websocket-session-handshake-lifecycle

name

websocket-session-handshake-lifecycle

description

The order a WebSocket session has to be brought up and torn down — reader started before the first message because the answer comes back through it, the handshake settled on a deferred with a timeout, the close frame sent under a non-cancellable context, and an event buffer that suspends rather than drops. Use when a socket connects but the session never becomes usable, when a deliberate disconnect leaves the peer thinking you are still there, or when clients drift out of sync after a burst of traffic.

The handshake travels on the socket it is opening

A WebSocket has no request/response shape: the answer to your first message arrives on the same stream as everything else, through the same reader. So the reader has to exist before the message goes out, and something has to bridge "a frame arrived in the read loop" back to "the caller waiting for the handshake". A CompletableDeferred is that bridge.

// adapted — names generalized; the capability payload, the resume branch and the log lines elided
val live = client.webSocketSession(url)
session = live
val pending = CompletableDeferred<PeerCapabilities>()
handshake = pending
try {
    coroutineScope {
        val reader = launch { readFrames(live) }        // FIRST — it completes `pending`
        send(HELLO, ourCapabilities())
        val caps = withTimeoutOrNull(HANDSHAKE_TIMEOUT) { pending.await() }
            ?: throw IllegalStateException("Handshake timed out after $HANDSHAKE_TIMEOUT")
        onReady(caps)
        val pinger = launch { pingLoop() }
        reader.join()                                    // returns when the socket closes
        pinger.cancel()
    }
} finally {
    handshake = null
    session = null
    withContext(NonCancellable) { runCatching { live.close() } }
}

Traps

Sending before the reader is running is a deadlock you cannot see. The peer answers immediately; with nothing collecting incoming, the answer sits in the transport and the await runs out. What the logs show is a socket that opened fine and a timeout afterwards, which reads as a slow peer. The ordering is the fix and it is one line — but it is also the line a later refactor moves, so say why it is where it is.

Bound the handshake, and treat "connected" as the handshake completing rather than the socket opening. A peer that accepts the connection and then goes quiet is indistinguishable from a live one if your readiness flag is session != null. Define it as socket active and handshake completed, and let the timeout throw so the failure lands on the reconnect path instead of hanging a coroutine for the life of the process. That readiness definition is also where a retry budget must refill — see retry-needs-backoff-and-cap.

The close frame is a suspending call, so a cancelled job never sends it. The deliberate-disconnect path cancels the connection job and then wants a clean close; with an ordinary finally the close aborts at its first suspension point and the peer is left holding a session it thinks is alive, to be reaped by a read deadline minutes later. withContext(NonCancellable) around the close is what makes the polite path actually polite. Same reasoning for anything else in that finally that suspends.

Let the event buffer suspend; do not give it a dropping overflow policy. One lost protocol frame is a silent desync — the state machine simply never learns about a track change and stays confidently wrong. A generous buffer plus the default suspending behaviour trades a momentarily slower reader for correctness. Dropping is right for UI notifications and wrong for anything a state machine consumes.

A runCatching in the read loop cannot continue. Kotlin forbids break/continue inside a lambda even when it is inlined, so getOrElse { continue } does not compile and the tempting fix — folding the whole body into the lambda — silently changes what happens after a bad frame. Unwrap to a nullable first, then branch in the loop body. While you are there, skip frames of the wrong kind explicitly rather than letting them fall into the decoder.

Capture the resume token as frames pass through, not by asking the layer above. The token that makes a drop survivable arrives inside ordinary messages, often before anything upstream has subscribed. Read it in the frame handler, replay it on the next connection, and drop it on every path where it is spent: being removed by the peer, giving up on reconnecting, and the user deliberately leaving. That last one reads as bookkeeping and is not — skip it and the token outlives the departure, so the next connect resumes instead of joining. Replaying an expired token gets an error instead of a session, which then looks like the reconnect logic failing.

A send that returns false means not sent, and nothing will resend it. Reconnection replays the token, not the traffic. Callers have to handle the false — clear the optimistic state they just set, or surface it — because the alternative is a UI stuck waiting for a reply to a message that never left. pending-state-makes-waiting-legible is the shape for that.

Clear the session and handshake handles in the same finally that closes, not at the next connect. Both are what readiness is computed from, so a stale pair reports a healthy connection over a dead socket: every send is attempted against a closed session, returns false, and the caller above sees an inexplicable run of "not sent" with no disconnect anywhere. Nulling them at the start of the next connect is too late by exactly the interval nobody is connecting.

Give the ping loop both jobs and size it against the peer's read deadline. It is a keepalive (any inbound message refreshes the peer's deadline, so the interval must sit comfortably under it) and it is also the sample source for clock calibration, which wants its first few close together before opening up — see monotonic-clock-offset-sync. A send failure in that loop is the earliest signal the socket is gone; return from it rather than looping on a dead session.

Verifying it

grep -rn 'webSocketSession\|install(WebSockets)' --include='*.kt' . | grep -v '/build/'
grep -rn 'NonCancellable' --include='*.kt' . | grep -v '/build/'
grep -rn 'extraBufferCapacity\|onBufferOverflow\|BufferOverflow\.' --include='*.kt' . | grep -v '/build/'

The first is the census of socket-owning clients — three lines each, an import, an install and a construction site — and audit them separately, because these classes get copied. The second lists every place cancellation is suspended, and the useful reading is the comparison: cross off the clients from the first list, and any that does not appear here closes only by accident, on the paths where nothing happened to cancel it. Most of this list will be unrelated cleanup elsewhere in the tree; only the crossing-off matters. Finding just some of the first list's clients here is the normal result, and every one of them missing is its own finding. The third shows the buffer declarations: read the capacity, and note that a stream with no onBufferOverflow hit is using the default suspending policy, which is the one you want here — a BufferOverflow.DROP_* next to a protocol stream is the desync.

Then exercise the two paths that differ. Point the client at a peer that accepts connections and never answers (a bare listener will do): the timeout must fire once, the state must not report ready, and the reconnect path must take over. Then disconnect deliberately while traffic is flowing and check from the peer's side that a close frame arrived rather than the session lingering until its read deadline — that is the only observation that distinguishes a correct teardown from one that merely stopped.