Some client messages have bare ids (as opposed to proxies/resources),
eg. as in pw_registry_bind/destroy. If the client is processing
messages late, these may refer to an object that was already removed,
and the id may now refers to a differnt objects. I.e. the following
race condition needs to be resolved:
server client
Global 1 (gen. 1)
Global 1
Global 1 remove
Global 1 (gen. 2)
Bind/destroy 1
Where the client would bind/destroy the wrong global, since it did not
yet see the messages for the second one.
To keep track of which object the client means, the server keeps track of
the "generation number" of its global registry, and what generation
the client is at.
Each global remembers at what generation of registry they were
registered. When processing the messages that use bare ids, check the
registry generation of the client, to know whether the message refers to
a stale global that was already removed.
Messages where client sends bare ids to server are:
pw_registry_bind, pw_registry_destroy, metadata_set_property
In pw_registry_* do the staleness check directly. Also add staleness
check in pw_impl_client_check_permissions, so that also the metadata
case is handled.
The generation numbers are passed around in message footers, but only if
they have changed. When the generation number changes on server, we
send the updated value to the client in a message footer. When client
has received an update value, it will send the value back in the footer
of the next message it sends to the server.
Based on: Wim Taymans <wtaymans@redhat.com> "impl-core: check serial number"
Extend version 3 protocol with message footers, which are for passing
around global state data that is not addressed to a specific object.
The extension is backward compatible with previous v3 clients, and won't
e.g. result to error spam in logs.
The footer is a single SPA POD, appended after the main message POD.
Because both the protocol message and the message POD record their
length, it's possible to append trailing data. Earlier clients will
ignore any data trailing the message POD.
The footer POD contains a sequence [Id opcode, Struct {...}]*,
so there is room to extend with new opcodes later as necessary.
There are separate marshal/demarshal routines for messages aimed at
resources and proxies.
