Make key lookups fail after request exhaustion instead of providing default

Returning the own node as a default does not make sense in all contexts:
Especially for bootstrap joining this can be harmful, so signalling
instead that the lookup failed makes distinguishing on a case by case
basis possible.

Also contributes to #57

src/Hash2Pub/DHTProtocol.hs

@@ -49,6 +49,8 @@ import           Control.Concurrent.STM.TQueue
 import           Control.Concurrent.STM.TVar
 import           Control.Exception
 import           Control.Monad                  (foldM, forM, forM_, void, when)
+import Control.Monad.IO.Class (MonadIO(..))
+import Control.Monad.Except (MonadError(..), runExceptT)
 import qualified Data.ByteString                as BS
 import           Data.Either                    (rights)
 import           Data.Foldable                  (foldl', foldr')
@@ -533,9 +535,10 @@ requestJoin toJoinOn ownStateSTM = do
 
 
 -- | Send a 'QueryID' 'Request' for getting the node that handles a certain key ID.
-requestQueryID :: LocalNodeState s        -- ^ NodeState of the querying node
+requestQueryID :: (MonadIO m, MonadError String m)
+               => LocalNodeState s        -- ^ NodeState of the querying node
                -> NodeID                -- ^ target key ID to look up
-               -> IO RemoteNodeState    -- ^ the node responsible for handling that key
+               -> m RemoteNodeState    -- ^ the node responsible for handling that key
 -- 1. do a local lookup for the l closest nodes
 -- 2. create l sockets
 -- 3. send a message async concurrently to all l nodes
@@ -543,23 +546,23 @@ requestQueryID :: LocalNodeState s        -- ^ NodeState of the querying node
 -- 5. repeat until FOUND (problem: new entries not necessarily already in cache, explicitly compare with closer results)
 -- TODO: deal with lookup failures
 requestQueryID ns targetID = do
-    firstCacheSnapshot <- readTVarIO . nodeCacheSTM $ ns
+    firstCacheSnapshot <- liftIO . readTVarIO . nodeCacheSTM $ ns
     -- TODO: make maxAttempts configurable
     queryIdLookupLoop firstCacheSnapshot ns 50 targetID
 
 -- | like 'requestQueryID, but allows passing of a custom cache, e.g. for joining
-queryIdLookupLoop :: NodeCache -> LocalNodeState s -> Int -> NodeID -> IO RemoteNodeState
+queryIdLookupLoop :: (MonadIO m, MonadError String m) => NodeCache -> LocalNodeState s -> Int -> NodeID -> m RemoteNodeState
 -- return node itself as default fallback value against infinite recursion.
 -- TODO: consider using an Either instead of a default value
-queryIdLookupLoop _ ns 0 _ = pure $ toRemoteNodeState ns
+queryIdLookupLoop _ ns 0 _ = throwError "exhausted maximum lookup attempts"
 queryIdLookupLoop cacheSnapshot ns maxAttempts targetID = do
     let localResult = queryLocalCache ns cacheSnapshot (lNumBestNodes ns) targetID
     -- FOUND can only be returned if targetID is owned by local node
     case localResult of
       FOUND thisNode -> pure thisNode
       FORWARD nodeSet -> do
-          responseEntries <- sendQueryIdMessages targetID ns Nothing (remoteNode <$> Set.elems nodeSet)
+          responseEntries <- liftIO $ sendQueryIdMessages targetID ns Nothing (remoteNode <$> Set.elems nodeSet)
-          now <- getPOSIXTime
+          now <- liftIO getPOSIXTime
           -- check for a FOUND and return it
           case responseEntries of
             FOUND foundNode -> pure foundNode

src/Hash2Pub/FediChord.hs

@@ -166,6 +166,7 @@ fediChordBootstrapJoin nsSTM bootstrapNode = do
         currentlyResponsible <- liftEither lookupResp
         liftIO . putStrLn $ "Trying to join on " <> show (getNid currentlyResponsible)
         -- 2. then send a join to the currently responsible node
+        liftIO $ putStrLn "send a bootstrap Join"
         joinResult <- liftIO $ requestJoin currentlyResponsible nsSTM
         liftEither joinResult
 
@@ -244,26 +245,24 @@ bootstrapQueryId nsSTM (bootstrapHost, bootstrapPort) targetID = do
                            Just (FORWARD resultset) -> foldr' (addCacheEntryPure now) cacheAcc resultset
                               )
                            initCache resp
-               currentlyResponsible <- queryIdLookupLoop bootstrapCache ns 50 $ getNid ns
+               currentlyResponsible <- runExceptT $ queryIdLookupLoop bootstrapCache ns 50 $ getNid ns
-               pure $ Right currentlyResponsible
+               pure currentlyResponsible
 
