Problems with rollback management

We do not handle rollbacks correctly:
* a client would not now what to do with a rollback event
* we do not handle these events correctly

[#185](#185) is about
dealing appropriately with the rollback.

With [#185](#185)
we will be able to really solve the issues in an appropriate way for
both, the client using a hydra-node and the internals of our system.
In the mean time, we remove the rollback logic from Hydra head so that:
* we do not send rollback events to the client
* we _bet_ that the seen transactions will be seen again in the new fork
  of the chain

TODO:
* add the chainState to the rollback event so that we remove all the rollback
  logic from HeadLogic

hydra-node/src/Hydra/HeadLogic.hs

@@ -970,7 +970,16 @@ onCurrentChainRollback ::
   ChainSlot ->
   Outcome tx
 onCurrentChainRollback currentState slot =
-  NewState (rollback slot currentState) [ClientEffect RolledBack]
+  let atTheTimeState = rollback slot currentState
+   in case atTheTimeState of
+        Idle IdleState{chainState} ->
+          NewState (updateChainState chainState currentState) []
+        Initial InitialState{chainState} ->
+          NewState (updateChainState chainState currentState) []
+        Open OpenState{chainState} ->
+          NewState (updateChainState chainState currentState) []
+        Closed ClosedState{chainState} ->
+          NewState (updateChainState chainState currentState) []
  where
   -- TODO use slot instead of local rollbackSlot argument
   rollback rollbackSlot hs
@@ -984,6 +993,14 @@ onCurrentChainRollback currentState slot =
           rollback rollbackSlot previousRecoverableState
         Closed ClosedState{previousRecoverableState} ->
           rollback rollbackSlot previousRecoverableState
+  updateChainState chainState = \case
+    Idle s@IdleState{} -> Idle s{chainState}
+    Initial s@InitialState{} ->
+      Initial s{chainState}
+    Open s@OpenState{} ->
+      Open s{chainState}
+    Closed s@ClosedState{} ->
+      Closed s{chainState}
 
 -- | The "pure core" of the Hydra node, which handles the 'Event' against a
 -- current 'HeadState'. Resulting new 'HeadState's are retained and 'Effect'

hydra-node/test/Hydra/BehaviorSpec.hs

@@ -455,7 +455,7 @@ spec = parallel $ do
 
     roundtripAndGoldenSpecs (Proxy @(HydraNodeLog SimpleTx))
 
-  describe "rolling back & forward" $ do
+  describe "rolling back & forward does not make the node crash" $ do
     it "does work for rollbacks past init" $
       shouldRunInSim $ do
         withSimulatedChainAndNetwork $ \chain ->
@@ -464,8 +464,9 @@ spec = parallel $ do
             waitUntil [n1] $ HeadIsInitializing testHeadId (fromList [alice])
             -- We expect the Init to be rolled back and forward again
             rollbackAndForward chain 1
-            waitUntil [n1] RolledBack
-            waitUntil [n1] $ HeadIsInitializing testHeadId (fromList [alice])
+            -- We expect the node to still work and let us commit
+            send n1 (Commit (utxoRef 1))
+            waitUntil [n1] $ Committed testHeadId alice (utxoRef 1)
 
     it "does work for rollbacks past open" $
       shouldRunInSim $ do
@@ -479,9 +480,9 @@ spec = parallel $ do
             -- We expect one Commit AND the CollectCom to be rolled back and
             -- forward again
             rollbackAndForward chain 2
-            waitUntil [n1] RolledBack
-            waitUntil [n1] $ Committed testHeadId alice (utxoRef 1)
-            waitUntil [n1] $ HeadIsOpen{headId = testHeadId, utxo = utxoRefs [1]}
+            -- We expect the node to still work and let us post L2 transactions
+            send n1 (NewTx (aValidTx 42))
+            waitUntil [n1] $ TxValid testHeadId (aValidTx 42)
 
 -- | Wait for some output at some node(s) to be produced /eventually/. See
 -- 'waitUntilMatch' for how long it waits.
@@ -543,7 +544,8 @@ data TestHydraNode tx m = TestHydraNode
 data ConnectToChain tx m = ConnectToChain
   { chainComponent :: HydraNode tx m -> m (HydraNode tx m)
   , tickThread :: Async m ()
-  , rollbackAndForward :: Natural -> m ()
+  , -- TODO remove the following if we really don't use it anymore
+    rollbackAndForward :: Natural -> m ()
   }
 
 -- | With-pattern wrapper around 'simulatedChainAndNetwork' which does 'cancel'

hydra-node/test/Hydra/HeadLogicSpec.hs

@@ -48,8 +48,6 @@ import Hydra.Party (Party (..))
 import Hydra.Snapshot (ConfirmedSnapshot (..), Snapshot (..), getSnapshot)
 import Test.Aeson.GenericSpecs (roundtripAndGoldenSpecs)
 import Test.Hydra.Fixture (alice, aliceSk, bob, bobSk, carol, carolSk, cperiod)
-import Test.QuickCheck (forAll)
-import Test.QuickCheck.Monadic (monadicIO, run)
 
 spec :: Spec
 spec = do
@@ -286,12 +284,6 @@ spec = do
             s2 = update bobEnv ledger s1 stepTimePastDeadline
         s2 `hasEffect` ClientEffect (ReadyToFanout testHeadId)
 
-      it "notify user on rollback" $
-        forAll arbitrary $ \s -> monadicIO $ do
-          let rollback = OnChainEvent (Rollback $ ChainSlot 2)
-          let s' = update bobEnv ledger s rollback
-          void $ run $ s' `hasEffect` ClientEffect RolledBack
-
       it "contests when detecting close with old snapshot" $ do
         let snapshot = Snapshot 2 mempty []
             latestConfirmedSnapshot = ConfirmedSnapshot snapshot (aggregate [])

hydra-node/test/Hydra/Model.hs

@@ -192,11 +192,7 @@ instance StateModel WorldState where
           , (1, genAbort)
           ]
       Open{} -> do
-        -- FIXME: Generation of arbitrary NewTx disabled as we don't control
-        -- rollbacks in the MockChain and the hydra-node purges L2 state when
-        -- rolling back "past open".
-        void genNewTx
-        pure $ Error "NewTx disabled because of rollbacks past open"
+        genNewTx
       _ -> fmap Some genSeed
    where
     genCommit pending = do