First, mongo_write_concern_init() set structure to 0 all.
After I set write_concern->mode to "majority", call mongo_insert().

In mongo_insert(), call mongo_choose_write_concern().
In mongo_choose_write_concern()...

...
*write_concern = custom_write_concern;
...
if ( *write_concern && (*write_concern)->w < 1 ) {
write_concern = 0; / do not generate getLastError request */
}

so, write_concern set to NULL.

Check this please.