Also consider txindex for transactions in AlreadyHave() (#3126)

This avoids many false negatives where TXs are announced to us which are
already mined and then were spent later.

Author: codablock

src/net_processing.cpp

@@ -25,6 +25,7 @@
 #include "reverse_iterator.h"
 #include "scheduler.h"
 #include "tinyformat.h"
+#include "txdb.h"
 #include "txmempool.h"
 #include "ui_interface.h"
 #include "util.h"
@@ -1019,7 +1020,8 @@ bool static AlreadyHave(const CInv& inv) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
             return (recentRejects->contains(inv.hash) && !llmq::quorumInstantSendManager->IsLocked(inv.hash)) ||
                    mempool.exists(inv.hash) ||
                    pcoinsTip->HaveCoinInCache(COutPoint(inv.hash, 0)) || // Best effort: only try output 0 and 1
-                   pcoinsTip->HaveCoinInCache(COutPoint(inv.hash, 1));
+                   pcoinsTip->HaveCoinInCache(COutPoint(inv.hash, 1)) ||
+                   (fTxIndex && pblocktree->HasTxIndex(inv.hash));
         }
 
     case MSG_BLOCK:

src/txdb.cpp

@@ -240,6 +240,10 @@ bool CBlockTreeDB::WriteBatchSync(const std::vector<std::pair<int, const CBlockF
     return WriteBatch(batch, true);
 }
 
+bool CBlockTreeDB::HasTxIndex(const uint256& txid) {
+    return Exists(std::make_pair(DB_TXINDEX, txid));
+}
+
 bool CBlockTreeDB::ReadTxIndex(const uint256 &txid, CDiskTxPos &pos) {
     return Read(std::make_pair(DB_TXINDEX, txid), pos);
 }

src/txdb.h

@@ -121,6 +121,7 @@ class CBlockTreeDB : public CDBWrapper
     bool ReadLastBlockFile(int &nFile);
     bool WriteReindexing(bool fReindex);
     bool ReadReindexing(bool &fReindex);
+    bool HasTxIndex(const uint256 &txid);
     bool ReadTxIndex(const uint256 &txid, CDiskTxPos &pos);
     bool WriteTxIndex(const std::vector<std::pair<uint256, CDiskTxPos> > &list);
     bool ReadSpentIndex(CSpentIndexKey &key, CSpentIndexValue &value);