a failover scenario

I am considering pgsql as the RDBMS in a project with the following constraints:
- there's a master and reserve instance of the RDBMS on every remote location
- there's a master and reserve instance of the RDBMS on a central location
- the connections are DSL connections and therefore unreliable
- all changes have to be propagated to all servers (multimaster replication)
- if the connection between a remote location and the central location
fails, the local server continues working and resynchronizes with the
central server when the connection is restored
- if any master fails, the reserve instance takes over and the rest of
the system acts as though nothing happened

The master/reserve instance is, from what I read, standard
functionality, but I'm not so sure about the resynchronization part of
a failed link...I imagine something like WAL shipping might be of use
here, but it's just an uneducated guess.
Does code exist to support this on pgsql or is it considered
application specific functionality? Do other RDBMSs support similar
functionality?

TIA,
t.n.a.