On Tue, Dec 18, 2018 at 5:35 AM Chris Burkert <burkert.ch...@gmail.com> wrote:
>
> I have a couple of goroutines sending multiple results over a channel - a
> simple fan-in. They signal the completion on a done channel. Main selects on
> the results and done channel in parallel. As the select is random main
> sometimes misses to select the last result. What would be the idiomatic way
> to prevent this and completely drain the result channel?
>
> Here is a minmal example which sometimes prints one 0 but should always print
> two of them:
>
> package main
>
> import (
> "fmt"
> )
>
> func do(rc chan<- int, dc chan<- bool) {
> rc <- 0
> dc <- true
> }
>
> func main() {
> worker := 2
> rc := make(chan int, worker)
> done := 0
> dc := make(chan bool, worker)
> for i := 0; i < worker; i++ {
> go do(rc, dc)
> }
> for done < worker {
> select {
> case <-dc:
> done++
> case r := <-rc:
> fmt.Println(r)
> }
> }
> }
I assume the workers can generate multiple results, as otherwise the
done marker seems pointless. In general the simplest way to signal
completion on a channel is to call close. The simplest way to call
close on a fan-in is to have another goroutine that waits for the
other goroutines and closes the channel. That might look like
package main
import (
"fmt"
"sync"
)
func do(rc chan<- int) {
rc <- 0
}
func main() {
worker := 2
rc := make(chan int, worker)
var wg sync.WaitGroup
wg.Add(worker)
for i := 0; i < worker; i++ {
go func() {
defer wg.Done()
do(rc)
}()
}
go func() {
wg.Wait()
close(rc)
}()
for r := range rc {
fmt.Println(r)
}
}
Ian
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