Remove multicast reqSn step on decrementTx (#1536)

<!-- Describe your change here -->

This PR reduces the number of network messages in the presence of
decrementTx, by avoiding unnecessary snapshot signature round-trips
between peers.
> Removed **multicast reqSn** step **on** *decrementTx from chain*.

Since it is triggered by a chain observation, we expect/assume everyone
will eventually observe it and continue cooperating.
 
---

<!-- Consider each and tick it off one way or the other -->
* [x] CHANGELOG updated or not needed
* [x] Documentation updated or not needed
* [x] Haddocks updated or not needed
* [x] No new TODOs introduced or explained herafter

hydra-node/src/Hydra/HeadLogic.hs

@@ -828,14 +828,13 @@ onOpenNetworkReqDec env ledger ttl openState decommitTx =
 -- __Transition__: 'OpenState' → 'OpenState'
 onOpenChainDecrementTx ::
   IsTx tx =>
-  Environment ->
   OpenState tx ->
   -- | New open state version
   SnapshotVersion ->
   -- | Outputs removed by the decrement
   [TxOutType tx] ->
   Outcome tx
-onOpenChainDecrementTx Environment{party} openState newVersion distributedTxOuts =
+onOpenChainDecrementTx openState newVersion distributedTxOuts =
   -- Spec: if outputs(txω) = 𝑈ω
   case decommitTx of
     Nothing -> Error $ AssertionFailed "decrement observed but no decommit pending"
@@ -845,28 +844,11 @@ onOpenChainDecrementTx Environment{party} openState newVersion distributedTxOuts
           --       v  ← v
           newState DecommitFinalized{newVersion}
             <> cause (ClientEffect $ ServerOutput.DecommitFinalized{headId, decommitTxId = txId tx})
-            -- Spec: if ŝ = ̅S.s ∧ leader(̅S.s + 1) = i
-            --         multicast (reqSn, v, ̅S.s + 1, T̂ , txω )
-            & maybeRequestSnapshot
       | otherwise -> Error $ AssertionFailed "decrement not matching pending decommit"
  where
-  maybeRequestSnapshot outcome =
-    if seenSn == confirmedSn && isLeader parameters party nextSn
-      then
-        outcome
-          <> newState SnapshotRequestDecided{snapshotNumber = nextSn}
-          <> cause (NetworkEffect $ ReqSn newVersion nextSn (txId <$> localTxs) Nothing)
-      else outcome
-
-  OpenState{parameters, coordinatedHeadState, headId} = openState
-
-  CoordinatedHeadState{confirmedSnapshot, localTxs, decommitTx, seenSnapshot} = coordinatedHeadState
-
-  seenSn = seenSnapshotNumber seenSnapshot
-
-  Snapshot{number = confirmedSn} = getSnapshot confirmedSnapshot
+  OpenState{coordinatedHeadState, headId} = openState
 
-  nextSn = confirmedSn + 1
+  CoordinatedHeadState{decommitTx} = coordinatedHeadState
 
 isLeader :: HeadParameters -> Party -> SnapshotNumber -> Bool
 isLeader HeadParameters{parties} p sn =
@@ -1119,7 +1101,7 @@ update env ledger st ev = case (st, ev) of
   (Open openState@OpenState{headId = ourHeadId}, ChainInput Observation{observedTx = OnDecrementTx{headId, newVersion, distributedOutputs}})
     -- TODO: What happens if observed decrement tx get's rolled back?
     | ourHeadId == headId ->
-        onOpenChainDecrementTx env openState newVersion distributedOutputs
+        onOpenChainDecrementTx openState newVersion distributedOutputs
     | otherwise ->
         Error NotOurHead{ourHeadId, otherHeadId = headId}
   -- Closed

hydra-node/test/Hydra/HeadLogicSpec.hs

@@ -241,14 +241,6 @@ spec =
           let reqSn = ReqSn{snapshotVersion = 0, snapshotNumber = 1, transactionIds = [], decommitTx = Just decommitTx'}
           s1 `hasEffect` NetworkEffect reqSn
 
-        it "emits snapshot onDecrementTx with cleared decommitTx" $ do
-          let decommitTx = SimpleTx 1 mempty (utxoRef 1)
-              s0 = inOpenState' threeParties coordinatedHeadState{decommitTx = Just decommitTx}
-              leaderEnv = aliceEnv
-          o <- runHeadLogic leaderEnv ledger s0 $ do
-            step (observeTx OnDecrementTx{headId = testHeadId, newVersion = 1, distributedOutputs = [1]})
-          o `hasEffect` NetworkEffect (ReqSn 1 1 [] Nothing)
-
       describe "Tracks Transaction Ids" $ do
         it "keeps transactions in allTxs given it receives a ReqTx" $ do
           let s0 = inOpenState threeParties

spec/fig_offchain_prot.tex

@@ -134,9 +134,6 @@
 							\If{$\mathsf{outputs}(\tx_{\omega}) = U_{\omega}$}{
 								$\tx_{\omega} \gets \bot$ \;
 								$\hatv \gets v$ \;
-								\If{$\hats = \bar{\mc S}.s \land \hpLdr(\bar{\mc S}.s + 1) = i$}{
-									\Multi{} $(\hpRS,\hatv,\bar{\mc S}.s+1,\hatmT, \tx_{\omega})$ \;
-								}
 							}
 						}