mdbx++: поддержка вложенных пишущих транзакций.

mdbx.h++

@@ -3654,7 +3654,8 @@ public:
   enum mode {
     readonly,        ///< \copydoc MDBX_RDONLY
     write_file_io,   // don't available on OpenBSD
-    write_mapped_io ///< \copydoc MDBX_WRITEMAP
+    write_mapped_io, ///< \copydoc MDBX_WRITEMAP
+    nested_transactions = write_file_io
   };
 
   /// \brief Durability level.
@@ -4197,6 +4198,9 @@ public:
   /// \brief Creates but not start read transaction.
   inline txn_managed prepare_read() const;
 
+  /// \brief Starts write (read-write) transaction.
+  inline txn_managed start_write(txn &parent);
+
   /// \brief Starts write (read-write) transaction.
   inline txn_managed start_write(bool dont_wait = false);
 
@@ -6404,6 +6408,14 @@ inline txn_managed env::start_write(bool dont_wait) {
   return txn_managed(ptr);
 }
 
+inline txn_managed env::start_write(txn &parent) {
+  ::MDBX_txn *ptr;
+  error::success_or_throw(
+      ::mdbx_txn_begin(handle_, parent, MDBX_TXN_READWRITE, &ptr));
+  assert(ptr != nullptr);
+  return txn_managed(ptr);
+}
+
 inline txn_managed env::try_start_write() { return start_write(true); }
 
 //------------------------------------------------------------------------------