[Serusers] Rewriting URI in the Contact field

[Jiri Kuthan]

that's correct.

>1. Check that _m->somefield is NULL and call parse_headers() with the
>appropriate HDR_FOO flag.

yes. you can skip the first check, parse_headers will do it;
don't forget to handle errors if parse_headers returns -1
(most likely mem alloc failure)

>2. Check that _m->somefield is non-NULL and return if false.

  yes, that means, that the header field is not in message.

  note too that some header fields may occur multiple times; in which case
  msg->hdr_foo points to the first occurence; if you wish to
  process all of them, you need to traverse the list
  msg->headers

example from record-routing (XXX are my extra comments):

int find_first_route(struct sip_msg* _m)
{
    /* XXX don't look at _m->route ... parse_headers will do it for you */
    if (parse_headers(_m, HDR_ROUTE, 0) == -1) {
        /* XXX -1 is some bad error, most likely lack of memory  -- leave! */
        LOG(L_ERR, "find_first_route(): Error while parsing headers\n");
        return -1;
    } else {

        if (_m->route) {
            return 0;
        } else { /* XXX not found .... the header field is not there */
            DBG("find_first_route(): No Route headers found\n");
            return 1;
        }
    }
}



>3. Modify _m->somefield according to the type of that field.
>4. Using del_lump() mark original version of the header for deletion and
>using insert_new_lump() indicate where modified version should be placed
>before sending a message out.

3 is the same as 4, isn't it. you modify a header field by creating
a delete lump refering the old value and an insert lump with the new
value. the new value must be created using pkg_malloc. the replace 
action in textops module is a good example. (The lump lists, sort 
of "diffs", is processed when later on the message is printed and forwarded.)

-Jiri