flush(9P) - Plan 9 from User Space

| FLUSH(9P) | FLUSH(9P)
| NAME
| flush – abort a message
SYNOPSIS
| size[4] `Tflush` tag[2] oldtag[2] size[4] `Rflush` tag[2]
DESCRIPTION
| 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 `Tflush`es be received for a pending request, they must be answered in order. A `Rflush` for any of the multiple `Tflush`es 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.
ENTRY POINTS
| The 9pclient(3) library does not generate `flush` transactions.. 9pserve(4) generates `flush` transactions to cancel transactions pending when a client hangs up.