change NodeCache protection to STM

- putting the NodeCache behind an IORef had been chose because those
  could've been read non-blocking
- the same is true for TVars. The performance characteristics are likely
  worse, but at the advantage of composability within STM monads

src/Hash2Pub/DHTProtocol.hs

@@ -27,6 +27,7 @@ module Hash2Pub.DHTProtocol
 
 import           Control.Concurrent.Async
 import           Control.Concurrent.STM
+import           Control.Concurrent.STM.TVar
 import           Control.Concurrent.STM.TBQueue
 import           Control.Concurrent.STM.TQueue
 import           Control.Exception
@@ -35,7 +36,6 @@ import qualified Data.ByteString                as BS
 import           Data.Either                    (rights)
 import           Data.Foldable                  (foldl', foldr')
 import           Data.Functor.Identity
-import           Data.IORef
 import           Data.IP                        (IPv6, fromHostAddress6,
                                                  toHostAddress6)
 import           Data.List                      (sortBy)
@@ -272,7 +272,7 @@ requestQueryID :: LocalNodeState        -- ^ 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 <- readIORef . nodeCacheRef $ ns
+    firstCacheSnapshot <- readTVarIO . nodeCacheSTM $ ns
     queryIdLookupLoop firstCacheSnapshot ns targetID
 
 -- | like 'requestQueryID, but allows passing of a custom cache, e.g. for joining

src/Hash2Pub/FediChord.hs

@@ -70,7 +70,6 @@ import           Crypto.Hash
 import qualified Data.ByteArray                as BA
 import qualified Data.ByteString               as BS
 import qualified Data.ByteString.UTF8          as BSU
-import           Data.IORef
 import           Data.IP                       (IPv6, fromHostAddress6,
                                                 toHostAddress6)
 import           Data.Typeable                 (Typeable (..), typeOf)
@@ -96,7 +95,7 @@ fediChordInit conf = do
 -- Separated from 'fediChordInit' to be usable in tests.
 nodeStateInit :: FediChordConf -> IO LocalNodeState
 nodeStateInit conf = do
-    cacheRef <- newIORef initCache
+    cacheSTM <- newTVarIO initCache
     q <- atomically newTQueue
     let
         containedState = RemoteNodeState {
@@ -109,7 +108,7 @@ nodeStateInit conf = do
                                             }
         initialState = LocalNodeState {
             nodeState = containedState
-          , nodeCacheRef = cacheRef
+          , nodeCacheSTM = cacheSTM
           , cacheWriteQueue = q
           , successors = []
           , predecessors = []
@@ -172,15 +171,11 @@ fediChordJoin cacheSnapshot nsSTM = do
 -- | 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 ()
-cacheWriter nsSTM = do
-    ns <- readTVarIO nsSTM
-    let writeQueue' = cacheWriteQueue ns
-    forever $ do
-      f <- atomically $ readTQueue writeQueue'
-      let
-          refModifier :: NodeCache -> (NodeCache, ())
-          refModifier nc = (f nc, ())
-      atomicModifyIORef' (nodeCacheRef ns) refModifier
+cacheWriter nsSTM =
+    forever $ atomically $ do
+        ns <- readTVar nsSTM
+        cacheModifier <- readTQueue $ cacheWriteQueue ns
+        modifyTVar' (nodeCacheSTM ns) cacheModifier
 
 -- | Receives UDP packets and passes them to other threads via the given TQueue.
 -- Shall be used as the single receiving thread on the server socket, as multiple

src/Hash2Pub/FediChordTypes.hs

@@ -47,7 +47,6 @@ import           Crypto.Hash
 import qualified Data.ByteArray                as BA
 import qualified Data.ByteString               as BS
 import qualified Data.ByteString.UTF8          as BSU
-import           Data.IORef
 import           Data.IP                       (IPv6, fromHostAddress6,
                                                 toHostAddress6)
 import           Data.Typeable                 (Typeable (..), typeOf)
@@ -127,7 +126,7 @@ data RemoteNodeState = RemoteNodeState
 data LocalNodeState = LocalNodeState
     { nodeState           :: RemoteNodeState
     -- ^ represents common data present both in remote and local node representations
-    , nodeCacheRef        :: IORef NodeCache
+    , nodeCacheSTM        :: TVar NodeCache
     -- ^ EpiChord node cache with expiry times for nodes
     , cacheWriteQueue     :: TQueue (NodeCache -> NodeCache)
     -- ^ cache updates are not written directly to the  'nodeCache' but queued and
@@ -206,7 +205,7 @@ instance NodeState LocalNodeState where
     toRemoteNodeState = nodeState
 
 -- | defining Show instances to be able to print NodeState for debug purposes
-instance Typeable a => Show (IORef a) where
+instance Typeable a => Show (TVar a) where
     show x = show (typeOf x)
 
 instance Typeable a => Show (TQueue a) where

test/FediChordSpec.hs

@@ -4,7 +4,6 @@ module FediChordSpec where
 import           Control.Exception
 import           Data.ASN1.Parse       (runParseASN1)
 import qualified Data.ByteString       as BS
-import           Data.IORef
 import qualified Data.Map.Strict       as Map
 import           Data.Maybe            (fromJust)
 import qualified Data.Set              as Set