Fanout fix (#2105)

Make sure we can fanout if we have a decommit or commit that is in
progress.

Author: v0d1ch

CHANGELOG.md

@@ -14,6 +14,8 @@ changes.
 
 - Add `Environment` to `Greetings` message, enabling clients to access runtime settings.
 
+- Bugfix for incorrect logic around fanning out with decommit/commit in progress
+
 ## [0.22.2] - 2025.06.30
 
 * Fix wrong hydra-script-tx-ids in networks.json

hydra-node/src/Hydra/HeadLogic.hs

@@ -1268,15 +1268,21 @@ onClosedClientFanout closedState =
       { postChainTx =
           FanoutTx
             { utxo
-            , utxoToCommit =
-                -- NOTE: note that logic is flipped in the commit and decommit case here.
-                if toInteger snapshotVersion == max (toInteger version - 1) 0
-                  then utxoToCommit
-                  else Nothing
-            , utxoToDecommit =
-                if toInteger snapshotVersion == max (toInteger version - 1) 0
+            , -- XXX: Perhaps move this check down so it can be more readily
+              -- tested.
+              -- Commit:
+              -- Here we check that to include in the fanout; if the versions
+              -- are the same, the utxoToCommit has not been used on chain yet
+              -- so we disregard, so we must not fan it out.
+              utxoToCommit =
+                if snapshotVersion == version
                   then Nothing
-                  else utxoToDecommit
+                  else utxoToCommit
+            , -- For decommit, if it hasn't happened, we _must_ fan it out.
+              utxoToDecommit =
+                if snapshotVersion == version
+                  then utxoToDecommit
+                  else Nothing
             , headSeed
             , contestationDeadline
             }

hydra-node/test/Hydra/BehaviorSpec.hs

@@ -780,32 +780,27 @@ spec = parallel $ do
 
                   waitUntil [n1, n2] $ DecommitFinalized{headId = testHeadId, distributedUTxO = utxoRef 42}
 
-        it "can close with decommit in flight" $
+        it "can fanout with decommit in flight" $
           shouldRunInSim $ do
             withSimulatedChainAndNetwork $ \chain ->
               withHydraNode aliceSk [bob] chain $ \n1 -> do
                 withHydraNode bobSk [alice] chain $ \n2 -> do
                   openHead2 chain n1 n2
                   let decommitTx = SimpleTx 1 (utxoRef 2) (utxoRef 42)
                   send n2 (Decommit{decommitTx})
+                  -- Close while the decommit is still in flight
                   send n1 Close
-                  -- XXX: Can't assert DecommitFinalized here as it is not
-                  -- emitted because an incorrect simulation of the chain here.
-                  -- The OnDecrementTx only reaches the nodes HeadLogic after
-                  -- the OnCloseTx. This is impossible using a proper chain so
-                  -- we don't assert it here.
-                  -- waitUntil [n1, n2] $ DecommitFinalized{headId = testHeadId, distributedUTxO = utxoRef 42}
                   waitUntil [n1, n2] $ ReadyToFanout{headId = testHeadId}
                   send n1 Fanout
-                  waitUntil [n1, n2] $ HeadIsFinalized{headId = testHeadId, utxo = utxoRefs [1]}
+                  waitUntil [n1, n2] $ HeadIsFinalized{headId = testHeadId, utxo = utxoRefs [1, 42]}
 
-        it "fanout utxo is correct after a decommit" $
+        it "can fanout after a decommit" $
           shouldRunInSim $ do
             withSimulatedChainAndNetwork $ \chain ->
               withHydraNode aliceSk [bob] chain $ \n1 -> do
                 withHydraNode bobSk [alice] chain $ \n2 -> do
                   openHead2 chain n1 n2
-                  let decommitTx = SimpleTx 1 (utxoRef 1) (utxoRef 42)
+                  let decommitTx = SimpleTx 1 (utxoRef 2) (utxoRef 42)
                   send n2 (Decommit{decommitTx})
                   waitUntil [n1, n2] $
                     DecommitApproved
@@ -816,7 +811,7 @@ spec = parallel $ do
                   send n1 Close
                   waitUntil [n1, n2] $ ReadyToFanout{headId = testHeadId}
                   send n1 Fanout
-                  waitUntil [n1, n2] $ HeadIsFinalized{headId = testHeadId, utxo = utxoRefs [2]}
+                  waitUntil [n1, n2] $ HeadIsFinalized{headId = testHeadId, utxo = utxoRefs [1]}
 
         it "can fanout with empty utxo" $
           shouldRunInSim $ do