Add mdb_cursor_renew()

Allow cursors on read-only txns to be reused with later txns.

libraries/libmdb/mdb.c

@@ -5285,6 +5285,19 @@ mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **ret)
 	return MDB_SUCCESS;
 }
 
+int
+mdb_cursor_renew(MDB_txn *txn, MDB_cursor *mc)
+{
+	if (txn == NULL || mc == NULL || mc->mc_dbi >= txn->mt_numdbs)
+		return EINVAL;
+
+	if (txn->mt_cursors)
+		return EINVAL;
+
+	mdb_cursor_init(mc, txn, mc->mc_dbi, mc->mc_xcursor);
+	return MDB_SUCCESS;
+}
+
 /* Return the count of duplicate data items for the current key */
 int
 mdb_cursor_count(MDB_cursor *mc, size_t *countp)

libraries/libmdb/mdb.h

@@ -904,6 +904,23 @@ int  mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor);
 	 */
 void mdb_cursor_close(MDB_cursor *cursor);
 
+	/** @brief Renew a cursor handle.
+	 *
+	 * Cursors are associated with a specific transaction and database and
+	 * may not span threads. Cursors that are only used in read-only
+	 * transactions may be re-used, to avoid unnecessary malloc/free overhead.
+	 * The cursor may be associated with a new read-only transaction, and
+	 * referencing the same database handle as it was created with.
+	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
+	 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
+	 * @return A non-zero error value on failure and 0 on success. Some possible
+	 * errors are:
+	 * <ul>
+	 *	<li>EINVAL - an invalid parameter was specified.
+	 * </ul>
+	 */
+int  mdb_cursor_renew(MDB_txn *txn, MDB_cursor *cursor);
+
 	/** @brief Return the cursor's transaction handle.
 	 *
 	 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()