mdb_env_sync(,force=1): Override MDB_MAPASYNC.

With MDB_MAPASYNC, the API provided no way to ensure full sync.

libraries/libmdb/mdb.c

@@ -1523,7 +1523,8 @@ mdb_env_sync(MDB_env *env, int force)
 	int rc = 0;
 	if (force || !F_ISSET(env->me_flags, MDB_NOSYNC)) {
 		if (env->me_flags & MDB_WRITEMAP) {
-			int flags = (env->me_flags & MDB_MAPASYNC) ? MS_ASYNC : MS_SYNC;
+			int flags = ((env->me_flags & MDB_MAPASYNC) && !force)
+				? MS_ASYNC : MS_SYNC;
 			if (MDB_MSYNC(env->me_map, env->me_mapsize, flags))
 				rc = ErrCode();
 #ifdef _WIN32

libraries/libmdb/mdb.h

@@ -500,9 +500,9 @@ int  mdb_env_info(MDB_env *env, MDB_envinfo *stat);
 	 * the OS buffers upon commit as well, unless the environment was
 	 * opened with #MDB_NOSYNC.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
-	 * @param[in] force If non-zero, force the flush to occur. Otherwise
+	 * @param[in] force If non-zero, force a synchronous flush.  Otherwise
 	 *  if the environment has the #MDB_NOSYNC flag set the flushes
-	 *	will be omitted.
+	 *	will be omitted, and with #MDB_MAPASYNC they will be asynchronous.
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>