A session held exactly one transport, and prompt.submit, session.resume, session.activate, and the queued-prompt drain all rebound that slot. A second client therefore took the stream away from the first: the earlier client stopped receiving the turn it was already rendering, and either client disconnecting parked the whole session on the drop sentinel.
FanoutTransport goes in the same slot and satisfies the same Transport protocol, so write_json and every other reader of the slot are unchanged. It delivers each frame to a snapshot of its peers, concurrently when more than one peer is attached and the caller is not on an event loop, and prunes any peer that returns False or raises. A dead client is dropped; a slow one costs the emitter at most one write timeout per frame rather than one per peer. Request/response RPCs are unaffected: they still answer on the request's context-bound transport, so a client only ever sees replies to its own calls.
The rebind sites become attach sites through _attach_session_transport, whose ladder keeps the single-client shape identical. The same object already in the slot is a no-op; an empty, stdio, or parked slot is taken outright; only the arrival of a second live client wraps both. The queued-prompt drain is included because it pinned the drained turn to the queuer and silenced everyone else. A non-peer newcomer such as stdio or the drop sentinel never displaces a live client, so an activate dispatched without a bound websocket cannot silence the socket that owns the session.
Disconnect detaches first. A session that retains another client keeps streaming and is neither parked nor reaped, and only the clientless ones follow the existing close_on_disconnect and park-sentinel path, so a single-client disconnect, the orphan reaper, and its grace window behave as before. _ws_session_is_orphaned is unchanged: it still asks whether the drop sentinel is in the slot, and a fan-out is never the sentinel, so a session that still has a peer is never reported as orphaned.
Attach performs no entitlement check: any authenticated peer may mirror any session.
The fan-out architecture follows the approach in #40822 by @OmarB97.
What the slot's later history forces. _close_sessions_for_transport drops the #83716 rebind-to-the-most-recent-surviving-viewer, which fan-out membership subsumes — a pop-out window is a peer, so a session that still shows in one is never returned as clientless — and keeps the #77129 revalidation before parking, now expressed as a liveness check under _session_transport_lock so it is race-free against attach and detach. _transport_is_live_peer defers its last answer to _transport_is_dead: a socket that already latched _closed is a departed client, and admitting it would keep a session out of both the park and the reap. _transport_is_dead also learns the fan-out: a FanoutTransport with no live peer is dead, so a session whose peers were all pruned by failed writes cannot outlive the TTL and LRU reapers. The upstream test that pinned the #83716 rebind, test_close_transport_rebinds_session_to_remaining_viewer, is re-expressed in fan-out terms: both windows attached, the pop-out closes, the session stays with the main window unparked and still receiving frames.
Ryan Tucker
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