transporthandle ichan ochan = do
+transport r url transporthandle ichan ochan = do
 	v <- liftAnnex transporthandle $ git_annex_shell r "notifychanges" [] []
 	case v of
 		Nothing -> noop
-		Just (cmd, params) -> go cmd (toCommand params)
+		Just (cmd, params) -> robustly 1 $
+			connect cmd (toCommand params)
   where
-	go cmd params = do
-		(Just toh, Just fromh, _, pid) <- createProcess (proc cmd params)
+	connect cmd params = do
+		(Just toh, Just fromh, Just errh, pid) <-
+			createProcess (proc cmd params)
 			{ std_in = CreatePipe
 			, std_out = CreatePipe
+			, std_err = CreatePipe
 			}
 		
-		let shutdown = do
-			hClose toh
-			hClose fromh
-			void $ waitForProcess pid
-			send DISCONNECTED
+		-- Run all threads until one finishes and get the status
+		-- of the first to finish. Cancel the rest.
+		status <- catchDefaultIO (Right ConnectionClosed) $
+			handlestderr errh
+				`race` handlestdout fromh
+					`race` handlecontrol
 
-		let fromshell = forever $ do
-			l <- hGetLine fromh
-			case parseMessage l of
-				Just SshRemote.READY -> send CONNECTED
-				Just (SshRemote.CHANGED shas) ->
-					whenM (checkNewShas transporthandle shas) $
-						fetch
-				Nothing -> shutdown
+		send (DISCONNECTED url)
+		hClose toh
+		hClose fromh
+		void $ waitForProcess pid
 
-		-- The only control message that matters is STOP.
-		--
-		-- Note that a CHANGED control message is not handled;
-		-- we don't push to the ssh remote. The assistant
-		-- and git-annex sync both handle pushes, so there's no
-		-- need to do it here.
-		let handlecontrol = forever $ do
-			msg <- readChan ichan
-			case msg of
-				STOP -> ioError (userError "done")
-				_ -> noop
+		return $ either (either id id) id status
 
-		-- Run both threads until one finishes.
-		void $ tryIO $ concurrently fromshell handlecontrol
-		shutdown
-
-	send msg = writeChan ochan (msg remotename)
+	send msg = writeChan ochan msg
 
 	fetch = do
-		send SYNCING
+		send (SYNCING url)
 		ok <- inLocalRepo transporthandle $
-			runBool [Param "fetch", Param remotename]
-		send (DONESYNCING ok)
+			runBool [Param "fetch", Param $ Git.repoDescribe r]
+		send (DONESYNCING url ok)
+		
+	handlestdout fromh = do
+		l <- hGetLine fromh
+		case parseMessage l of
+			Just SshRemote.READY -> do
+				send (CONNECTED url)
+				handlestdout fromh
+			Just (SshRemote.CHANGED shas) -> do
+				whenM (checkNewShas transporthandle shas) $
+					fetch
+				handlestdout fromh
+			-- avoid reconnect on protocol error
+			Nothing -> return Stopping
+	
+	handlecontrol = do
+		msg <- readChan ichan
+		case msg of
+			STOP -> return Stopping
+			_ -> handlecontrol
+
+	-- Old versions of git-annex-shell that do not support
+	-- the notifychanges command will exit with a not very useful
+	-- error message. Detect that error, and avoid reconnecting.
+	-- Propigate all stderr.
+	handlestderr errh = do
+		s <- hGetSomeString errh 1024
+		hPutStr stderr s
+		hFlush stderr
+		if "git-annex-shell: git-shell failed" `isInfixOf` s
+			then do
+				send $ WARNING url $ unwords
+					[ "Remote", Git.repoDescribe r
+					, "needs its git-annex upgraded"
+					, "to 5.20140405 or newer"
+					]
+				return Stopping
+			else handlestderr errh
+
+data Status = Stopping | ConnectionClosed
+
+{- Make connection robustly, with exponentioal backoff on failure. -}
+robustly :: Int -> IO Status -> IO ()
+robustly backoff a = handle =<< catchDefaultIO ConnectionClosed a
+  where
+	handle Stopping = return ()
+	handle ConnectionClosed = do
+		threadDelaySeconds (Seconds backoff)
+		robustly increasedbackoff a
+
+	increasedbackoff
+		| b2 > maxbackoff = maxbackoff
+		| otherwise = b2
+	  where
+	  	b2 = backoff * 2
+		maxbackoff = 3600 -- one hour