[N4] Whitelist of Free Contracts

src/Neo/SmartContract/ApplicationEngine.cs

@@ -276,6 +276,14 @@ protected static void OnSysCall(ExecutionEngine engine, Instruction instruction)
     /// <param name="datoshi">The amount of GAS, in the unit of datoshi, 1 datoshi = 1e-8 GAS, to be added.</param>
     protected internal void AddFee(long datoshi)
     {
+        // Check whitelist
+
+        if (CurrentContext?.GetState<ExecutionContextState>()?.WhiteListed == true)
+        {
+            // The execution is whitelisted
+            return;
+        }
+
         FeeConsumed = checked(FeeConsumed + datoshi);
         if (FeeConsumed > _feeAmount)
             throw new InvalidOperationException("Insufficient GAS.");
@@ -319,6 +327,14 @@ private ExecutionContext CallContractInternal(ContractState contract, ContractMe
                 throw new InvalidOperationException($"Cannot Call Method {method.Name} Of Contract {contract.Hash} From Contract {CurrentScriptHash}");
         }
 
+        // Check whitelist
+
+        if (NativeContract.Policy.IsWhitelistFeeContract(SnapshotCache, contract.Hash, method.Name, method.Parameters.Length, out var fixedFee))
+        {
+            AddFee(fixedFee.Value);
+            state.WhiteListed = true;
+        }
+
         if (invocationCounter.TryGetValue(contract.Hash, out var counter))
         {
             invocationCounter[contract.Hash] = counter + 1;

src/Neo/SmartContract/ExecutionContextState.cs

@@ -48,4 +48,9 @@ public class ExecutionContextState
     public int NotificationCount { get; set; }
 
     public bool IsDynamicCall { get; set; }
+
+    /// <summary>
+    /// True if the execution is whitelisted by committee
+    /// </summary>
+    public bool WhiteListed { get; set; }
 }

src/Neo/SmartContract/Native/ContractManagement.cs

@@ -353,7 +353,10 @@ private ContractTask Update(ApplicationEngine engine, byte[]? nefFile, byte[]? m
             contract.Manifest = manifestNew;
         }
         Helper.Check(new Script(contract.Nef.Script, true), contract.Manifest.Abi);
-        contract.UpdateCounter++; // Increase update counter
+        // Increase update counter
+        contract.UpdateCounter++;
+        // Clean whitelist (emit event if exists)
+        Policy.CleanWhitelist(engine, contract.Hash);
         return OnDeployAsync(engine, contract, data, true);
     }
 
@@ -374,6 +377,8 @@ private async ContractTask Destroy(ApplicationEngine engine)
             engine.SnapshotCache.Delete(key);
         // lock contract
         await Policy.BlockAccountInternal(engine, hash);
+        // Clean whitelist (emit event if exists)
+        Policy.CleanWhitelist(engine, contract.Hash);
         // emit event
         Notify(engine, "Destroy", hash);
     }

src/Neo/SmartContract/Native/NativeContract.cs

@@ -387,6 +387,10 @@ private protected StorageKey CreateStorageKey(byte prefix, params IEnumerable<IS
         return builder;
     }
 
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private protected StorageKey CreateStorageKey(byte prefix, UInt160 hash, string methodName, int bigEndianKey)
+        => new KeyBuilder(Id, prefix) { hash, bigEndianKey, methodName.ToStrictUtf8Bytes() };
+
     #endregion
 
