implement join-retry on new cache entries

closes #42

app/Main.hs

@@ -36,8 +36,9 @@ main = do
 
         putStrLn $ err <> " Error joining, start listening for incoming requests anyways"
         print =<< readTVarIO thisNode
+        -- launch thread attempting to join on new cache entries
+        _ <- forkIO $ joinOnNewEntriesThread thisNode
         wait =<< async (fediMainThreads serverSock thisNode)
-        -- TODO: periodic retry
            )
            (\joinedNS -> do
         -- launch main eventloop with successfully joined state

src/Hash2Pub/FediChord.hs

@@ -46,6 +46,7 @@ module Hash2Pub.FediChord (
   , mkSendSocket
   , resolve
   , cacheWriter
+  , joinOnNewEntriesThread
                            ) where
 
 import           Control.Applicative           ((<|>))
@@ -166,6 +167,7 @@ fediChordJoin cacheSnapshot nsSTM = do
     -- get routed to the currently responsible node, based on the response
     -- from the bootstrapping node
     currentlyResponsible <- queryIdLookupLoop cacheSnapshot ns 50 $ getNid ns
+    putStrLn $ "Trying to join on " <> show (getNid currentlyResponsible)
     -- 2. then send a join to the currently responsible node
     joinResult <- requestJoin currentlyResponsible nsSTM
     case joinResult of
@@ -173,6 +175,36 @@ fediChordJoin cacheSnapshot nsSTM = do
       Right joinedNS -> pure . Right $ joinedNS
 
 
+-- | Wait for new cache entries to appear and then try joining on them.
+-- Exits after successful joining.
+joinOnNewEntriesThread :: LocalNodeStateSTM -> IO ()
+joinOnNewEntriesThread nsSTM = loop
+  where
+    loop = do
+        nsSnap <- readTVarIO nsSTM
+        (lookupResult, cache) <- atomically $ do
+            cache <- readTVar $ nodeCacheSTM nsSnap
+            case queryLocalCache nsSnap cache 1 (getNid nsSnap) of
+              -- empty cache, block until cache changes and then retry
+              (FORWARD s) | Set.null s -> retry
+              result -> pure (result, cache)
+        case lookupResult of
+          -- already joined
+          FOUND _ -> do
+              print =<< readTVarIO nsSTM
+              pure ()
+          -- otherwise try joining
+          FORWARD _ -> do
+              joinResult <- fediChordJoin cache  nsSTM
+              either
+                -- on join failure, sleep and retry
+                -- TODO: make delay configurable
+                (const $ threadDelay (30 * 10^6) >> loop)
+                (const $ pure ()) 
+                joinResult
+    emptyset = Set.empty    -- because pattern matches don't accept qualified names
+
+
 -- | cache updater thread that waits for incoming NodeCache update instructions on
 -- the node's cacheWriteQueue and then modifies the NodeCache as the single writer.
 cacheWriter :: LocalNodeStateSTM -> IO ()