在select {case：channel}中更改频道

Question

I use a Ticker to execute tasks periodically, but I've got some problems when changing it. I'll change the ticker to a new one on receiving some messages and change the interval. Here's the sample code which will repro this problem:

package main

import (
    "fmt"
    "time"
)

type A struct {
    ticker *time.Ticker
}

func (a *A) modify() {
    a.ticker.Stop()
    a.ticker = time.NewTicker(time.Second)
}
func main() {
    a := new(A)
    a.ticker = time.NewTicker(time.Second)
    go func() {
        for {
            select {
            case <-a.ticker.C:
                fmt.Println("now")
                go a.modify()
                /*
                    default:
                        //fmt.Println("default")
                        time.Sleep(time.Millisecond * 100)
                */
            }
        }
    }()
    time.Sleep(time.Second * 60)
}

"now" will be printed only once. But it will be printed continously if I remove the "go", like this:

package main

import (
    "fmt"
    "time"
)

type A struct {
    ticker *time.Ticker
}

func (a *A) modify() {
    a.ticker.Stop()
    a.ticker = time.NewTicker(time.Second)
}
func main() {
    a := new(A)
    a.ticker = time.NewTicker(time.Second)
    go func() {
        for {
            select {
            case <-a.ticker.C:
                fmt.Println("now")
                a.modify()
                /*
                    default:
                        //fmt.Println("default")
                        time.Sleep(time.Millisecond * 100)
                */
            }
        }
    }()
    time.Sleep(time.Second * 60)
}

Also, if I leave the default clause un-commented, "now" can be printed continously. Can anyone explain how would this happen?

Answer 1

The problem is that goroutine runs asynchronously. your code behave like this when using a.modify():

1. get tick from a.ticker.C
2. stop old ticker, and create new a.ticker.C
3. wait for a.ticker.C using select

In this case, newly created a.ticker.C in 2. is identical to channel waiting in 3.

If you do 2. in goroutine. it may be done in following order

1. get tick from a.ticker.C
2. wait for a.ticker.C using select
3. stop old ticker, and create new a.ticker.C

In this case channel waiting in 2. is different from newly created channel in 3. . Since selecting channel is old one which is stopped, it never gets any tick.

You can confirm this behavior inserting some fmt.Printf and watch for the address of a.ticker.C.

func (a *A) modify() {
    a.ticker.Stop()
    fmt.Printf("ticker stopped: %p
", &a.ticker.C)
    a.ticker = time.NewTicker(time.Second)
    fmt.Printf("new ticker created: %p
", &a.ticker.C)
}
func main() {
    a := new(A)
    a.ticker = time.NewTicker(time.Second)
    go func() {
        for {
            fmt.Printf("waiting for ticker: %p
", &a.ticker.C)
            select {
        ....

with a.modify():

waiting for ticker: 0xc420010100
ticker stopped: 0xc420010100
new ticker created: 0xc420068000
waiting for ticker: 0xc420068000
ticker stopped: 0xc420068000
new ticker created: 0xc420068080
waiting for ticker: 0xc420068080

with go a.modify():

waiting for ticker: 0xc420010100
waiting for ticker: 0xc420010100
ticker stopped: 0xc420010100
new ticker created: 0xc420066040

you can see that with go a.modify() you are not waiting for a newly created channel.

UPDATE for behavior of default:

When using default: with go a.modify(), it will behave like this.

1. wait for a.ticker.C, got tick, call go a.modify() which does 3.
2. wait for a.ticker.C, got nothing, so fallback to default and sleep 100ms.
3. stop old ticker, update a.ticker.C
4. wait for a.ticker.C, got nothing, so fallback to default and sleep 100ms.
5. wait for a.ticker.C, got nothing, so fallback to default and sleep 100ms.
6. wait for a.ticker.C, got nothing, so fallback to default and sleep 100ms.

.....

1. wait for a.ticker.C, got tick, call go a.modify()

The point is that for{} loop can keep on going even when you got nothing from a.ticker.C. You can confirm the behavior with same code.

waiting ticker: 0xc420010100     <-- 1.
now                              <-- 1.
waiting ticker: 0xc420010100     <-- 2.
default                          <-- 2.
ticker stopped: 0xc420010100     <-- 3.
new ticker created: 0xc420066240 <-- 3.
waiting ticker: 0xc420066240     <-- 4.
default                          <-- 4.