Why does Go panic on writing to a closed channel?

Issue

Why does Go panic on writing to a closed channel?

While one can use the value, ok := <-channel idiom for reading from channels, and thus the ok result can be tested for hitting a closed channel:

// reading from closed channel

package main

import "fmt"

func main() {
    ch := make(chan int, 1)
    ch <- 2
    close(ch)

    read(ch)
    read(ch)
    read(ch)
}

func read(ch <-chan int) {
    i,ok := <- ch   
    if !ok {
        fmt.Printf("channel is closed\n")
        return
    }
    fmt.Printf("read %d from channel\n", i)
}

Output:

read 2 from channel
channel is closed
channel is closed

Run “reading from closed channel” on Playground

Writing to a possibly closed channel is more convoluted, because Go will panic if you simply try to write when the channel is closed:

//writing to closed channel

package main

import (
    "fmt"
)

func main() {
    output := make(chan int, 1) // create channel
    write(output, 2)
    close(output) // close channel
    write(output, 3)
    write(output, 4)
}

// how to write on possibly closed channel
func write(out chan int, i int) (err error) {

    defer func() {
        // recover from panic caused by writing to a closed channel
        if r := recover(); r != nil {
            err = fmt.Errorf("%v", r)
            fmt.Printf("write: error writing %d on channel: %v\n", i, err)
            return
        }

        fmt.Printf("write: wrote %d on channel\n", i)
    }()

    out <- i // write on possibly closed channel

    return err
}

Output:

write: wrote 2 on channel
write: error writing 3 on channel: send on closed channel
write: error writing 4 on channel: send on closed channel

Run “writing to closed channel” on Playground

As far as I know, there is not a simpler idiom for writing into a possibly closed channel without panicking. Why not? What is the reasoning behind such an asymmetric behavior between read and write?

Solution

From the Go Language Spec:

For a channel c, the built-in function close(c) records that no more
values will be sent on the channel. It is an error if c is a
receive-only channel. Sending to or closing a closed channel causes a
run-time panic. Closing the nil channel also causes a run-time panic.
After calling close, and after any previously sent values have been
received, receive operations will return the zero value for the
channel’s type without blocking. The multi-valued receive operation
returns a received value along with an indication of whether the
channel is closed.

If you write to a closed channel, your program will panic. You could potentially catch this error with recover if you really want to do that, but being in a situation where you don’t know whether the channel you are writing to is open is usually a sign of a bug in the program.

Some quotes:

Here is a motivation:

A channel “close” is really just a send of a special value on a
channel. It is a special value that promises that no more values will
be sent. Attempting to send a value on a channel after it has been
closed will panic, since actually sending the value would violate the
guarantee provided by close. Since a close is just a special kind of
send, it is also not permitted after the channel has been closed.

Here is another:

The only use of channel close is to signal to the reader that there
are no more values to come. That only makes sense when there is a
single source of values, or when multiple sources coordinate. There
is no reasonable program in which multiple goroutines close a channel
without communicating. That would imply that multiple goroutines
would know that there are no more values to send–how could they
determine that if they don’t communicate?

(Ian Lance Taylor)

Here is another:

Closing a channel releases it as a resource. It makes no more sense to
close a channel multiple times than it makes to close a file
descriptor multiple times, or free a block of allocated memory
multiple times. Such actions imply the code is broken, which is why
closing a closed channel triggers a panic.

(Rob Pike)

Source: Go design detail rationale question – channel close

Answered By – Justlike

Answer Checked By – Gilberto Lyons (GoLangFix Admin)

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