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.TH FLUSH 5
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.SH NAME
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flush \- abort a message
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.SH SYNOPSIS
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.ta \w'\fLTflush 'u
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.IR size [4]
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.B Tflush
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.IR tag [2]
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.IR oldtag [2]
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.br
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.IR size [4]
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.B Rflush
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.IR tag [2]
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.SH DESCRIPTION
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When the response to a request is no longer needed, such as when
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a user interrupts a process doing a
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.IR read (2),
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a
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.B Tflush
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request is sent to the server to purge the pending response.
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The message being flushed is identified by
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.IR oldtag .
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The semantics of
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.B flush
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depends on messages arriving in order.
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.PP
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The server should answer the
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.B flush
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message immediately.
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If it recognizes
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.I oldtag
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as the tag of a pending transaction, it should abort any pending response
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and discard that tag.
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In either case, it should respond with an
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.B Rflush
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echoing the
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.I tag
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(not
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.IR oldtag )
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of the
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.B Tflush
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message.
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A
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.B Tflush
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can never be responded to by an
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.B Rerror
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message.
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.PP
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The server may respond to the pending request before
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responding to the
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.BR Tflush .
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It is possible for a client to send multiple
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.B Tflush
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messages for a particular pending request.  Each
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subsequent
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.B Tflush
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must contain as
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.I oldtag
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the tag of the pending request (not a previous
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.BR Tflush ).
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Should multiple
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.BR Tflush es
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be received for a pending request, they must be answered in
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order.  A
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.B Rflush
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for any of the multiple
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.BR Tflush es
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implies an answer for all previous ones.  Therefore, should
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a server receive a request and then multiple flushes for that
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request, it need respond only to the last flush.
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.PP
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When the client sends a
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.BR Tflush ,
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it must wait to receive the corresponding
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.B Rflush
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before reusing
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.I oldtag
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for subsequent messages.
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If a response to the flushed request is received before the
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.BR Rflush ,
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the client must honor the response
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as if it had not been flushed,
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since the completed request may signify a state change in the server.
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For instance,
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.B Tcreate
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may have created a file and
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.B Twalk
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may have allocated a fid.
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If no response is received before the
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.BR Rflush ,
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the flushed transaction is considered to have been canceled,
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and should be treated as though it had never been sent.
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.PP
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Several exceptional conditions are handled correctly by the above specification:
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sending multiple flushes for a single tag,
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flushing after a transaction is completed,
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flushing a
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.BR Tflush ,
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and flushing an invalid tag.