How to test if a channel is close and only send to it when it's not closed

Question

In Go, if a channel channel is closed, I can still read from it using the following syntax and I can test ok to see if it's closed.

value, ok := <- channel
if !ok {
    // channel was closed and drained
}

However, if I don't know whether a channel is closed and blindly write to it, I may got an error. I want to know if there is any way that I can test the channel and only write to it when it's not closed. I ask this question is because sometimes I don't know if a channel is closed or not in a goroutine.

Answer 1

You can't. The rule of thumb here is that only writers should close channels, this way you know that you shouldn't write to that channel anymore.

Some simple code would look like this:

for i := 0; i < 100; i++ {
    value := calculateSomeValue()
    channel <- value
}

close(channel) //indicate that we will no more send values

Answer 2

If few goroutins write to channel you can also nil it instead of close and use select to read and write. Something like this

ch := make(chan int, 1)
var value int
ch <- 5
select {
case value = <-ch:
    fmt.Println("value", value)
default:
    fmt.Println("oops")
}
ch = nil
select {
case ch <- 5:
default:
    fmt.Println("don't panic")
}
select {
case value = <-ch:
    fmt.Println("value", value)
default:
    fmt.Println("oops")
}

Try it works https://play.golang.org/p/sp8jk961TB

Answer 3

You can't. You could split up the work where messages are sent to a channel and another go routine that reads from a channel.

You should could add a donechannel to signal when the reader is done.

Example.

package main

import (
    "fmt"
)

func main() {

  mychan := make(chan int)
  donechannel := make(chan struct{})
  go pushchannel(mychan)
  go drainchan(mychan, donechannel)
  _, ok := <-donechannel; if !ok {
     fmt.Println("can not read from donechannel this means donechannel is closed, means we are done :)")
  }
  fmt.Println("Done")
}

func pushchannel(ch chan int) {
 fmt.Println("pushing to chan")
 for i:=0; i<=10; i++ {
     fmt.Printf("pushed %v\n",i)
     ch<-i
 }
 close(ch)
}

func drainchan(ch chan int, donechannel chan struct{}) {
  fmt.Println("draining")
  for {
    res, ok := <- ch
    if !ok {
       fmt.Println("result channel is closed, we can signal the donechannel now.")
        close(donechannel)
       break 
    } else {
      fmt.Printf("got result %v\n", res)
    }
  }
}

https://play.golang.org/p/BMyMkrqWF7s