     /// <summary>
@@ -425,8 +429,12 @@ internal async void Invoke(ApplicationEngine engine, byte version)
             var state = context.GetState<ExecutionContextState>();
             if (!state.CallFlags.HasFlag(method.RequiredCallFlags))
                 throw new InvalidOperationException($"Cannot call this method with the flag {state.CallFlags}.");
-            // In the unit of datoshi, 1 datoshi = 1e-8 GAS
-            engine.AddFee(method.CpuFee * engine.ExecFeeFactor + method.StorageFee * engine.StoragePrice);
+            // Check native-whitelist
+            if (!Policy.IsWhitelistFeeContract(engine.SnapshotCache, Hash, method.Name, method.Parameters.Length, out var fixedFee))
+            {
+                // In the unit of datoshi, 1 datoshi = 1e-8 GAS
+                engine.AddFee(method.CpuFee * engine.ExecFeeFactor + method.StorageFee * engine.StoragePrice);
+            }
             List<object?> parameters = new();
             if (method.NeedApplicationEngine) parameters.Add(engine);
             if (method.NeedSnapshot) parameters.Add(engine.SnapshotCache);

src/Neo/SmartContract/Native/PolicyContract.cs

@@ -11,16 +11,24 @@
 
 #pragma warning disable IDE0051
 
+using Neo.Extensions.IO;
 using Neo.Network.P2P.Payloads;
 using Neo.Persistence;
 using Neo.SmartContract.Iterators;
+using System.Diagnostics.CodeAnalysis;
 using System.Numerics;
 
 namespace Neo.SmartContract.Native;
 
 /// <summary>
 /// A native contract that manages the system policies.
 /// </summary>
+[ContractEvent(0, name: WhitelistChangedEventName,
+    "contract", ContractParameterType.Hash160,
+    "method", ContractParameterType.String,
+    "argCount", ContractParameterType.Integer,
+    "fee", ContractParameterType.Any
+)]
 public sealed class PolicyContract : NativeContract
 {
     /// <summary>
@@ -64,8 +72,9 @@
     /// </summary>
     public const uint MaxStoragePrice = 10000000;
 
-    private const byte Prefix_BlockedAccount = 15;
     private const byte Prefix_FeePerByte = 10;
+    private const byte Prefix_BlockedAccount = 15;
+    private const byte Prefix_WhitelistedFeeContracts = 16;
     private const byte Prefix_ExecFeeFactor = 18;
     private const byte Prefix_StoragePrice = 19;
     private const byte Prefix_AttributeFee = 20;
@@ -74,6 +83,8 @@
     private readonly StorageKey _execFeeFactor;
     private readonly StorageKey _storagePrice;
 
+    private const string WhitelistChangedEventName = "WhitelistFeeChanged";
+
     internal PolicyContract()
     {
         _feePerByte = CreateStorageKey(Prefix_FeePerByte);
@@ -237,6 +248,125 @@
         return true;
     }
 
