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fsm.go
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fsm.go
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// Copyright 2017 VMware, Inc. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package telnet
import log "github.com/Sirupsen/logrus"
type state int
const (
dataState state = iota
optionNegotiationState
cmdState
subnegState
subnegEndState
errorState
)
type fsm struct {
curState state
tc *Conn
}
func newFSM() *fsm {
f := &fsm{
curState: dataState,
}
return f
}
func (fsm *fsm) start() {
defer func() {
log.Infof("FSM closed")
}()
for {
b := make([]byte, 4096)
n, err := fsm.readFromRawConnection(b)
if n > 0 {
for i := 0; i < n; i++ {
ch := b[i]
ns := fsm.nextState(ch)
fsm.curState = ns
}
}
if err != nil {
log.Debugf("connection read: %v", err)
fsm.tc.close()
break
}
}
}
func (fsm *fsm) readFromRawConnection(b []byte) (int, error) {
return fsm.tc.conn.Read(b)
}
// this function returns what the next state is and performs the appropriate action
func (fsm *fsm) nextState(ch byte) state {
var nextState state
b := []byte{ch}
switch fsm.curState {
case dataState:
if ch != Iac {
fsm.tc.dataRW.Write(b)
fsm.tc.dataWrittenCh <- true
nextState = dataState
} else {
nextState = cmdState
}
case cmdState:
if ch == Iac { // this is an escaping of IAC to send it as data
fsm.tc.dataRW.Write(b)
fsm.tc.dataWrittenCh <- true
nextState = dataState
} else if ch == Do || ch == Dont || ch == Will || ch == Wont {
fsm.tc.cmdBuffer.WriteByte(ch)
nextState = optionNegotiationState
} else if ch == Sb {
fsm.tc.cmdBuffer.WriteByte(ch)
nextState = subnegState
} else { // anything else
fsm.tc.cmdBuffer.WriteByte(ch)
fsm.tc.cmdHandlerWrapper(fsm.tc.handlerWriter, &fsm.tc.cmdBuffer)
fsm.tc.cmdBuffer.Reset()
nextState = dataState
}
case optionNegotiationState:
fsm.tc.cmdBuffer.WriteByte(ch)
opt := ch
cmd := fsm.tc.cmdBuffer.Bytes()[0]
fsm.tc.optCallback(cmd, opt)
fsm.tc.cmdBuffer.Reset()
nextState = dataState
case subnegState:
if ch == Iac {
nextState = subnegEndState
} else {
nextState = subnegState
fsm.tc.cmdBuffer.WriteByte(ch)
}
case subnegEndState:
if ch == Se {
fsm.tc.cmdBuffer.WriteByte(ch)
fsm.tc.cmdHandlerWrapper(fsm.tc.handlerWriter, &fsm.tc.cmdBuffer)
fsm.tc.cmdBuffer.Reset()
nextState = dataState
} else if ch == Iac { // escaping IAC
nextState = subnegState
fsm.tc.cmdBuffer.WriteByte(ch)
} else {
nextState = errorState
}
case errorState:
nextState = dataState
log.Infof("Finite state machine is in an error state. This should not happen for correct telnet protocol syntax")
}
return nextState
}