how to select for input on a dynamic list of channels in Go?

Go has a mechanism to do a blocking read from one of several channels, the select statement. So you can say

select {
    case <- c1:
    case <- c2:
}

will block until we get input from either of these two channels. Very nice.

But this requires that I specify in the source code how many channels I want to poll. What if I have a slice or array of channels and I want to block until I get input on any of them?

Since go1.1, there's a proper API to dynamically do select sets.

Here's a complete and usable example:

package main

import (
	"log"
	"reflect"
)

func sendToAny(ob int, chs []chan int) int {
	set := []reflect.SelectCase{}
	for _, ch := range chs {
		set = append(set, reflect.SelectCase{
			Dir:  reflect.SelectSend,
			Chan: reflect.ValueOf(ch),
			Send: reflect.ValueOf(ob),
		})
	}
	to, _, _ := reflect.Select(set)
	return to
}

func recvFromAny(chs []chan int) (val int, from int) {
	set := []reflect.SelectCase{}
	for _, ch := range chs {
		set = append(set, reflect.SelectCase{
			Dir:  reflect.SelectRecv,
			Chan: reflect.ValueOf(ch),
		})
	}
	from, valValue, _ := reflect.Select(set)
	val = valValue.Interface().(int)
	return
}

func main() {
	channels := []chan int{}
	for i := 0; i < 5; i++ {
		channels = append(channels, make(chan int))
	}

	go func() {
		for i := 0; i < 10; i++ {
			x := sendToAny(i, channels)
			log.Printf("Sent %v to ch%v", i, x)
		}
	}()

	for i := 0; i < 10; i++ {
		v, x := recvFromAny(channels)
		log.Printf("Received %v from ch%v", v, x)
	}
}

You can play around with it interactively on the playground

Just a thought, but you could use a multiplexing pattern, where you spawn off a goroutine with 2 channels that blocks on both and sends the output to a new channel. Then you can just build up a tree of these dynamically from your list that funnels everything down to a single channel, which you then read on.

package main

import "fmt"

func main() {
    c1 := make(chan int)
    c2 := make(chan int)

    go func() { c1 <- 1 }()
    go func() { c2 <- 2 }()

    cs := []chan int{c1, c2}
    cm := make(chan [2]int)

    for idx, c := range(cs) {
        go func(idx int, c chan int) {
            cm <- [2]int{idx, <-c}
        }(idx, c)
    }

    fmt.Print(<-cm)
    fmt.Print(<-cm)
}

prints [0 1][1 2] (or maybe [1 2][0 1]).

Perhaps something like this may apply?

// multiplex takes a slice of chan ints and returns a channel
// that multiplexes between all of them.
func multiplex(chs []<-chan int) <-chan int {
	c := make(chan int)
	d := make(chan bool)
	for _, ch := range chs {
		go func(ch <-chan int) {
			for r := range ch {
				c <- r
			}
			d <- true
		}(ch)
	}
	go func() {
		for i := 0; i < len(chs); i++ {
			<-d
		}
		close(c)
	}()
	return c
}