Re: [go-nuts] Re: An old problem: lack of priority select cases

[Robert Engels]

Because ctx.Done() and v being ready for read are independent events. You can not impose ordering on them unless there is an outer mutex that both events are subject to.

As an aside, this is why I think the best 'concurrent software' developers are those that have been exposed to at least some hardware design. Many programmers think in terms of 1 and 0 and everything being ordered. This is certainly not the case in hardware, nor concurrent software. (For example, in computer hardware, the clock/sync line is what is used as the outer controlling event, but still things like propagation times, etc. make even this simple statement not fully correct).

-----Original Message-----
From: T L
Sent: Oct 4, 2019 2:44 PM
To: golang-nuts
Subject: Re: [go-nuts] Re: An old problem: lack of priority select cases

On Friday, October 4, 2019 at 3:32:31 PM UTC-4, Robert Engels wrote:

You still are not understanding  proper concurrent design. Priority select cases do not matter in the case of asynchronous external events. 

It at least avoids code verbosity and improves code quantity..

BTW, I don't understand what you said. Could you elaborate more?

On Oct 4, 2019, at 1:46 PM, T L <tapi...@gmail.com> wrote:



I just found an example in the "context" package docs:

    //  // Stream generates values with DoSomething and sends them to out
    //  // until DoSomething returns an error or ctx.Done is closed.
    //  func Stream(ctx context.Context, out chan<- Value) error {
    //  	for {
    //  		v, err := DoSomething(ctx)
    //  		if err != nil {
    //  			return err
    //  		}
    //  		select {
    //  		case <-ctx.Done():
    //  			return ctx.Err()
    //  		case out <- v:
    //  		}
    //  	}
    //  }

It looks the send "out <- v" still has a possibility to be executed,
even if "ctx.Done()" is closed.
But if Go supports select case priority, then this will never happen.

On Wednesday, August 28, 2019 at 12:06:33 PM UTC-4, T L wrote:

The old thread: https://groups.google.com/forum/#!topic/golang-nuts/ZrVIhHCrR9o

Go channels are flexible, but in practice, I often encountered some situations in which channel are hard to use.

Given an example:

import "math/rand"

type Producer struct {
    data   chan int
    closed chan struct{}
}

func NewProducer() *Producer {
    p := &Producer {
        data:   make(chan int),
        closed: make(chan struct{}),
    }
   
    go p.run()
   
    return p
}

func (p *Produce) Stream() chan int {
    return p.data
}

func (p *Producer) run() {
    for {
        // If non-blocking cases are selected by their appearance order,
        // then the following slect block is a perfect use.
        select {
        case(0) <-p.closed: return
        case p.data <- rand.Int():
        }
    }
}

func (p *Produce) Clsoe() {
    close(p.closed)
    close(p.data)
}

func main() {
    p := NewProducer()
    for n := p.Stream() {
        // use n ...
    }
}

If the first case in the select block in the above example has a higher priority than the second one,

then coding will be much happier for the use cases like the above one.

In short, the above use case requires:

* for receivers, data streaming end is notified by the close of a channel.
* for senders, data will never be sent to closed channel.

But, as Go 1 doesn't support priority select cases, it is much tedious to implement the code

satisfying the above listed requirements. The final implementation is often very ugly and inefficient.

Does anyone else also experience the pain?

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