feat: add SubscribeOpt.Receipt - #148
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Conn.Subscribe returns as soon as the SUBSCRIBE frame has been handed to the I/O goroutine, so a message published right afterwards can be lost: the server may not have finished creating the subscription (and any underlying queue/binding) yet. SubscribeOpt.Receipt asks for a RECEIPT on the SUBSCRIBE frame and waits for it before returning, bounded by ConnOpt.SubscribeReceiptTimeout (default 30s, ErrSubscribeReceiptTimeout on expiry). The confirmation is routed via a new writeRequest.ReceiptC rather than writeRequest.C, because for a SUBSCRIBE request C is already the subscription's own frame channel, which the RECEIPT would otherwise consume and close. If the confirmation never arrives, the subscription is unsubscribed again before returning the error. The SUBSCRIBE frame is already on its way to the server at that point and the caller gets no handle back, so otherwise the subscription's readLoop goroutine would leak, and any message the server did deliver for it would eventually fill the unreachable Subscription.C and block processLoop for every other subscription on the connection. Subscribe now also releases closeMutex once the request has been handed over, instead of holding it across the receipt wait, so a slow server does not stall every other operation on the connection. On the (rare) path where the request never reaches processLoop, the frame channel is closed so readLoop does not block on it forever. Reply-to (temporary queue) subscriptions are never sent to the server - processLoop registers the channel locally and drops the frame - so nothing can produce a RECEIPT for them. The receipt header is dropped for those rather than failing every such call after the timeout.
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Some feedback. I'm still thinking about Subscribe changes and abandon()
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This PR adds an option to request the receipt for the SUBSCRIBE command, so the client can wait until the subscription is confirmed.