GlobalPartitionedMessageTopology.Except<T>() (#3867 follow-up)

[jeremydmiller]

Follow-up to #3867 (parts 1 and 2 merged in #3868). Small addition, and it closes a real loop we hit immediately after adopting global partitioning.

What

GlobalPartitionedMessageTopology.Except<T>() / Except(Type) — carve a message type, or a whole family when given an interface or base class, out of a topology even when a broader rule like MessagesImplementing<T>() would otherwise match it. Exclusions are checked first and win outright, so the result does not depend on declaration order.

opts.MessagePartitioning.GlobalPartitioned(topology =>
{
    topology.UseShardedRabbitQueues("critterwatch", 5);
    topology.MessagesImplementing<ICritterWatchMessage>();

    // This family is what we forward to the browser; without the exemption our own
    // re-publish re-enters the topology and comes back to the handler that sent it.
    topology.Except<ICritterStackWebSocketMessage>();
});

Why

A message type can legitimately belong to the topology on the way in while the receiving application also re-publishes it on its way somewhere else. Because each side configures its own topology, excluding it on the receiving side keeps inbound partitioning intact while stopping that application's own re-publish from re-entering the topology and arriving back at the handler that published it.

The loop is worth describing because it is invisible in configuration and shows up only as amplified load. In CritterWatch a handler ends with bus.PublishAsync(message) to forward the message to the UI over SignalR. The message implements an interface that extends the marker the topology matches, so that re-publish drew two routes — the intended SignalR one, and the topology one, which came straight back to the same handler.

Measured, same driver and rate, over two minutes:

event type	appends	published	amplification
the re-publishing type	2,607	~425	6.1x
sibling type	60	~425	1.0x
sibling type	61	~425	1.0x

With Except in place the same measurement is 429 appends from ~429 published — exactly 1.0x.

Note MessageRouter.DeduplicateRoutes does not catch this: it only removes explicit routes to sticky-handler local queues, never a transport route coexisting with a GlobalPartitionedRoute.

Two things worth a reviewer's eye

Excluding a type does not stop the application listening for it on the topology's slots — the companion-queue bridge is wired per endpoint, not per message type. Only the publishing rules are affected. That asymmetry is the whole point, and it is documented on the method.

Message(Type) now skips seeding the name cache for an already-excluded type, and Except removes it. Without both halves, MatchesByMessageTypeName — the pre-deserialization path Kafka uses — would disagree with Matches() under one of the two declaration orders. There is a test for each ordering.

Tests

7 new in CoreTests.Acceptance.GlobalPartitionedExclusionTests: broad-rule carve-out, both declaration orders, interface-family exclusion, the name-cache agreement under both orders, no effect on unrelated types, null guard.

Partitioning + batching suites on net9.0: 207 passed / 0 failed. Full CoreTests on the previous base: 2,322 passed / 0 failed.