text: // Copyright (c) 2018, The Monero Project
  // All rights reserved.
  //
  // Redistribution and use in source and binary forms, with or without modification, are
  // permitted provided that the following conditions are met:
  //
  // 1. Redistributions of source code must retain the above copyright notice, this list of
  //    conditions and the following disclaimer.
  //
  // 2. Redistributions in binary form must reproduce the above copyright notice, this list
  //    of conditions and the following disclaimer in the documentation and/or other
  //    materials provided with the distribution.
  //
  // 3. Neither the name of the copyright holder nor the names of its contributors may be
  //    used to endorse or promote products derived from this software without specific
  //    prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
  // EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
  // MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
  // THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  // PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  // INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  // STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
  // THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  #pragma once
  
  #include "external/db_drivers/liblmdb/lmdb.h"
  #include <memory>
  
  #include "src/lmdb/error.h"
  
  //! Uses C++ type system to differentiate between cursors
  #define MONERO_CURSOR(name)                                    \
      struct close_ ## name : ::lmdb::close_cursor {};           \
      using name = std::unique_ptr< MDB_cursor, close_ ## name >;
  
  namespace lmdb
  {
      struct abort_txn
      {
          void operator()(MDB_txn* ptr) const noexcept
          {
              if (ptr)
                  mdb_txn_abort(ptr);
          }
      };
  
      /*!
          Only valid if used via `create_read_txn()`. Decrements active count in
          associated `context`, and aborts a LMDB transaction (`mdb_txn_abort`).
      */
      struct release_read_txn
      { 
          void operator()(MDB_txn* ptr) const noexcept;
          // implementation in database.cpp
      };
  
      /*!
          Only valid if used via `create_write_txn()`. Decrements active count in
          associated `context`, and aborts a LMDB transaction (`mdb_txn_abort`).
      */
      struct abort_write_txn
      {
          void operator()(MDB_txn* ptr) const noexcept
          {
              release_read_txn{}(ptr);
          }
      };
  
      struct close_cursor
      {
          void operator()(MDB_cursor* ptr) const noexcept
          {
              if (ptr)
                  mdb_cursor_close(ptr);
          }
      };
  
      template<typename D>
      inline expect<std::unique_ptr<MDB_cursor, D>>
      open_cursor(MDB_txn& txn, MDB_dbi tbl) noexcept
      {
          MDB_cursor* cur = nullptr;
          MONERO_LMDB_CHECK(mdb_cursor_open(&txn, tbl, &cur));
          return std::unique_ptr<MDB_cursor, D>{cur};
      }
  
      // The below use the C++ type system to designate `MDB_txn` status.
  
      using suspended_txn = std::unique_ptr<MDB_txn, abort_txn>;
      using read_txn = std::unique_ptr<MDB_txn, release_read_txn>;
      using write_txn = std::unique_ptr<MDB_txn, abort_write_txn>;
  } // lmdb
  