 
 -- | join a node to the DHT using the global node cache
 -- node's position.
-fediChordVserverJoin :: Service s (RealNodeSTM s)
+fediChordVserverJoin :: (MonadError String m, MonadIO m, Service s (RealNodeSTM s))
                      => LocalNodeStateSTM s                    -- ^ the local 'NodeState'
-                     -> IO (Either String (LocalNodeStateSTM s))  -- ^ the joined 'NodeState' after a
+                     -> m (LocalNodeStateSTM s)  -- ^ the joined 'NodeState' after a
                                                     -- successful join, otherwise an error message
 fediChordVserverJoin nsSTM = do
-    ns <- readTVarIO nsSTM
+    ns <- liftIO $ readTVarIO nsSTM
     -- 1. get routed to the currently responsible node
     currentlyResponsible <- requestQueryID ns $ getNid ns
-    putStrLn $ "Trying to join on " <> show (getNid currentlyResponsible)
+    liftIO . putStrLn $ "Trying to join on " <> show (getNid currentlyResponsible)
     -- 2. then send a join to the currently responsible node
-    joinResult <- requestJoin currentlyResponsible nsSTM
+    joinResult <- liftIO $ requestJoin currentlyResponsible nsSTM
-    case joinResult of
+    liftEither joinResult
-      Left err       -> pure . Left $ "Error joining on " <> err
-      Right joinedNS -> pure . Right $ joinedNS
 
 fediChordVserverLeave :: (MonadError String m, MonadIO m, Service s (RealNodeSTM s)) => LocalNodeState s -> m ()
 fediChordVserverLeave ns = do
@@ -323,7 +322,7 @@ joinOnNewEntriesThread nsSTM = loop
               pure ()
           -- otherwise try joining
           FORWARD _ -> do
-              joinResult <- fediChordVserverJoin nsSTM
+              joinResult <- runExceptT $ fediChordVserverJoin nsSTM
               either
                 -- on join failure, sleep and retry
                 -- TODO: make delay configurable
@@ -504,18 +503,26 @@ stabiliseThread nsSTM = forever $ do
     -- try looking up additional neighbours if list too short
     forM_ [(length $ predecessors updatedNs)..(kNeighbours updatedNs)] (\_ -> do
         ns' <- readTVarIO nsSTM
-        nextEntry <- requestQueryID ns' $ pred . getNid $ lastDef (toRemoteNodeState ns') (predecessors ns')
+        nextEntry <- runExceptT . requestQueryID ns' $ pred . getNid $ lastDef (toRemoteNodeState ns') (predecessors ns')
-        atomically $ do
+        either 
+            (const $ pure ())
+            (\entry -> atomically $ do
                 latestNs <- readTVar nsSTM
-            writeTVar nsSTM $ addPredecessors [nextEntry] latestNs
+                writeTVar nsSTM $ addPredecessors [entry] latestNs
+            )
+            nextEntry
                                                                        )
 
     forM_ [(length $ successors updatedNs)..(kNeighbours updatedNs)] (\_ -> do
         ns' <- readTVarIO nsSTM
-        nextEntry <- requestQueryID ns' $ succ . getNid $ lastDef (toRemoteNodeState ns') (successors ns')
+        nextEntry <- runExceptT . requestQueryID ns' $ succ . getNid $ lastDef (toRemoteNodeState ns') (successors ns')
-        atomically $ do
+        either
+            (const $ pure ())
+            (\entry -> atomically $ do
                 latestNs <- readTVar nsSTM
-            writeTVar nsSTM $ addSuccessors [nextEntry] latestNs
+                writeTVar nsSTM $ addSuccessors [entry] latestNs
+            )
+            nextEntry
                                                                      )
 
     newNs <- readTVarIO nsSTM
@@ -638,7 +645,7 @@ requestMapPurge :: MVar RequestMap -> IO ()
 requestMapPurge mapVar = forever $ do
     rMapState <- takeMVar mapVar
     now <- getPOSIXTime
-    putMVar mapVar $ Map.filter (\entry@(RequestMapEntry _ _ ts)  ->
+    putMVar mapVar $ Map.filter (\(RequestMapEntry _ _ ts)  ->
         now - ts < responsePurgeAge
                                 ) rMapState
     threadDelay $ round responsePurgeAge * 2 * 10^6
@@ -757,7 +764,7 @@ getKeyResponsibility nodeSTM lookupKey = do
 -- new entry.
 -- If no vserver is active in the DHT, 'Nothing' is returned.
 updateLookupCache :: RealNodeSTM s -> NodeID -> IO (Maybe (String, PortNumber))
-updateLookupCache nodeSTM lookupKey = do
+updateLookupCache nodeSTM keyToLookup = do
     (node, lookupSource) <- atomically $ do
         node <- readTVar nodeSTM
         let firstVs = headMay (vservers node)
@@ -767,18 +774,25 @@ updateLookupCache nodeSTM lookupKey = do
         pure (node, lookupSource)
     maybe (do
         -- if no local node available, delete cache entry and return Nothing
-        atomically $ modifyTVar' (lookupCacheSTM node) $ Map.delete lookupKey
+        atomically $ modifyTVar' (lookupCacheSTM node) $ Map.delete keyToLookup
         pure Nothing
           )
           (\n -> do
         -- start a lookup from the node, update the cache with the lookup result and return it
-        newResponsible <- requestQueryID n lookupKey
+        -- TODO: better retry management, because having no vserver joined yet should
-        let newEntry = (getDomain newResponsible, getServicePort newResponsible)
+        -- be treated differently than other reasons for not getting a result.
+        newResponsible <- runExceptT $ requestQueryID n keyToLookup
+        either 
+            (const $ pure Nothing)
+            (\result -> do
+                let newEntry = (getDomain result, getServicePort result)
                 now <- getPOSIXTime
                 -- atomic update against lost updates
                 atomically $ modifyTVar' (lookupCacheSTM node) $
-            Map.insert lookupKey (CacheEntry False newEntry now)
+                    Map.insert keyToLookup (CacheEntry False newEntry now)
                 pure $ Just newEntry
+            )
+            newResponsible
            ) lookupSource