Prevent empty decommits (#1534)

fix #1502 

Add a test that convinces us empty inputs in decommit tx are prevented.

---

<!-- 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

@@ -732,7 +732,6 @@ onOpenNetworkReqDec ::
   tx ->
   Outcome tx
 onOpenNetworkReqDec env ledger ttl openState decommitTx =
-  -- TODO: require outputs(tx) ≠ ∅ to prevent decommit spam? See hydra#1502
   -- Spec: wait txω =⊥ ∧ L̂ ◦ tx ≠ ⊥
   waitOnApplicableDecommit $ \newLocalUTxO -> do
     -- Spec: L̂ ← L̂ ◦ tx \ outputs(tx)

hydra-node/test/Hydra/HeadLogicSpec.hs

@@ -12,11 +12,13 @@ import Hydra.Prelude
 import Test.Hydra.Prelude
 
 import Cardano.Api.UTxO qualified as UTxO
+import Cardano.Ledger.Api (bodyTxL, inputsTxBodyL)
+import Control.Lens ((.~))
 import Data.List qualified as List
 import Data.Map (notMember)
 import Data.Set qualified as Set
 import Hydra.API.ServerOutput (DecommitInvalidReason (..), ServerOutput (..))
-import Hydra.Cardano.Api (genTxIn, mkVkAddress, txOutValue, unSlotNo, pattern TxValidityUpperBound)
+import Hydra.Cardano.Api (fromLedgerTx, genTxIn, mkVkAddress, toLedgerTx, txOutValue, unSlotNo, pattern TxValidityUpperBound)
 import Hydra.Chain (
   ChainEvent (..),
   HeadParameters (..),
@@ -670,6 +672,34 @@ spec =
               } -> null localTxs
           _ -> False
 
+    prop "empty inputs in decommit tx are prevented" $ \tx -> do
+      let ledger = cardanoLedger Fixture.defaultGlobals Fixture.defaultLedgerEnv
+      let st =
+            Open
+              OpenState
+                { parameters = HeadParameters defaultContestationPeriod threeParties
+                , coordinatedHeadState =
+                    CoordinatedHeadState
+                      { localUTxO = mempty
+                      , allTxs = mempty
+                      , localTxs = []
+                      , confirmedSnapshot = InitialSnapshot testHeadId mempty
+                      , seenSnapshot = NoSeenSnapshot
+                      , decommitTx = Nothing
+                      , version = 0
+                      }
+                , chainState = Prelude.error "should not be used"
+                , headId = testHeadId
+                , headSeed = testHeadSeed
+                , currentSlot = ChainSlot 1
+                }
+
+      let tx' = fromLedgerTx (toLedgerTx tx & bodyTxL . inputsTxBodyL .~ mempty)
+      let input = receiveMessage $ ReqDec{transaction = tx'}
+      update bobEnv ledger st input `shouldSatisfy` \case
+        Wait WaitOnNotApplicableDecommitTx{notApplicableReason = DecommitTxInvalid{}} _ -> True
+        _ -> False
+
 -- * Properties
 
 prop_ignoresUnrelatedOnInitTx :: Property