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0036. Drop final broadcast recipients by sid

Status: Accepted · 2026-08-14 · #263 Governed by: 0010, 0031, 0032

TL;DR DroppingAdapter deterministically removes known sids after normal broadcast routing and volatile filtering. It changes event delivery and new acknowledgement collection only, then clears each sid on whole-socket removal.

Decision​

Socket.IO has no built-in deterministic recipient-dropping adapter. Smocket adds one test-only routing affordance through the existing adapter seam without changing default Socket.IO-compatible delivery.

setDropped(sid, dropped = true) toggles a currently known namespace sid and isDropped(sid) observes that state. Unknown and removed sids are no-ops. The core first completes room union, exclusions, sender exclusion, roster lookup, and volatile pre-connect filtering, then gives an optional adapter hook the ordered final ids. The hook may only narrow: additions, duplicates, and reordering do not affect core order.

A dropped sid receives no broadcast event, contributes no new acknowledgement, and has no outgoing catch-all observation for that delivery. Direct server-Socket emits, client-to-server events, rooms, lifecycle, and acknowledgements already in flight remain unchanged. Removing every recipient uses the existing empty-recipient collector result.

Whole-socket removal clears drop state through the 0031 lifecycle hook. Each namespace has its own adapter, and reconnect creates a fresh sid, so neither path inherits a drop. The adapter wraps another adapter and forwards membership, scheduling, tracing, and cleanup. TracingAdapter also forwards narrowing, so either wrapper order records the post-drop final recipients while DelayingAdapter keeps retained-recipient FIFO.

Alternatives rejected​

  • Override socketsIn. Exclusion lookup and empty-room broadcasts make that unsafe.
  • Random or next-N loss. Probability, seed, and sequence semantics are separate work.
  • Drop direct Socket delivery. The use case is final broadcast routing, not a general transport failure model.
  • Let filters add or reorder recipients. That would reopen routing and FIFO contracts instead of providing deterministic narrowing.