[9.2] Confusion over CacheRegisterSyscacheCallback

Commit b5282aa893: "Revise sinval code to remove no-longer-used tuple TID from inval messages."

dropped ItemPointer from callbacks and replaced it with "hashValue".
There seems to be 2 ways that new backend code calculates it:

- hashoid(oid), which seems to assume too much?
- CatalogCacheComputeTupleHashValue(), which is 'static'.

So my question is that after doing generic SearchSysCache() like:
  tup = SearchSysCache(USERMAPPINGUSERSERVER,       ObjectIdGetDatum(user_mapping->userid),
ObjectIdGetDatum(foreign_server->serverid),      0, 0);
 

what is the proper way to calculate the hashValue that 
will be given to callback?

-- 
marko