Implement economic adjustments for Conflicts attribute

src/Neo/Ledger/Blockchain.cs

@@ -206,6 +206,8 @@ private void OnFillMemoryPool(IEnumerable<Transaction> transactions)
             {
                 if (NativeContract.Ledger.ContainsTransaction(snapshot, tx.Hash))
                     continue;
+                if (NativeContract.Ledger.ContainsConflictHash(snapshot, tx.Hash))
+                    continue;
                 // First remove the tx if it is unverified in the pool.
                 system.MemPool.TryRemoveUnVerified(tx.Hash, out _);
                 // Add to the memory pool
@@ -337,6 +339,7 @@ private VerifyResult OnNewExtensiblePayload(ExtensiblePayload payload)
         private VerifyResult OnNewTransaction(Transaction transaction)
         {
             if (system.ContainsTransaction(transaction.Hash)) return VerifyResult.AlreadyExists;
+            if (system.ContainsConflictHash(transaction.Hash)) return VerifyResult.HasConflicts;
             return system.MemPool.TryAdd(transaction, system.StoreView);
         }
 
@@ -391,8 +394,9 @@ private void OnTransaction(Transaction tx)
         {
             if (system.ContainsTransaction(tx.Hash))
                 SendRelayResult(tx, VerifyResult.AlreadyExists);
-            else
-                system.TxRouter.Forward(new TransactionRouter.Preverify(tx, true));
+            else if (system.ContainsConflictHash(tx.Hash))
+                SendRelayResult(tx, VerifyResult.HasConflicts);
+            else system.TxRouter.Forward(new TransactionRouter.Preverify(tx, true));
         }
 
         private void Persist(Block block)

src/Neo/Ledger/MemoryPool.cs

@@ -55,6 +55,10 @@ public class MemoryPool : IReadOnlyCollection<Transaction>
         /// </summary>
         private readonly Dictionary<UInt256, PoolItem> _unsortedTransactions = new();
         /// <summary>
+        /// Store transaction hashes that conflict with verified mempooled transactions.
+        /// </summary>
+        private readonly Dictionary<UInt256, List<UInt256>> _conflicts = new();
+        /// <summary>
         /// Stores the verified sorted transactions currently in the pool.
         /// </summary>
         private readonly SortedSet<PoolItem> _sortedTransactions = new();
@@ -275,7 +279,8 @@ private PoolItem GetLowestFeeTransaction(out Dictionary<UInt256, PoolItem> unsor
             }
         }
 
-        // Note: this must only be called from a single thread (the Blockchain actor)
+        // Note: this must only be called from a single thread (the Blockchain actor) and
+        // doesn't take into account conflicting transactions.
         internal bool CanTransactionFitInPool(Transaction tx)
         {
             if (Count < Capacity) return true;
@@ -293,23 +298,39 @@ internal VerifyResult TryAdd(Transaction tx, DataCache snapshot)
             _txRwLock.EnterWriteLock();
             try
             {
-                VerifyResult result = tx.VerifyStateDependent(_system.Settings, snapshot, VerificationContext);
+                if (!CheckConflicts(tx, out List<PoolItem> conflictsToBeRemoved)) return VerifyResult.HasConflicts;
+                VerifyResult result = tx.VerifyStateDependent(_system.Settings, snapshot, VerificationContext, conflictsToBeRemoved.Select(c => c.Tx));
                 if (result != VerifyResult.Succeed) return result;
 
                 _unsortedTransactions.Add(tx.Hash, poolItem);
                 VerificationContext.AddTransaction(tx);
                 _sortedTransactions.Add(poolItem);
+                foreach (var conflict in conflictsToBeRemoved)
+                {
+                    if (TryRemoveVerified(conflict.Tx.Hash, out var _))
+                        VerificationContext.RemoveTransaction(conflict.Tx);
+                }
+                removedTransactions = conflictsToBeRemoved.Select(itm => itm.Tx).ToList();
+                foreach (var attr in tx.GetAttributes<Conflicts>())
+                {
+                    if (!_conflicts.TryGetValue(attr.Hash, out var pooled))
+                    {
+                        pooled = new List<UInt256>();
+                    }
+                    pooled.Add(tx.Hash);
+                    _conflicts.AddOrSet(attr.Hash, pooled);
+                }
 
                 if (Count > Capacity)
-                    removedTransactions = RemoveOverCapacity();
+                    removedTransactions.AddRange(RemoveOverCapacity());
             }
             finally
             {
                 _txRwLock.ExitWriteLock();
             }
 
