[db] Create separate database for txindex.

The new TxIndexDB class will be used by a future commit in this
change set.

src/dbwrapper.h

@@ -224,6 +224,9 @@ public:
     CDBWrapper(const fs::path& path, size_t nCacheSize, bool fMemory = false, bool fWipe = false, bool obfuscate = false);
     ~CDBWrapper();
 
+    CDBWrapper(const CDBWrapper&) = delete;
+    CDBWrapper& operator=(const CDBWrapper&) = delete;
+
     template <typename K, typename V>
     bool Read(const K& key, V& value) const
     {

src/txdb.cpp

@@ -424,3 +424,35 @@ bool CCoinsViewDB::Upgrade() {
     LogPrintf("[%s].\n", ShutdownRequested() ? "CANCELLED" : "DONE");
     return !ShutdownRequested();
 }
+
+TxIndexDB::TxIndexDB(size_t n_cache_size, bool f_memory, bool f_wipe) :
+    CDBWrapper(GetDataDir() / "indexes" / "txindex", n_cache_size, f_memory, f_wipe)
+{}
+
+bool TxIndexDB::ReadTxPos(const uint256 &txid, CDiskTxPos& pos) const
+{
+    return Read(std::make_pair(DB_TXINDEX, txid), pos);
+}
+
+bool TxIndexDB::WriteTxs(const std::vector<std::pair<uint256, CDiskTxPos>>& v_pos)
+{
+    CDBBatch batch(*this);
+    for (const auto& tuple : v_pos) {
+        batch.Write(std::make_pair(DB_TXINDEX, tuple.first), tuple.second);
+    }
+    return WriteBatch(batch);
+}
+
+bool TxIndexDB::ReadBestBlock(CBlockLocator& locator) const
+{
+    bool success = Read(DB_BEST_BLOCK, locator);
+    if (!success) {
+        locator.SetNull();
+    }
+    return success;
+}
+
+bool TxIndexDB::WriteBestBlock(const CBlockLocator& locator)
+{
+    return Write(DB_BEST_BLOCK, locator);
+}

src/txdb.h

@@ -9,6 +9,7 @@
 #include <coins.h>
 #include <dbwrapper.h>
 #include <chain.h>
+#include <primitives/block.h>
 
 #include <map>
 #include <memory>
@@ -112,9 +113,6 @@ class CBlockTreeDB : public CDBWrapper
 public:
     explicit CBlockTreeDB(size_t nCacheSize, bool fMemory = false, bool fWipe = false);
 
-    CBlockTreeDB(const CBlockTreeDB&) = delete;
-    CBlockTreeDB& operator=(const CBlockTreeDB&) = delete;
-
     bool WriteBatchSync(const std::vector<std::pair<int, const CBlockFileInfo*> >& fileInfo, int nLastFile, const std::vector<const CBlockIndex*>& blockinfo);
     bool ReadBlockFileInfo(int nFile, CBlockFileInfo &info);
     bool ReadLastBlockFile(int &nFile);
@@ -127,4 +125,32 @@ public:
     bool LoadBlockIndexGuts(const Consensus::Params& consensusParams, std::function<CBlockIndex*(const uint256&)> insertBlockIndex);
 };
 
+/**
+ * Access to the txindex database (indexes/txindex/)
+ *
+ * The database stores a block locator of the chain the database is synced to
+ * so that the TxIndex can efficiently determine the point it last stopped at.
+ * A locator is used instead of a simple hash of the chain tip because blocks
+ * and block index entries may not be flushed to disk until after this database
+ * is updated.
+ */
+class TxIndexDB : public CDBWrapper
+{
+public:
+    explicit TxIndexDB(size_t n_cache_size, bool f_memory = false, bool f_wipe = false);
+
+    /// Read the disk location of the transaction data with the given hash. Returns false if the
+    /// transaction hash is not indexed.
+    bool ReadTxPos(const uint256& txid, CDiskTxPos& pos) const;
+
+    /// Write a batch of transaction positions to the DB.
+    bool WriteTxs(const std::vector<std::pair<uint256, CDiskTxPos>>& v_pos);
+
+    /// Read block locator of the chain that the txindex is in sync with.
+    bool ReadBestBlock(CBlockLocator& locator) const;
+
+    /// Write block locator of the chain that the txindex is in sync with.
+    bool WriteBestBlock(const CBlockLocator& locator);
+};
+
 #endif // BITCOIN_TXDB_H