clean P2P protocol shutdown on EOF

Avoids "git-annex-shell: <stdin>: hGetChar: end of file"
being displayed by the test suite, due to the way it
runs git-annex-shell without using ssh.

git-annex-shell over ssh was not affected because git-annex hangs up the
ssh connection and so never sees the error message that git-annnex-shell
probably did emit.

This commit was sponsored by Ryan Newton on Patreon.

P2P/IO.hs

@@ -38,6 +38,7 @@ import Annex.ChangedRefs
 import Control.Monad.Free
 import Control.Monad.IO.Class
 import System.Exit (ExitCode(..))
+import System.IO.Error
 import Network.Socket
 import Control.Concurrent
 import Control.Concurrent.Async
@@ -159,9 +160,11 @@ runNet runst conn runner f = case f of
 			Left e -> return (Left (show e))
 			Right () -> runner next
 	ReceiveMessage next -> do
-		v <- liftIO $ tryNonAsync $ getProtocolLine (connIhdl conn)
+		v <- liftIO $ tryIOError $ getProtocolLine (connIhdl conn)
 		case v of
-			Left e -> return (Left (show e))
+			Left e
+				| isEOFError e -> runner (next (Just ProtocolEOF))
+				| otherwise -> return (Left (show e))
 			Right Nothing -> return (Left "protocol error")
 			Right (Just l) -> case parseMessage l of
 				Just m -> do

P2P/Protocol.hs

@@ -83,6 +83,7 @@ data Message
 	| DATA Len -- followed by bytes of data
 	| VALIDITY Validity
 	| ERROR String
+	| ProtocolEOF
 	deriving (Show)
 
 instance Proto.Sendable Message where
@@ -108,6 +109,7 @@ instance Proto.Sendable Message where
 	formatMessage (VALIDITY Invalid) = ["INVALID"]
 	formatMessage (DATA len) = ["DATA", Proto.serialize len]
 	formatMessage (ERROR err) = ["ERROR", Proto.serialize err]
+	formatMessage (ProtocolEOF) = []
 
 instance Proto.Receivable Message where
 	parseCommand "AUTH" = Proto.parse2 AUTH
@@ -367,6 +369,8 @@ serverLoop :: (Message -> Proto (ServerHandler a)) -> Proto (Maybe a)
 serverLoop a = do
 	mcmd <- net receiveMessage
 	case mcmd of
+		-- Stop loop at EOF
+		Just ProtocolEOF -> return Nothing
 		-- When the client sends ERROR to the server, the server
 		-- gives up, since it's not clear what state the client
 		-- is in, and so not possible to recover.

Utility/SimpleProtocol.hs

@@ -113,6 +113,8 @@ dupIoHandles = do
  - This implementation is not super efficient, but as long as the Handle
  - supports buffering, it avoids reading a character at a time at the
  - syscall level.
+ -
+ - Throws isEOFError when no more input is available.
  -}
 getProtocolLine :: Handle -> IO (Maybe String)
 getProtocolLine h = go (32768 :: Int) []