             TransactionAdded?.Invoke(this, poolItem.Tx);
-            if (removedTransactions != null)
+            if (removedTransactions.Count() > 0)
                 TransactionRemoved?.Invoke(this, new()
                 {
                     Transactions = removedTransactions,
@@ -320,6 +341,49 @@ internal VerifyResult TryAdd(Transaction tx, DataCache snapshot)
             return VerifyResult.Succeed;
         }
 
+        /// <summary>
+        /// Checks whether there is no mismatch in Conflicts attributes between the current transaction
+        /// and mempooled unsorted transactions. If true, then these unsorted transactions will be added
+        /// into conflictsList.
+        /// </summary>
+        /// <param name="tx">The <see cref="Transaction"/>current transaction needs to be checked.</param>
+        /// <param name="conflictsList">The list of conflicting verified transactions that should be removed from the pool if tx fits the pool.</param>
+        /// <returns>True if transaction fits the pool, otherwise false.</returns>
+        private bool CheckConflicts(Transaction tx, out List<PoolItem> conflictsList)
+        {
+            conflictsList = new();
+            long conflictsFeeSum = 0;
+            // Step 1: check if `tx` was in Conflicts attributes of unsorted transactions.
+            if (_conflicts.TryGetValue(tx.Hash, out var conflicting))
+            {
+                foreach (var hash in conflicting)
+                {
+                    var unsortedTx = _unsortedTransactions[hash];
+                    if (unsortedTx.Tx.Signers.Select(s => s.Account).Contains(tx.Sender))
+                        conflictsFeeSum += unsortedTx.Tx.NetworkFee;
+                    conflictsList.Add(unsortedTx);
+                }
+            }
+            // Step 2: check if unsorted transactions were in `tx`'s Conflicts attributes.
+            foreach (var hash in tx.GetAttributes<Conflicts>().Select(p => p.Hash))
+            {
+                if (_unsortedTransactions.TryGetValue(hash, out PoolItem unsortedTx))
+                {
+                    if (!tx.Signers.Select(p => p.Account).Contains(unsortedTx.Tx.Sender)) return false;
+                    conflictsFeeSum += unsortedTx.Tx.NetworkFee;
+                    conflictsList.Add(unsortedTx);
+                }
+            }
+            // Network fee of tx have to be larger than the sum of conflicting txs network fees.
+            if (conflictsFeeSum != 0 && conflictsFeeSum >= tx.NetworkFee)
+                return false;
+
+            // Step 3: take into account sender's conflicting transactions while balance check,
+            // this will be done in VerifyStateDependant.
+
+            return true;
+        }
+
         private List<Transaction> RemoveOverCapacity()
         {
             List<Transaction> removedTransactions = new();
@@ -332,7 +396,10 @@ private List<Transaction> RemoveOverCapacity()
                 removedTransactions.Add(minItem.Tx);
 
                 if (ReferenceEquals(sortedPool, _sortedTransactions))
+                {
+                    RemoveConflictsOfVerified(minItem);
                     VerificationContext.RemoveTransaction(minItem.Tx);
+                }
             } while (Count > Capacity);
 
             return removedTransactions;
@@ -347,9 +414,27 @@ private bool TryRemoveVerified(UInt256 hash, out PoolItem item)
             _unsortedTransactions.Remove(hash);
             _sortedTransactions.Remove(item);
 
+            RemoveConflictsOfVerified(item);
+
             return true;
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void RemoveConflictsOfVerified(PoolItem item)
+        {
+            foreach (var h in item.Tx.GetAttributes<Conflicts>().Select(attr => attr.Hash))
+            {
+                if (_conflicts.TryGetValue(h, out List<UInt256> conflicts))
+                {
+                    conflicts.Remove(item.Tx.Hash);
+                    if (conflicts.Count() == 0)
+                    {
+                        _conflicts.Remove(h);
+                    }
+                }
+            }
+        }
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal bool TryRemoveUnVerified(UInt256 hash, out PoolItem item)
         {
@@ -374,28 +459,60 @@ internal void InvalidateVerifiedTransactions()
             _unsortedTransactions.Clear();
             VerificationContext = new TransactionVerificationContext();
             _sortedTransactions.Clear();
+            _conflicts.Clear();
         }
 
