[sr-dev] presence module (API)

[Andrew Mortensen]

On Jan 23, 2014, at 9:19 AM, Jason Penton <jason.penton at gmail.com> wrote:

> Hey all,
> 
> I was taking a look at the presence module API code, specfically the API code for the hash table. I see that when we delete a subscription according to hash.c:
> [snip]
> Why are we only searching on to-tag? What if there is a collision on the hash AND the to-tag is the same for 2+ different subscriptions. This is even more likely of happening considering that the hash calculation is based only on the callid and to-tag...

At least nominally the tag should be globally unique, per RFC3261:

19.3 ... When a tag is generated by a UA for insertion into a request or
   response, it MUST be globally unique and cryptographically random
   with at least 32 bits of randomness.

But I agree that poorly behaved UAs could cause problems here. (We actually ran into duplicate to-tags recently with vendor handsets booted simultaneously.)

It looks to me like we should be calling search_shtable, which compares the dialog's full tag set. The problem is that unlike other similar calls, delete_shtable doesn't take a subs_t pointer as an argument, so the only point of reference  is the to-tag. Fortunately it looks like there's only one place delete_shtable's called in the presence module, so it shouldn't be much work to start using search_shtable to find the subscription to delete.

To complicate things further, though, delete_shtable is exported as part of the presence API, so we'd need to update all API consumers as well. That doesn't seem unreasonable, as a quick git grep indicates the only current user of the presence API is the rls module.

andrew