onClose callbacks are synchronous, so suspending cleanup has nowhere to run

[rnett]

Affected version / component: io.modelcontextprotocol:kotlin-sdk 0.14.0 — shared/Transport.kt, shared/AbstractTransport.kt, server/ServerSession.kt, server/Server.kt

Every close callback in the server stack is a plain () -> Unit. Transport.onClose, ServerSession.onClose, and Server.onClose all take a non-suspending block, and Server.close() invokes the registered chain synchronously. That leaves nowhere to run suspending teardown — closing pooled resources, or cancelling and then joining a child CoroutineScope — without wrapping it in runBlocking, which can stall or deadlock the close path.

The contract is synchronous at each layer. AbstractTransport composes the registered blocks as () -> Unit, and invokeOnCloseCallback() runs them inline inside runCatching, swallowing exceptions. ServerSession.onClose chains the same way, and its onClose() hook fires the chain synchronously. Server.onClose does likewise, and Server.close() calls it directly:

public suspend fun close() {
    notificationService.close()
    sessions.forEach { (_, session) -> session.close() }
    _onClose() // synchronous; no suspension point for teardown
}

close() is itself a suspend function, so the synchronous callback is a limitation of the API contract rather than something the implementation forces.

For context, #778 (shipped in 0.13.0) made the stdio transport's own close() suspending, with a graceful drain of in-flight outbound messages, and moved message handlers to Dispatchers.Default. It did not touch the user-facing close callbacks, which are still non-suspending everywhere, and there is no awaitClose() in 0.14.0.

Suggested fix

Offer a suspending close path — for example an onClose(block: suspend () -> Unit) overload, or a suspend close hook that Server.close() and session/transport teardown await. Either lets a server with suspending resources shut down deterministically instead of bridging through runBlocking.

Workaround

Keep the onClose block limited to synchronous work (flipping flags, cancelling a scope without joining it) and move all suspending teardown into a separate suspend function that callers invoke and await after close. Alternatively, treat the callback purely as a signal that resumes a coroutine, and run the suspending cleanup outside it:

suspend fun awaitClose(session: ServerSession) = suspendCoroutine { cont ->
    session.onClose { cont.resume(Unit) }
}
// awaitClose(session); mySuspendingCleanup()

Both work, but they push lifecycle management onto every caller, which is easy to get wrong.