make some requests optional, simplify and future-proof protocol more

Remote/External.hs

@@ -152,7 +152,8 @@ safely a = go =<< tryAnnex a
 		return False
 
 {- Sends a Request to the external remote, and waits for it to generate
- - a Response that the responsehandler accepts.
+ - a Response. That is fed into the responsehandler, which should return
+ - the action to run for it (or Nothing if there's a protocol error).
  -
  - While the external remote is processing the Request, it may send
  - any number of RemoteRequests, that are handled here.
@@ -291,7 +292,7 @@ getCost external r gc = go =<< remoteCost' gc
 	go Nothing = do
 		c <- handleRequest external GETCOST Nothing $ \req -> case req of
 			COST c -> Just $ return c
-			COST_UNKNOWN -> Just $ return expensiveRemoteCost
+			UNSUPPORTED_REQUEST -> Just $ return expensiveRemoteCost
 			_ -> Nothing
 		setRemoteCost r c
 		return c

Remote/External/Types.hs

@@ -118,10 +118,9 @@ data Response
 	| REMOVE_SUCCESS Key
 	| REMOVE_FAILURE Key ErrorMsg
 	| COST Cost
-	| COST_UNKNOWN
 	| INITREMOTE_SUCCESS
 	| INITREMOTE_FAILURE ErrorMsg
-	| UNKNOWN_REQUEST
+	| UNSUPPORTED_REQUEST
 	deriving (Show)
 
 instance Receivable Response where
@@ -134,10 +133,9 @@ instance Receivable Response where
 	parseCommand "REMOVE-SUCCESS" = parse1 REMOVE_SUCCESS
 	parseCommand "REMOVE-FAILURE" = parse2 REMOVE_FAILURE
 	parseCommand "COST" = parse1 COST
-	parseCommand "COST_UNKNOWN" = parse0 COST_UNKNOWN
 	parseCommand "INITREMOTE-SUCCESS" = parse0 INITREMOTE_SUCCESS
 	parseCommand "INITREMOTE-FAILURE" = parse1 INITREMOTE_FAILURE
-	parseCommand "UNKNOWN-REQUEST" = parse0 UNKNOWN_REQUEST
+	parseCommand "UNSUPPORTED-REQUEST" = parse0 UNSUPPORTED_REQUEST
 	parseCommand _ = parseFail
 
 -- Requests that the external remote can send at any time it's in control.