MAX_WS_PER_SESSION was gated by a bare Map<sessionId,number> counter,
incremented on upgrade and decremented only on the old socket's async
close. A client that dropped and immediately reconnected could land its
new upgrade before the old socket's close fired, briefly over-counting and
tripping a spurious 4008 (-> HTTP fallback). The limit also counted raw
sockets, so a reconnecting client consumed a new slot instead of its own.
Replace the counter with WsConnectionRegistry (new pure, unit-tested module)
that tracks live sockets per session keyed by clientId. A same-cid upgrade
SUPERSEDES its own socket (evicts the stale one with close 4010, reuses the
slot, no net count change) -> a reconnect can never be rejected by the cap.
The reliable-input protocol (shouldApplyInput(cid,seq)) already assumes one
logical client per cid per session, so same-cid eviction is principled, not
a regression of multi-tab (which already collides on seq). Slots are freed
EAGERLY on error/terminate, not just async close; close is identity-matched
so a superseded socket's late close is a no-op. cid-less upgrades are
admitted anonymously up to the cap and never evict (backward-compat).
Client sends cid on the WS upgrade URL (?cid=, encoded, omitted if absent).
Tests: ws-connection-registry.test.ts (reconnect-reclaim at cap, rejects
N+1th distinct, eager-terminate frees slot, cid-less up-to-limit + no-evict,
late-close-no-evict, per-session isolation) + route integration in
ws-routes.test.ts (real upgrade through the cap). 45/45 across registry +
ws-routes + input-send-order + ws-reconnect-plan; tsc 0, build, prettier,
frontend-syntax clean.
Aamer Akhter
4ab89f9a4e