An EventSource that stops delivering does not always error. A proxy that
idle-closed the connection, a laptop resumed from sleep, a tailnet reconnect:
`onerror` never fires, the header dot stays green, and every SSE-driven surface
(tab status dots, sessions created on another device, renames) freezes until the
user reloads. Nothing on the client tracked stream liveness at all.
The server already wrote a keepalive every 15s, but as an SSE `:keepalive`
COMMENT, and comments are invisible to `EventSource` by spec, so there was
nothing a client could observe.
Server:
- `sse:heartbeat` under a new Transport category in the event registry
(155 constants now, both counts updated).
- `cleanupDeadClients()` writes that named frame (`{"t":<epoch ms>}`) instead of
the comment. Interval, tunnel padding and dead-socket eviction are unchanged.
The write stays per-client rather than going through `broadcast()`: the frame
carries no session data, so it needs no multi-user owner routing.
Client:
- `computeSseStale()` in constants.js, a pure policy beside
`computeConnectionLossUi`. Stale only when the transport believes it is
`connected`, the device is online, and no frame has arrived for 45s (three
missed heartbeats). The `connected`-only guard is also the loop breaker: a
forced reconnect leaves that state immediately, so the watchdog cannot re-fire
while one is in flight.
- The liveness stamp is applied inside `addListener` itself, so the
`_SSE_HANDLER_MAP` wrappers and the directly-registered listeners all feed it
from one place instead of three that can drift. The heartbeat's own listener
is a no-op that exists only to be registered, since `EventSource` drops named
events nobody listens for.
- A 5s watchdog forces `connectSSE()` when the policy says stale, and is cleared
at the top of `connectSSE()` and nowhere else (its only teardown path).
Recovery needs no new sync path: the reconnect re-runs `handleInit`, which
already rebuilds from the server. `visibilitychange` -> visible checks too,
riding the existing listener, since a background tab's timers are throttled
and a wake is exactly when a stream comes back zombie.
- The forced reconnect logs one diagnostic line: if a middlebox ever strips or
delays heartbeats, the failure mode is "silently reconnects every 45s", which
is undebuggable from a field report without it.
Tests: `test/sse-staleness.test.ts` (node VM over constants.js, threshold
boundaries and every not-stale guard) and `test/sse-heartbeat.test.ts` (drives
`cleanupDeadClients()` with fake replies: named frame not a comment, parseable
payload, padding only with a tunnel, dead clients still evicted).
Verified end to end on an isolated instance: with the stream closed client-side
(no `onerror`), a rename sticks, an out-of-band session stays invisible, then
the watchdog reconnects on its own and it appears without a reload.
Event names are part of the stable API contract, so this is a MINOR bump.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
