how to avoid sending on a closed channel

[long...@gmail.com]

I can't find a design to avoid sending on a closed channel. Consider following scenario:

There are 3 go routines, producer, consumer, main. And a channel C. 

"producer" sends new task to channel C

"consumer" receives a task from channel C, and processing it. 

"main" is responsible for run cleanup when shutdown.

Following execution sequence may cause a "send on closed channel" panic.

1. "consumer" fetched a job from channel C then processing, the it is slow

1. "producer" trying to send, but blocked on C because there is no buffer available in channel C

2. service is going to shutdown, so "main" close the channel C

3. when the channel C is closed, the "producer" is waked up, and panic. If no recover() called, the job is lost too.

The code can be found here:

http://play.golang.org/p/MCQ2U33v2m

Do I have to wrap every send operation with a recover() to detect the panic? 

[Ian Davis]

Generally the producer should be in charge of closing the channel so it knows not to send on it. Main could signal the producer to close the channel.

 

Ian

[Jérôme Champion]

The procducer should close the channel.

I think "main" should notify "producer" that it has to stop. Producer can have a select like:

select {

case c <- 42:

break

case <-quit:

break

[Jérôme Champion]

And of course closing the channel in the <-quit case.

[Roberto Zanotto]

Normally you'd want to structure it like this: https://play.golang.org/p/CEO_flcINB

It's common to use a separate channel to signal when you are done. In the example I wrote, if we closed "c" instead of using quit, the consumer would have received a zero value message, which is what happens when you read from a closed channel. So we wouldn't be able to distinguish between receiving 0 and the channel being closed.

It's fun to write concurrent programs in Go, but at first it takes a bit of time to get comfortable with asynchronous/synchronous communication, goroutines and co. I'd suggest looking for some videos and tutorials online.

Cheers

[Matt Harden]

No, that's not idiomatic. It is possible to distinguish between a zero value sent on a channel and the channel closing by using comma-ok syntax:

v, ok := <-mychannel

For a consumer of a single channel as in your example, it would be more usual to use range(), which exits the for loop when the channel is closed. https://play.golang.org/p/3wQNexfISO

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[Roberto Zanotto]

Ooops, thanks for the corrections :)