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// Copyright 2020 The gVisor Authors.
//
// 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 time
import (
"sync"
"time"
)
// AfterFunc waits for duration to elapse according to clock then runs fn.
// The timer is started immediately and will fire exactly once.
func AfterFunc(clock Clock, duration time.Duration, fn func()) *VariableTimer {
timer := &VariableTimer{
clock: clock,
}
timer.notifier = functionNotifier{
fn: func() {
// tcpip.Timer.Stop() explicitly states that the function is called in a
// separate goroutine that Stop() does not synchronize with.
// Timer.Destroy() synchronizes with calls to Listener.NotifyTimer().
// This is semantically meaningful because, in the former case, it's
// legal to call tcpip.Timer.Stop() while holding locks that may also be
// taken by the function, but this isn't so in the latter case. Most
// immediately, Timer calls Listener.NotifyTimer() while holding
// Timer.mu. A deadlock occurs without spawning a goroutine:
// T1: (Timer expires)
// => Timer.Tick() <- Timer.mu.Lock() called
// => Listener.NotifyTimer()
// => Timer.Stop()
// => Timer.Destroy() <- Timer.mu.Lock() called, deadlock!
//
// Spawning a goroutine avoids the deadlock:
// T1: (Timer expires)
// => Timer.Tick() <- Timer.mu.Lock() called
// => Listener.NotifyTimer() <- Launches T2
// T2:
// => Timer.Stop()
// => Timer.Destroy() <- Timer.mu.Lock() called, blocks
// T1:
// => (returns) <- Timer.mu.Unlock() called
// T2:
// => (continues) <- No deadlock!
go func() {
timer.Stop()
fn()
}()
},
}
timer.Reset(duration)
return timer
}
// VariableTimer is a resettable timer with variable duration expirations.
// Implements tcpip.Timer, which does not define a Destroy method; instead, all
// resources are released after timer expiration and calls to Timer.Stop.
//
// Must be created by AfterFunc.
type VariableTimer struct {
// clock is the time source. clock is immutable.
clock Clock
// notifier is called when the Timer expires. notifier is immutable.
notifier functionNotifier
// mu protects t.
mu sync.Mutex
// t stores the latest running Timer. This is replaced whenever Reset is
// called since Timer cannot be restarted once it has been Destroyed by Stop.
//
// This field is nil iff Stop has been called.
t *Timer
}
// Stop implements tcpip.Timer.Stop.
func (r *VariableTimer) Stop() bool {
r.mu.Lock()
defer r.mu.Unlock()
if r.t == nil {
return false
}
_, lastSetting := r.t.Swap(Setting{})
r.t.Destroy()
r.t = nil
return lastSetting.Enabled
}
// Reset implements tcpip.Timer.Reset.
func (r *VariableTimer) Reset(d time.Duration) {
r.mu.Lock()
defer r.mu.Unlock()
if r.t == nil {
r.t = NewTimer(r.clock, &r.notifier)
}
r.t.Swap(Setting{
Enabled: true,
Period: 0,
Next: r.clock.Now().Add(d),
})
}
// functionNotifier is a TimerListener that runs a function.
//
// functionNotifier cannot be saved or loaded.
type functionNotifier struct {
fn func()
}
// NotifyTimer implements ktime.TimerListener.NotifyTimer.
func (f *functionNotifier) NotifyTimer(uint64, Setting) (Setting, bool) {
f.fn()
return Setting{}, false
}
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