ITS#7994 Access to current transaction ID.

I, David Barbour, hereby place the following modifications to OpenLDAP
Software (and only these modifications) into the public domain. Hence,
these modifications may be freely used and/or redistributed for any
purpose with or without attribution and/or other notice.

libraries/liblmdb/lmdb.h

@@ -226,6 +226,9 @@ typedef struct MDB_env MDB_env;
  */
 typedef struct MDB_txn MDB_txn;
 
+/** @brief Unique identifier for an active or recent transaction. */
+typedef size_t MDB_txnid_t;
+
 /** @brief A handle for an individual database in the DB environment. */
 typedef unsigned int	MDB_dbi;
 
@@ -448,7 +451,7 @@ typedef struct MDB_envinfo {
 	void	*me_mapaddr;			/**< Address of map, if fixed */
 	size_t	me_mapsize;				/**< Size of the data memory map */
 	size_t	me_last_pgno;			/**< ID of the last used page */
-	size_t	me_last_txnid;			/**< ID of the last committed transaction */
+	MDB_txnid_t  me_last_txnid;		/**< ID of the last committed transaction */
 	unsigned int me_maxreaders;		/**< max reader slots in the environment */
 	unsigned int me_numreaders;		/**< max reader slots used in the environment */
 } MDB_envinfo;
@@ -950,6 +953,19 @@ int  mdb_txn_begin(MDB_env *env, MDB_txn *parent, unsigned int flags, MDB_txn **
 	 */
 MDB_env *mdb_txn_env(MDB_txn *txn);
 
+	/** @brief Return the transaction's #MDB_txnid_t
+	 *
+	 * This returns the identifier associated with this transaction. For a
+	 * read-only transaction, this corresponds to the snapshot being read;
+	 * concurrent readers will frequently have the same transaction ID. For
+	 * a write transaction, this is always the snapshot read plus one. When
+	 * a write transaction aborts, the next transaction ID will be reused.
+	 *
+	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
+	 * @return A transaction ID, valid if input is an active transaction.
+	 */
+MDB_txnid_t mdb_txn_id(MDB_txn *txn);
+
 	/** @brief Commit all the operations of a transaction into the database.
 	 *
 	 * The transaction handle is freed. It and its cursors must not be used

libraries/liblmdb/mdb.c

@@ -2775,6 +2775,13 @@ mdb_txn_env(MDB_txn *txn)
 	return txn->mt_env;
 }
 
+MDB_txnid_t
+mdb_txn_id(MDB_txn *txn)
+{
+    if(!txn) return (txnid_t)-1;
+    return txn->mt_txnid;
+}
+
 /** Export or close DBI handles opened in this txn. */
 static void
 mdb_dbis_update(MDB_txn *txn, int keep)