         // Note: this must only be called from a single thread (the Blockchain actor)
         internal void UpdatePoolForBlockPersisted(Block block, DataCache snapshot)
         {
+            var conflictingItems = new List<Transaction>();
             _txRwLock.EnterWriteLock();
             try
             {
+                HashSet<UInt256> conflicts = new HashSet<UInt256>();
                 // First remove the transactions verified in the block.
+                // No need to modify VerificationContext as it will be reset afterwards.
                 foreach (Transaction tx in block.Transactions)
                 {
-                    if (TryRemoveVerified(tx.Hash, out _)) continue;
-                    TryRemoveUnVerified(tx.Hash, out _);
+                    if (!TryRemoveVerified(tx.Hash, out _)) TryRemoveUnVerified(tx.Hash, out _);
+                    foreach (var h in tx.GetAttributes<Conflicts>().Select(a => a.Hash))
+                    {
+                        conflicts.Add(h);
+                    }
+                }
+
+                // Then remove the transactions conflicting with the accepted ones.
+                // No need to modify VerificationContext as it will be reset afterwards.
+                var persisted = block.Transactions.Select(t => t.Hash);
+                var stale = new List<UInt256>();
+                foreach (var item in _sortedTransactions)
+                {
+                    if (conflicts.Contains(item.Tx.Hash) || item.Tx.GetAttributes<Conflicts>().Select(a => a.Hash).Intersect(persisted).Any())
+                    {
+                        stale.Add(item.Tx.Hash);
+                        conflictingItems.Add(item.Tx);
+                    }
+                }
+                foreach (var h in stale)
+                {
+                    if (!TryRemoveVerified(h, out _)) TryRemoveUnVerified(h, out _);
                 }
 
-                // Add all the previously verified transactions back to the unverified transactions
+                // Add all the previously verified transactions back to the unverified transactions and clear mempool conflicts list.
                 InvalidateVerifiedTransactions();
             }
             finally
             {
                 _txRwLock.ExitWriteLock();
             }
+            if (conflictingItems.Count() > 0)
+            {
+                TransactionRemoved?.Invoke(this, new()
+                {
+                    Transactions = conflictingItems.ToArray(),
+                    Reason = TransactionRemovalReason.Conflict,
+                });
+            }
 
             // If we know about headers of future blocks, no point in verifying transactions from the unverified tx pool
             // until we get caught up.
@@ -431,10 +548,31 @@ private int ReverifyTransactions(SortedSet<PoolItem> verifiedSortedTxPool,
                 // Since unverifiedSortedTxPool is ordered in an ascending manner, we take from the end.
                 foreach (PoolItem item in unverifiedSortedTxPool.Reverse().Take(count))
                 {
-                    if (item.Tx.VerifyStateDependent(_system.Settings, snapshot, VerificationContext) == VerifyResult.Succeed)
+                    if (CheckConflicts(item.Tx, out List<PoolItem> conflictsToBeRemoved) &&
+                        item.Tx.VerifyStateDependent(_system.Settings, snapshot, VerificationContext, conflictsToBeRemoved.Select(c => c.Tx)) == VerifyResult.Succeed)
                     {
                         reverifiedItems.Add(item);
-                        VerificationContext.AddTransaction(item.Tx);
+                        if (_unsortedTransactions.TryAdd(item.Tx.Hash, item))
+                        {
+                            verifiedSortedTxPool.Add(item);
+                            foreach (var attr in item.Tx.GetAttributes<Conflicts>())
+                            {
+                                if (!_conflicts.TryGetValue(attr.Hash, out var pooled))
+                                {
+                                    pooled = new List<UInt256>();
+                                }
+                                pooled.Add(item.Tx.Hash);
+                                _conflicts.AddOrSet(attr.Hash, pooled);
+                            }
+                            VerificationContext.AddTransaction(item.Tx);
+                            foreach (var conflict in conflictsToBeRemoved)
+                            {
+                                if (TryRemoveVerified(conflict.Tx.Hash, out var _))
+                                    VerificationContext.RemoveTransaction(conflict.Tx);
+                                invalidItems.Add(conflict);
+                            }
+
+                        }
                     }
                     else // Transaction no longer valid -- it will be removed from unverifiedTxPool.
                         invalidItems.Add(item);
@@ -450,18 +588,14 @@ private int ReverifyTransactions(SortedSet<PoolItem> verifiedSortedTxPool,
                 var rebroadcastCutOffTime = TimeProvider.Current.UtcNow.AddMilliseconds(-_system.Settings.MillisecondsPerBlock * blocksTillRebroadcast);
                 foreach (PoolItem item in reverifiedItems)
                 {
-                    if (_unsortedTransactions.TryAdd(item.Tx.Hash, item))
+                    if (_unsortedTransactions.ContainsKey(item.Tx.Hash))
                     {
-                        verifiedSortedTxPool.Add(item);
-
                         if (item.LastBroadcastTimestamp < rebroadcastCutOffTime)
                         {
                             _system.LocalNode.Tell(new LocalNode.RelayDirectly { Inventory = item.Tx }, _system.Blockchain);
                             item.LastBroadcastTimestamp = TimeProvider.Current.UtcNow;
                         }
                     }
-                    else
-                        VerificationContext.RemoveTransaction(item.Tx);
 
