respond to and handle Leave requests

contributes to #28

src/Hash2Pub/DHTProtocol.hs

@@ -38,7 +38,7 @@ import           Data.Foldable                  (foldl', foldr')
 import           Data.Functor.Identity
 import           Data.IP                        (IPv6, fromHostAddress6,
                                                  toHostAddress6)
-import           Data.List                      (sortBy)
+import           Data.List                      (delete, sortBy)
 import qualified Data.Map                       as Map
 import           Data.Maybe                     (fromJust, fromMaybe, mapMaybe,
                                                  maybe)
@@ -209,6 +209,32 @@ handleIncomingRequest nsSTM sendQ msgSet sourceAddr = do
 
 -- TODO: could all these respond* functions be in STM instead of IO?
 
+-- | Respond to a Leave request by removing the leaving node from local data structures
+-- and confirming with response.
+-- TODO: copy over key data from leaver and confirm
+respondLeave :: LocalNodeStateSTM -> Set.Set FediChordMessage -> IO (Map.Map Integer BS.ByteString)
+respondLeave nsSTM msgSet = do
+    responseMsg <- atomically $ do
+        nsSnap <- readTVar nsSTM
+        let
+            aRequestPart = Set.elemAt 0 msgSet
+            senderID = getNid . sender $ aRequestPart
+        -- remove leaving node from successors, predecessors and NodeCache
+        writeTQueue (cacheWriteQueue nsSnap) $ deleteCacheEntry senderID
+        writeTVar nsSTM $
+            setPredecessors (delete senderID $ predecessors nsSnap)
+          . setSuccessors (delete senderID $ successors nsSnap) $ nsSnap
+        -- TODO: handle handover of key data
+        let leaveResponse = Response {
+                responseTo = requestID aRequestPart
+              , senderID = getNid nsSnap
+              , part = if Set.size msgSet == 1 then 1 else fromIntegral $ Set.size msgSet + 1
+              , isFinalPart = False
+              , action = Leave
+              , payload = Just LeaveResponsePayload
+                                     }
+        pure leaveResponse
+    pure $ serialiseMessage sendMessageSize responseMsg
 
 -- | respond to stabilise requests by returning successor and predecessor list
 respondStabilise :: LocalNodeStateSTM -> Set.Set FediChordMessage -> IO (Map.Map Integer BS.ByteString)