Fix mempool corruption at zero-inputs

When node accepts transaction or receives block, it tries to load all
inputs to mempool. Trying to load zero-inputs to mempool leads to
corruption - valid transaction may be removed from mempool during
conflicts check. To prevent this, inputs selection algorithms modified
to ignore zero-inputs. Conflicts check ignores them too.

Sources/Stratis.Bitcoin.Features.Consensus/Rules/CommonRules/DeStreamLoadCoinviewRule.cs

 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.Extensions.Logging;
 using NBitcoin;
@@ -45,5 +46,32 @@ namespace Stratis.Bitcoin.Features.Consensus.Rules.CommonRules
             // The task is not awaited so will not stall main validation process.
             this.TryPrefetchAsync(context.Flags);
         }
+
+        /// <inheritdoc />
+        protected override uint256[] GetIdsToFetch(Block block, bool enforceBIP30)
+        {
+            this.Logger.LogTrace("({0}:'{1}',{2}:{3})", nameof(block), block.GetHash(), nameof(enforceBIP30), enforceBIP30);
+
+            var ids = new HashSet<uint256>();
+            foreach (Transaction tx in block.Transactions)
+            {
+                if (enforceBIP30)
+                {
+                    uint256 txId = tx.GetHash();
+                    ids.Add(txId);
+                }
+
+                if (tx.IsCoinBase) continue;
+                
+                foreach (TxIn input in tx.Inputs.RemoveChangePointer())
+                {
+                    ids.Add(input.PrevOut.Hash);
+                }
+            }
+
+            uint256[] res = ids.ToArray();
+            this.Logger.LogTrace("(-):*.{0}={1}", nameof(res.Length), res.Length);
+            return res;
+        }
     }
 }
\ No newline at end of file

Sources/Stratis.Bitcoin.Features.Consensus/Rules/CommonRules/LoadCoinviewRule.cs

@@ -58,7 +58,7 @@ namespace Stratis.Bitcoin.Features.Consensus.Rules.CommonRules
         /// <param name="block">The block with the transactions.</param>
         /// <param name="enforceBIP30">Whether to enforce look up of the transaction id itself and not only the reference to previous transaction id.</param>
         /// <returns>A list of transaction ids to fetch from store</returns>
-        protected uint256[] GetIdsToFetch(Block block, bool enforceBIP30)
+        protected virtual uint256[] GetIdsToFetch(Block block, bool enforceBIP30)
         {
             this.Logger.LogTrace("({0}:'{1}',{2}:{3})", nameof(block), block.GetHash(), nameof(enforceBIP30), enforceBIP30);
 

Sources/Stratis.Bitcoin.Features.MemoryPool/DeStreamMempoolCoinView.cs

@@ -23,7 +23,7 @@ namespace Stratis.Bitcoin.Features.MemoryPool
         public override async Task LoadViewAsync(Transaction trx)
         {
             // lookup all ids (duplicate ids are ignored in case a trx spends outputs from the same parent).
-            List<uint256> ids = trx.Inputs.Select(n => n.PrevOut.Hash).Distinct().Concat(new[] { trx.GetHash() }).ToList();
+            List<uint256> ids = trx.Inputs.RemoveChangePointer().Select(n => n.PrevOut.Hash).Distinct().Concat(new[] { trx.GetHash() }).ToList();
             FetchCoinsResponse coins = await this.Inner.FetchCoinsAsync(ids.ToArray());
             // find coins currently in the mempool
             List<Transaction> mempoolcoins = await this.mempoolLock.ReadAsync(() =>

Sources/Stratis.Bitcoin.Features.MemoryPool/TxMemPool.cs

@@ -838,7 +838,7 @@ namespace Stratis.Bitcoin.Features.MemoryPool
         {
             // Remove transactions which depend on inputs of tx, recursively
             //LOCK(cs);
-            foreach (TxIn txInput in tx.Inputs)
+            foreach (TxIn txInput in tx.Inputs.RemoveChangePointer())
             {
                 NextTxPair it = this.MapNextTx.FirstOrDefault(p => p.OutPoint == txInput.PrevOut);
                 if (it != null)