txindex: use a new block locator for downgrade safety

The hashed txindex entries cannot be found by older nodes. Record sync
progress under a new locator key so a downgraded node will not rely on
entries indexed by upgraded nodes, and instead continue syncing from the
legacy locator.

src/index/base.h

@@ -66,13 +66,14 @@ protected:
     public:
         DB(const fs::path& path, size_t n_cache_size,
            bool f_memory = false, bool f_wipe = false, bool f_obfuscate = false, bool f_bloom = true);
+        virtual ~DB() = default;
 
         /// Read block locator of the chain that the index is in sync with.
         /// Note, the returned locator will be empty if no record exists.
-        CBlockLocator ReadBestBlock() const;
+        virtual CBlockLocator ReadBestBlock() const;
 
         /// Write block locator of the chain that the index is in sync with.
-        void WriteBestBlock(CDBBatch& batch, const CBlockLocator& locator);
+        virtual void WriteBestBlock(CDBBatch& batch, const CBlockLocator& locator);
     };
 
 private:

src/index/txindex.cpp

@@ -30,6 +30,7 @@
 #include <vector>
 
 constexpr uint8_t DB_TXINDEX{'t'};
+const std::string DB_BEST_BLOCK_V2{"best_block_v2"};
 
 std::unique_ptr<TxIndex> g_txindex;
 
@@ -46,6 +47,9 @@ public:
 
     /// Write a batch of transaction positions to the DB.
     void WriteTxs(const std::vector<std::pair<Txid, CDiskTxPos>>& v_pos);
+
+    CBlockLocator ReadBestBlock() const override;
+    void WriteBestBlock(CDBBatch& batch, const CBlockLocator& locator) override;
 };
 
 TxIndex::DB::DB(size_t n_cache_size, bool f_memory, bool f_wipe) :
@@ -57,6 +61,21 @@ bool TxIndex::DB::ReadTxPos(const Txid& txid, CDiskTxPos& pos) const
     return Read(std::make_pair(DB_TXINDEX, txid.ToUint256()), pos);
 }
 
+CBlockLocator TxIndex::DB::ReadBestBlock() const
+{
+    CBlockLocator locator;
+    if (Read(DB_BEST_BLOCK_V2, locator)) {
+        return locator;
+    }
+    // If we don't have a locator yet, start from the legacy best block.
+    return BaseIndex::DB::ReadBestBlock();
+}
+
+void TxIndex::DB::WriteBestBlock(CDBBatch& batch, const CBlockLocator& locator)
+{
+    batch.Write(DB_BEST_BLOCK_V2, locator);
+}
+
 void TxIndex::DB::WriteTxs(const std::vector<std::pair<Txid, CDiskTxPos>>& v_pos)
 {
     CDBBatch batch(*this);