tailscale/syncs/watchdog.go

95 lines
2.1 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
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
}