start restructuring joinOnNewEntries flow

- overview comment on possible flow
- cache query
- doesn't compile yet

src/Hash2Pub/FediChord.hs

@@ -127,9 +127,16 @@ fediChordInit initConf serviceRunner = do
         either (\err -> do
             -- handle unsuccessful join
             putStrLn $ err <> " Error joining, start listening for incoming requests anyways"
+            -- add an unjoined placeholder vserver to be able to listen for
+            -- incoming request
+            placeholderVS <- nodeStateInit realNodeSTM 0
+            placeholderVSSTM <- newTVarIO placeholderVS
+            atomically . modifyTVar' realNodeSTM $
+                addVserver (getNid placeholderVS, placeholderVSSTM)
+            -- TODO: on which bootstrap node vserver to join? (#77)
             -- launch thread attempting to join on new cache entries
             -- TODO: adjust joinOnNewEntriesThread to k-choices
-            --_ <- forkIO $ joinOnNewEntriesThread firstVSSTM
+            _ <- forkIO $ joinOnNewEntriesThread realNodeSTM
             async (fediMainThreads serverSock realNodeSTM)
                )
                (\_ -> do
@@ -370,9 +377,8 @@ tryBootstrapJoining nodeSTM = do
         joinResult <- tryJoining bss (fediChordBootstrapJoin firstVSSTM)
         either
             (pure . Left)
-            (\_ -> fmap Right . atomically . modifyTVar' nodeSTM $ (\rn -> rn
-                { vservers = HMap.insert (getNid firstVS) firstVSSTM (vservers rn) }
-                                                      )
+            (\_ -> fmap Right . atomically . modifyTVar' nodeSTM $
+                addVserver (getNid firstVS, firstVSSTM)
             )
             (joinResult :: Either String ())
 
@@ -473,24 +479,44 @@ fediChordVserverLeave ns = do
 
 -- | Wait for new cache entries to appear and then try joining on them.
 -- Exits after successful joining.
-joinOnNewEntriesThread :: Service s (RealNodeSTM s) => LocalNodeStateSTM s -> IO ()
-joinOnNewEntriesThread nsSTM = loop
+joinOnNewEntriesThread :: Service s (RealNodeSTM s) => RealNodeSTM s -> IO ()
+joinOnNewEntriesThread nodeSTM = loop
   where
+    -- situation 1: not joined yet -> vservers == empty
+    -- problem: empty vservers means not responding to incoming requests, so
+    -- another node cannot join on us an we not on them (as they're still
+    -- unjoined as well)
+    -- solution: on failure still join a dummy node, also add it as vserver
+    -- problem: once another node has joined on the dummy vserver, we shouldn't
+    -- just delete it again as it now relies on it as a neighbour
+    -- => either trigger a kChoicesNodeJoin with the flag that **not** at least one
+    -- single node needs to be joined (read vservers initially), or do an accelerated
+    -- periodic rebalance
+    -- TODO: document this approach in the docs
     loop = do
-        nsSnap <- readTVarIO nsSTM
-        (lookupResult, parentNode) <- atomically $ do
-            cache <- readTVar $ nodeCacheSTM nsSnap
-            parentNode <- readTVar $ parentRealNode 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, parentNode)
+        lookupResult <- atomically $ do
+            nodeSnap <- readTVar nodeSTM
+            case headMay (HMap.toList $ vservers nodeSnap) of
+              Nothing -> retry
+              Just vsSTM -> do
+                -- take any active vserver as heuristic for whether this node has
+                -- successfully joined:
+                -- If the node hasn't joined yet, only a single placeholder node
+                -- is active…
+                firstVS <- readTVar vsSTM
+                cache <- readTVar $ globalNodeCacheSTM nodeSnap
+                case queryLocalCache firstVS cache 1 (getNid firstVS) of
+                  -- …which, having no neighbours, returns an empty forward list
+                  -- -> block until cache changes and then retry
+                  (FORWARD s) | Set.null s -> retry
+                  result                   -> pure result
         case lookupResult of
           -- already joined
           FOUND _ ->
               pure ()
           -- otherwise try joining
           FORWARD _ -> do
+              -- do normal join, but without bootstrap nodes
               joinResult <- runExceptT $ fediChordVserverJoin nsSTM
               either
                 -- on join failure, sleep and retry

src/Hash2Pub/FediChordTypes.hs

@@ -22,6 +22,7 @@ module Hash2Pub.FediChordTypes
   , LoadStats (..)
   , emptyLoadStats
   , remainingLoadTarget
+  , addVserver
   , SegmentLoadStats (..)
   , setSuccessors
   , setPredecessors
@@ -170,6 +171,10 @@ data RealNode s = RealNode
     , nodeService           :: s (RealNodeSTM s)
     }
 
+-- | insert a new vserver mapping into a node
+addVserver :: (NodeID, LocalNodeStateSTM s) -> RealNode s -> RealNode s
+addVserver (key, nstate) node = node
+    { vservers = HMap.insert key nstate (vservers node) }
 
 type VSMap s = HashMap NodeID (LocalNodeStateSTM s)
 type RealNodeSTM s = TVar (RealNode s)