flush – abort a message

size[4] Tflush tag[2] oldtag[2]
size[4] Rflush tag[2]

When the response to a request is no longer needed, such as when a user interrupts a process doing a read(9p), a Tflush request is sent to the server to purge the pending response. The message being flushed is identified by oldtag. The semantics of flush depends on messages arriving in order.
The server should answer the flush message immediately. If it recognizes oldtag as the tag of a pending transaction, it should abort any pending response and discard that tag. In either case, it should respond with an Rflush echoing the tag (not oldtag) of the Tflush message. A Tflush can never be responded to by an Rerror message.
The server may respond to the pending request before responding to the Tflush. It is possible for a client to send multiple Tflush messages for a particular pending request. Each subsequent Tflush must contain as oldtag the tag of the pending request (not a previous Tflush). Should multiple Tflushes be received for a pending request, they must be answered in order. A Rflush for any of the multiple Tflushes implies an answer for all previous ones. Therefore, should a server receive a request and then multiple flushes for that request, it need respond only to the last flush.
When the client sends a Tflush, it must wait to receive the corresponding Rflush before reusing oldtag for subsequent messages. If a response to the flushed request is received before the Rflush, the client must honor the response as if it had not been flushed, since the completed request may signify a state change in the server. For instance, Tcreate may have created a file and Twalk may have allocated a fid. If no response is received before the Rflush, the flushed transaction is considered to have been canceled, and should be treated as though it had never been sent.
Several exceptional conditions are handled correctly by the above specification: sending multiple flushes for a single tag, flushing after a transaction is completed, flushing a Tflush, and flushing an invalid tag.

The 9pclient(3) library does not generate flush transactions.. 9pserve(4) generates flush transactions to cancel transactions pending when a client hangs up.

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