+    internal bool IsWhitelistFeeContract(DataCache snapshot, UInt160 contractHash, string method, int argCount, [NotNullWhen(true)] out long? fixedFee)
+    {
+        // Check contract existence
+
+        var currentContract = ContractManagement.GetContract(snapshot, contractHash);
+
+        if (currentContract != null)
+        {
+            // Check state existence
+
+            var item = snapshot.TryGet(CreateStorageKey(Prefix_WhitelistedFeeContracts, contractHash, method, argCount));
+
+            if (item != null)
+            {
+                fixedFee = (long)(BigInteger)item;
+                return true;
+            }
+        }
+
+        fixedFee = null;
+        return false;
+    }
+
+    /// <summary>
+    /// Remove whitelisted Fee contracts
+    /// </summary>
+    /// <param name="engine">The execution engine.</param>
+    /// <param name="contractHash">The contract to set the whitelist</param>
+    /// <param name="method">Method</param>
+    /// <param name="argCount">Argument count</param>
+    [ContractMethod(CpuFee = 1 << 15, RequiredCallFlags = CallFlags.States | CallFlags.AllowNotify)]
+    private void RemoveWhitelistFeeContract(ApplicationEngine engine, UInt160 contractHash, string method, int argCount)
+    {
+        if (!CheckCommittee(engine)) throw new InvalidOperationException("Invalid committee signature");
+
+        var key = CreateStorageKey(Prefix_WhitelistedFeeContracts, contractHash, method, argCount);
+
+        if (!engine.SnapshotCache.Contains(key)) throw new InvalidOperationException("Whitelist not found");
+
+        engine.SnapshotCache.Delete(key);
+
+        // Emit event
+        engine.SendNotification(Hash, WhitelistChangedEventName,
+            [new VM.Types.ByteString(contractHash.ToArray()), new VM.Types.ByteString(method.ToStrictUtf8Bytes()),
+                new VM.Types.Integer(argCount), VM.Types.StackItem.Null]);
+    }
+
+    internal int CleanWhitelist(ApplicationEngine engine, UInt160 contractHash)
+    {
+        var count = 0;
+        var searchKey = CreateStorageKey(Prefix_WhitelistedFeeContracts, contractHash);
+
+        foreach ((var key, _) in engine.SnapshotCache.Find(searchKey, SeekDirection.Forward))
+        {
+            engine.SnapshotCache.Delete(key);
+            count++;
+
+            // Emit event recovering the values from the Key
+
+            var keyData = key.ToArray().AsSpan();
+            // TODO: Require a unwrap
+            (var method, var argCount) = StorageKey.ReadMethodAndArgCount(key.ToArray().AsSpan());
+
+            engine.SendNotification(Hash, WhitelistChangedEventName,
+                [new VM.Types.ByteString(contractHash.ToArray()), new VM.Types.ByteString(method.ToStrictUtf8Bytes()),
+                    new VM.Types.Integer(argCount), VM.Types.StackItem.Null]);
+        }
+
+        return count;
+    }
+
+    /// <summary>
+    /// Set whitelisted Fee contracts
+    /// </summary>
+    /// <param name="engine">The execution engine.</param>
+    /// <param name="contractHash">The contract to set the whitelist</param>
+    /// <param name="method">Method</param>
+    /// <param name="argCount">Argument count</param>
+    /// <param name="fixedFee">Fixed execution fee</param>
+    [ContractMethod(CpuFee = 1 << 15, RequiredCallFlags = CallFlags.States | CallFlags.AllowNotify)]
+    internal void SetWhitelistFeeContract(ApplicationEngine engine, UInt160 contractHash, string method, int argCount, long fixedFee)
+    {
+        ArgumentOutOfRangeException.ThrowIfNegative(fixedFee, nameof(fixedFee));
+
+        if (!CheckCommittee(engine)) throw new InvalidOperationException("Invalid committee signature");
+
+        var key = CreateStorageKey(Prefix_WhitelistedFeeContracts, contractHash, method, argCount);
+
+        // Validate methods
+        var contract = ContractManagement.GetContract(engine.SnapshotCache, contractHash)
+                ?? throw new InvalidOperationException("Is not a valid contract");
+
+        if (contract.Manifest.Abi.GetMethod(method, argCount) is null)
+            throw new InvalidOperationException($"{method} with {argCount} args is not a valid method of {contractHash}");
+
+        // Set
+        var entry = engine.SnapshotCache
+                .GetAndChange(key, () => new StorageItem(fixedFee));
+
+        entry.Set(fixedFee);
+
+        // Emit event
+
+        engine.SendNotification(Hash, WhitelistChangedEventName,
+            [new VM.Types.ByteString(contractHash.ToArray()), new VM.Types.ByteString(method.ToStrictUtf8Bytes()),
+                new VM.Types.Integer(argCount), new VM.Types.Integer(fixedFee)]);
+    }
+
+    [ContractMethod(CpuFee = 1 << 15, RequiredCallFlags = CallFlags.ReadStates)]
+    internal StorageIterator GetWhitelistFeeContracts(DataCache snapshot)
+    {
+        const FindOptions options = FindOptions.RemovePrefix | FindOptions.KeysOnly;
+        var enumerator = snapshot
+            .Find(CreateStorageKey(Prefix_WhitelistedFeeContracts), SeekDirection.Forward)
+            .GetEnumerator();
+
+        return new StorageIterator(enumerator, 1, options);
+    }
+
     [ContractMethod(CpuFee = 1 << 15, RequiredCallFlags = CallFlags.ReadStates)]
     private StorageIterator GetBlockedAccounts(DataCache snapshot)
     {