                     _unverifiedTransactions.Remove(item.Tx.Hash);
                     unverifiedSortedTxPool.Remove(item);

src/Neo/Ledger/TransactionRemovalReason.cs

@@ -16,13 +16,18 @@ namespace Neo.Ledger
     public enum TransactionRemovalReason : byte
     {
         /// <summary>
-        /// The transaction was ejected since it was the lowest priority transaction and the memory pool capacity was exceeded.
+        /// The transaction was rejected since it was the lowest priority transaction and the memory pool capacity was exceeded.
         /// </summary>
         CapacityExceeded,
 
         /// <summary>
-        /// The transaction was ejected due to failing re-validation after a block was persisted.
+        /// The transaction was rejected due to failing re-validation after a block was persisted.
         /// </summary>
         NoLongerValid,
+
+        /// <summary>
+        /// The transaction was rejected due to conflict with higher priority transactions with Conflicts attribute.
+        /// </summary>
+        Conflict,
     }
 }

src/Neo/Ledger/TransactionVerificationContext.cs

@@ -12,6 +12,7 @@
 using Neo.Persistence;
 using Neo.SmartContract.Native;
 using System.Collections.Generic;
+using System.Linq;
 using System.Numerics;
 
 namespace Neo.Ledger
@@ -50,15 +51,18 @@ public void AddTransaction(Transaction tx)
         /// Determine whether the specified <see cref="Transaction"/> conflicts with other transactions.
         /// </summary>
         /// <param name="tx">The specified <see cref="Transaction"/>.</param>
+        /// <param name="conflictingTxs">The list of <see cref="Transaction"/> that conflicts with the specified one and are to be removed from the pool.</param>
         /// <param name="snapshot">The snapshot used to verify the <see cref="Transaction"/>.</param>
         /// <returns><see langword="true"/> if the <see cref="Transaction"/> passes the check; otherwise, <see langword="false"/>.</returns>
-        public bool CheckTransaction(Transaction tx, DataCache snapshot)
+        public bool CheckTransaction(Transaction tx, IEnumerable<Transaction> conflictingTxs, DataCache snapshot)
         {
             BigInteger balance = NativeContract.GAS.BalanceOf(snapshot, tx.Sender);
             senderFee.TryGetValue(tx.Sender, out var totalSenderFeeFromPool);
 
-            BigInteger fee = tx.SystemFee + tx.NetworkFee + totalSenderFeeFromPool;
-            if (balance < fee) return false;
+            BigInteger expectedFee = tx.SystemFee + tx.NetworkFee + totalSenderFeeFromPool;
+            foreach (var conflictTx in conflictingTxs.Where(c => c.Sender.Equals(tx.Sender)))
+                expectedFee -= (conflictTx.NetworkFee + conflictTx.SystemFee);
+            if (balance < expectedFee) return false;
 
             var oracle = tx.GetAttribute<OracleResponse>();
             if (oracle != null && oracleResponses.ContainsKey(oracle.Id))

src/Neo/Ledger/VerifyResult.cs

@@ -77,6 +77,11 @@ public enum VerifyResult : byte
         /// </summary>
         PolicyFail,
 
+        /// <summary>
+        /// Indicates that the <see cref="Transaction"/> failed to verify because it conflicts with on-chain or mempooled transactions.
+        /// </summary>
+        HasConflicts,
+
         /// <summary>
         /// Indicates that the <see cref="IInventory"/> failed to verify due to other reasons.
         /// </summary>

src/Neo/NeoSystem.cs

@@ -278,5 +278,15 @@ public bool ContainsTransaction(UInt256 hash)
             if (MemPool.ContainsKey(hash)) return true;
             return NativeContract.Ledger.ContainsTransaction(StoreView, hash);
         }
+
+        /// <summary>
+        /// Determines whether the specified transaction conflicts with some on-chain transaction.
+        /// </summary>
+        /// <param name="hash">The hash of the transaction</param>
+        /// <returns><see langword="true"/> if the transaction conflicts with on-chain transaction; otherwise, <see langword="false"/>.</returns>
+        public bool ContainsConflictHash(UInt256 hash)
+        {
+            return NativeContract.Ledger.ContainsConflictHash(StoreView, hash);
+        }
     }
 }