wait.go

/*
Copyright 2014 Google 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 wait

import (
	"errors"
	"math/rand"
	"time"
)

// Jitter returns a time.Duration between duration and duration + maxFactor * duration,
// to allow clients to avoid converging on periodic behavior.  If maxFactor is 0.0, a
// suggested default value will be chosen.
func Jitter(duration time.Duration, maxFactor float64) time.Duration {
	if maxFactor <= 0.0 {
		maxFactor = 1.0
	}
	wait := duration + time.Duration(rand.Float64()*maxFactor*float64(duration))
	return wait
}

// ErrWaitTimeout is returned when the condition exited without success
var ErrWaitTimeout = errors.New("timed out waiting for the condition")

// ConditionFunc returns true if the condition is satisfied, or an error
// if the loop should be aborted.
type ConditionFunc func() (done bool, err error)

// Poll tries a condition func until it returns true, an error, or the timeout
// is reached. condition will always be invoked at least once but some intervals
// may be missed if the condition takes too long or the time window is too short.
// If you pass maxTimes = 0, Poll will loop until condition returns true or an
// error.
func Poll(interval, timeout time.Duration, condition ConditionFunc) error {
	return WaitFor(poller(interval, timeout), condition)
}

// WaitFunc creates a channel that receives an item every time a test
// should be executed and is closed when the last test should be invoked.
type WaitFunc func() <-chan struct{}

// WaitFor gets a channel from wait(), and then invokes c once for every value
// placed on the channel and once more when the channel is closed.  If c
// returns an error the loop ends and that error is returned, and if c returns
// true the loop ends and nil is returned. ErrWaitTimeout will be returned if
// the channel is closed without c every returning true.
func WaitFor(wait WaitFunc, c ConditionFunc) error {
	w := wait()
	for {
		_, open := <-w
		ok, err := c()
		if err != nil {
			return err
		}
		if ok {
			return nil
		}
		if !open {
			break
		}
	}
	return ErrWaitTimeout
}

// poller returns a WaitFunc that will send to the channel every
// interval until timeout has elapsed and then close the channel.
// Over very short intervals you may receive no ticks before
// the channel is closed closed.  If maxTimes is 0, the channel
// will never be closed.
func poller(interval, timeout time.Duration) WaitFunc {
	return WaitFunc(func() <-chan struct{} {
		ch := make(chan struct{})
		go func() {
			tick := time.NewTicker(interval)
			defer tick.Stop()
			var after <-chan time.Time
			if timeout != 0 {
				after = time.After(timeout)
			}
			for {
				select {
				case <-tick.C:
					ch <- struct{}{}
				case <-after:
					close(ch)
					return
				}
			}
		}()
		return ch
	})
}