mirror of
https://github.com/tailscale/tailscale.git
synced 2024-12-05 07:55:36 +00:00
297b3d6fa4
Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
96 lines
2.2 KiB
Go
96 lines
2.2 KiB
Go
// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
|
|
// Use of this source code is governed by a BSD-style
|
|
// license that can be found in the LICENSE file.
|
|
|
|
package syncs
|
|
|
|
import (
|
|
"context"
|
|
"sync"
|
|
"time"
|
|
)
|
|
|
|
// Watch monitors mu for contention.
|
|
// On first call, and at every tick, Watch locks and unlocks mu.
|
|
// (Tick should be large to avoid adding contention to mu.)
|
|
// Max is the maximum length of time Watch will wait to acquire the lock.
|
|
// The time required to lock mu is sent on the returned channel.
|
|
// Watch exits when ctx is done, and closes the returned channel.
|
|
func Watch(ctx context.Context, mu sync.Locker, tick, max time.Duration) chan time.Duration {
|
|
// Set up the return channel.
|
|
c := make(chan time.Duration)
|
|
var (
|
|
closemu sync.Mutex
|
|
closed bool
|
|
)
|
|
sendc := func(d time.Duration) {
|
|
closemu.Lock()
|
|
defer closemu.Unlock()
|
|
if closed {
|
|
// Drop values written after c is closed.
|
|
return
|
|
}
|
|
c <- d
|
|
}
|
|
closec := func() {
|
|
closemu.Lock()
|
|
defer closemu.Unlock()
|
|
close(c)
|
|
closed = true
|
|
}
|
|
|
|
// check locks the mutex and writes how long it took to c.
|
|
// check returns ~immediately.
|
|
check := func() {
|
|
// Start a race between two goroutines.
|
|
// One locks the mutex; the other times out.
|
|
// Ensure that only one of the two gets to write its result.
|
|
// Since the common case is that locking the mutex is fast,
|
|
// let the timeout goroutine exit early when that happens.
|
|
var sendonce sync.Once
|
|
done := make(chan bool)
|
|
go func() {
|
|
start := time.Now()
|
|
mu.Lock()
|
|
mu.Unlock()
|
|
elapsed := time.Since(start)
|
|
if elapsed > max {
|
|
elapsed = max
|
|
}
|
|
close(done)
|
|
sendonce.Do(func() { sendc(elapsed) })
|
|
}()
|
|
go func() {
|
|
select {
|
|
case <-time.After(max):
|
|
// the other goroutine may not have sent a value
|
|
sendonce.Do(func() { sendc(max) })
|
|
case <-done:
|
|
// the other goroutine sent a value
|
|
}
|
|
}()
|
|
}
|
|
|
|
// Check once at startup.
|
|
// This is mainly to make testing easier.
|
|
check()
|
|
|
|
// Start the watchdog goroutine.
|
|
// It checks the mutex every tick, until ctx is done.
|
|
go func() {
|
|
ticker := time.NewTicker(tick)
|
|
for {
|
|
select {
|
|
case <-ctx.Done():
|
|
closec()
|
|
ticker.Stop()
|
|
return
|
|
case <-ticker.C:
|
|
check()
|
|
}
|
|
}
|
|
}()
|
|
|
|
return c
|
|
}
|