Add the parseReply function to the Confd utils

This function will be used by the Haskell Confd client to parse the replies
it receives from the server.

Unit test is included as well.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

htest/Test/Ganeti/Confd/Utils.hs

@@ -67,6 +67,21 @@ prop_req_sign key (NonNegative timestamp) (Positive bad_delta)
                     \ timestamp, got " ++ show ts_bad)
        (ts_bad ==? BasicTypes.Bad "Too old/too new timestamp or clock skew")
 
+-- | Tests that a ConfdReply can be properly encoded, signed and parsed using
+-- the proper salt, but fails parsing with the wrong salt.
+prop_rep_salt :: Hash.HashKey     -- ^ The hash key
+              -> Confd.ConfdReply -- ^ A Confd reply
+              -> Property
+prop_rep_salt hmac reply =
+  forAll arbitrary $ \salt1 ->
+  forAll (arbitrary `suchThat` (/= salt1)) $ \salt2 ->
+  let innerMsg = J.encode reply
+      msg = J.encode $ Confd.Utils.signMessage hmac salt1 innerMsg
+  in
+    Confd.Utils.parseReply hmac msg salt1 ==? BasicTypes.Ok (innerMsg, reply)
+      .&&. Confd.Utils.parseReply hmac msg salt2 ==?
+           BasicTypes.Bad "The received salt differs from the expected salt"
+
 -- | Tests that signing with a different key fails detects failure
 -- correctly.
 prop_bad_key :: String             -- ^ Salt
@@ -88,5 +103,6 @@ prop_bad_key salt crq =
 
 testSuite "Confd/Utils"
   [ 'prop_req_sign
+  , 'prop_rep_salt
   , 'prop_bad_key
   ]

htools/Ganeti/Confd/Utils.hs

@@ -30,6 +30,7 @@ module Ganeti.Confd.Utils
   ( getClusterHmac
   , parseSignedMessage
   , parseRequest
+  , parseReply
   , signMessage
   , getCurrentTime
   ) where
@@ -65,8 +66,8 @@ parseSignedMessage key str = do
            else Bad "HMAC verification failed"
   return (salt, msg, parsedMsg)
 
--- | Message parsing. This can either result in a good, valid message,
--- or fail in the Result monad.
+-- | Message parsing. This can either result in a good, valid request
+-- message, or fail in the Result monad.
 parseRequest :: HashKey -> String -> Integer
              -> Result (String, ConfdRequest)
 parseRequest hmac msg curtime = do
@@ -76,6 +77,17 @@ parseRequest hmac msg curtime = do
     then fail "Too old/too new timestamp or clock skew"
     else return (origmsg, request)
 
+-- | Message parsing. This can either result in a good, valid reply
+-- message, or fail in the Result monad.
+-- It also checks that the salt in the message corresponds to the one
+-- that is expected
+parseReply :: HashKey -> String -> String -> Result (String, ConfdReply)
+parseReply hmac msg expSalt = do
+  (salt, origmsg, reply) <- parseSignedMessage hmac msg
+  if salt /= expSalt
+    then fail "The received salt differs from the expected salt"
+    else return (origmsg, reply)
+
 -- | Signs a message with a given key and salt.
 signMessage :: HashKey -> String -> String -> SignedMessage
 signMessage key salt msg =