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fix a race condition in idle timeouts

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squigz on freenode reported an issue where bots were responding to PING
on time, but were occasionally being timed out regardless. This was a race
condition: timeout was detected as idleTime >= it.quitTimeout, but if
the client responded promptly to its PING message and sent no further messages,
but the main loop subsequently slept for longer than expected (i.e., significantly
longer than quitTimeout), this condition would be met through no fault of the
client's.

The fix here is to explicitly track the last time the ping was sent, then test
!lastSeen.After(lastPinged) instead (making use of time.Time's monotonicity).
It is sufficient that the measurement of lastPinged happens-before the PING is sent.
This commit is contained in:
Shivaram Lingamneni 2017-12-02 20:14:28 -05:00
parent f5e7ec525d
commit ad1e00629b
1 changed files with 8 additions and 11 deletions
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19
irc/idletimer.go
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@ -38,7 +38,6 @@ type IdleTimer struct {
// mutable
client *Client
state TimerState
lastSeen time.Time
}
@ -49,7 +48,6 @@ func NewIdleTimer(client *Client) *IdleTimer {
idleTimeout: IdleTimeout,
quitTimeout: QuitTimeout,
client: client,
state: TimerUnregistered,
}
return &it
}
@ -58,17 +56,18 @@ func NewIdleTimer(client *Client) *IdleTimer {
// it will eventually be stopped.
func (it *IdleTimer) Start() {
it.Lock()
it.state = TimerUnregistered
it.lastSeen = time.Now()
it.Unlock()
go it.mainLoop()
}
func (it *IdleTimer) mainLoop() {
state := TimerUnregistered
var lastPinged time.Time
for {
it.Lock()
client := it.client
state := it.state
lastSeen := it.lastSeen
it.Unlock()
@ -76,7 +75,8 @@ func (it *IdleTimer) mainLoop() {
return
}
idleTime := time.Now().Sub(lastSeen)
now := time.Now()
idleTime := now.Sub(lastSeen)
var nextSleep time.Duration
if state == TimerUnregistered {
@ -87,8 +87,8 @@ func (it *IdleTimer) mainLoop() {
nextSleep = it.registerTimeout - idleTime
}
} else if state == TimerIdle {
if idleTime < it.quitTimeout {
// new ping came in after we transitioned to TimerIdle,
if lastSeen.After(lastPinged) {
// new pong came in after we transitioned to TimerIdle,
// transition back to active and process deadlines below
state = TimerActive
} else {
@ -100,6 +100,7 @@ func (it *IdleTimer) mainLoop() {
nextSleep = it.idleTimeout - idleTime
if nextSleep <= 0 {
state = TimerIdle
lastPinged = now
client.Ping()
// grant the client at least quitTimeout to respond
nextSleep = it.quitTimeout
@ -113,10 +114,6 @@ func (it *IdleTimer) mainLoop() {
return
}
it.Lock()
it.state = state
it.Unlock()
time.Sleep(nextSleep)
}
}