The cliff-detector diagnostic logs from the user's two-phone repro
(commit f2205dc, run 15:06) showed the smoking gun — every tick from
the receiver-side cliff detector said `announced=0` even though an
Active announce had clearly been received earlier (the session-level
`pumpAnnounceWatch` logged it). The cliff detector only acts on
speakers that are in `_announcedSpeakers`, so it never tripped, even
when frames clearly stopped flowing for 6+ s.
Root cause: `MoqLiteSession.announce` builds a `MutableSharedFlow`
with `replay = 0` to bridge the bidi pump and the consumer's
`collect`. With Kotlin's `MutableSharedFlow(replay = 0)`, an `emit`
that happens BEFORE the first collector subscribes is silently
dropped — there's no buffer for items emitted into a flow with no
subscribers. The bidi pump is launched inside `announce()` and
starts collecting `bidi.incoming()` immediately; the caller (the
flow returned by `MoqLiteNestsListener.announces()`) attaches its
collector AFTER `session.announce()` returns. In between, if the
relay sends an Active update (which it does the moment the
broadcaster's session is registered), the pump's emit hits a flow
with no collectors and is lost.
In the user's logs both an internal session-level
`pumpAnnounceWatch` AND the VM-level `observeAnnounces` open
separate announce bidis. The internal one started first and won
the race for its bidi's Active. The VM-level one lost — the bidi
pump emitted Active before the consumer subscribed, the SharedFlow
dropped it, and `_announcedSpeakers` never populated.
Fix: change the announce SharedFlow to `replay = 64` with
`BufferOverflow.DROP_OLDEST`. Late-attaching collectors now see up
to the last 64 announces (far more than nests rooms have
speakers — typical stage size is ≤ 10). The bidi pump's emit
still never suspends, so QUIC-level backpressure can't propagate.
The replay cache evicts the oldest item if more than 64 announces
arrive before any collector attaches; for the production workload
this is unreachable.
Tests: nestsClient jvmTest passes 57/57 unchanged
(MoqLiteCodecTest, MoqLiteFrameBufferTest, MoqLitePathTest,
MoqLiteSessionTest, NestBroadcasterTest, NestPlayerTest). The
session-level pumpAnnounceWatch path used the same SharedFlow but
its caller subscribes from `scope.launch { pumpAnnounceWatch(...) }`
inside `ensureAnnounceWatchStarted` — close enough in time that it
won the race in production. The fix closes the race for both
callers regardless of timing.
Claude
d6517cf346