|
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.
|
