Removes a [key, value] pair from the database, and returns it. If the
database is empty, returns nil. The order in which values are removed/returned is not guaranteed.
static VALUE
fdbm_shift(VALUE obj)
{
datum key, val;
struct dbmdata *dbmp;
DBM *dbm;
VALUE keystr, valstr;
fdbm_modify(obj);
GetDBM2(obj, dbmp, dbm);
dbmp->di_size = -1;
key = dbm_firstkey(dbm);
if (!key.dptr) return Qnil;
val = dbm_fetch(dbm, key);
keystr = rb_tainted_str_new(key.dptr, key.dsize);
valstr = rb_tainted_str_new(val.dptr, val.dsize);
dbm_delete(dbm, key);
return rb_assoc_new(keystr, valstr);
}