don't send PREPARE before INITREMOTE

That complicated special remote programs, because they had to avoid making
PREPARE fail if some configuration is missing, because the remote might not
be initialized yet. Instead, complicate git-annex slightly by only sending
PREPARE immediately before some other request other than INITREMOTE (or
PREPARE of course).

Remote/External.hs

@@ -169,6 +169,8 @@ handleRequest external req mp responsehandler =
 
 handleRequest' :: ExternalLock -> External -> Request -> Maybe MeterUpdate -> (Response -> Maybe (Annex a)) -> Annex a
 handleRequest' lck external req mp responsehandler = do
+	when (needsPREPARE req) $
+		checkPrepared lck external
 	sendMessage lck external req
 	loop
   where
@@ -230,15 +232,13 @@ fromExternal lck external extractor a =
 		void $ liftIO $ atomically $ swapTMVar v st
 
 		{- Handle initial protocol startup; check the VERSION
-		 - the remote sends, and send it the PREPARE request. -}
+		 - the remote sends. -}
 		receiveMessage lck external
 			(const Nothing)
 			(checkVersion lck external)
 			(const Nothing)
-		handleRequest' lck external PREPARE Nothing $ \resp -> 
-			case resp of
-				PREPARE_SUCCESS -> Just $ run st
-				_ -> Nothing
+
+		run st
 
 	run st = a $ extractor st
 	v = externalState external
@@ -259,6 +259,7 @@ startExternal externaltype = liftIO $ do
 		{ externalSend = hin
 		, externalReceive = hout
 		, externalPid = pid
+		, externalPrepared = False
 		}
 
 stopExternal :: External -> Annex ()
@@ -282,6 +283,15 @@ checkVersion lck external (VERSION v) = Just $
 		else sendMessage lck external (ERROR "unsupported VERSION")
 checkVersion _ _ _ = Nothing
 
+checkPrepared :: ExternalLock -> External -> Annex ()
+checkPrepared lck external = 
+	fromExternal lck external externalPrepared $ \prepared ->
+		unless prepared $
+			handleRequest' lck external PREPARE Nothing $ \resp ->
+				case resp of
+					PREPARE_SUCCESS -> Just noop
+					_ -> Nothing
+
 {- Caches the cost in the git config to avoid needing to start up an
  - external special remote every time time just to ask it what its
  - cost is. -}

Remote/External/Types.hs

@@ -18,6 +18,7 @@ module Remote.External.Types (
 	Sendable(..),
 	Receivable(..),
 	Request(..),
+	needsPREPARE,
 	Response(..),
 	RemoteRequest(..),
 	RemoteResponse(..),
@@ -60,6 +61,7 @@ data ExternalState = ExternalState
 	{ externalSend :: Handle
 	, externalReceive :: Handle
 	, externalPid :: ProcessHandle
+	, externalPrepared :: Bool
 	}
 
 -- Constructor is not exported, and only created by newExternal.
@@ -98,6 +100,12 @@ data Request
 	| REMOVE Key
 	deriving (Show)
 
+-- Does PREPARE need to have been sent before this request?
+needsPREPARE :: Request -> Bool
+needsPREPARE PREPARE = False
+needsPREPARE INITREMOTE = False
+needsPREPARE _ = True
+
 instance Sendable Request where
 	formatMessage PREPARE = ["PREPARE"]
 	formatMessage INITREMOTE = ["INITREMOTE"]