Just realized what you meant about buffered channels. I'm wrong, and sorry
for the confusion.
+1 to your original solution (combined with setting channels to nil).
Diane Looney
On Tue, Dec 18, 2018 at 11:31 AM Chris Burkert <burkert.ch...@gmail.com>
wrote:
> Hello Ian, all,
> yes, the workers generate multiple results. I was able to use your
> proposal with the waiting goroutine which closes the channel. Unfortunately
> my initial minimal example was not so minimal. It is a little more
> complicated, as I have multiple "result" channels with different types and
> I don't know how many values I get. So I can't just range over one channel
> but instead use select with the ok from "val, ok := <-channel" to count the
> closed channels which finally ends the loop. Below is what I came up with
> (using Johns approach to run it multiple times) and it seems to be
> deterministic.
> Many thanks!
> PS: with buffered channels I have to disable closed channels with "channel
> = nil" to make it work.
>
> package main
>
> import (
> "fmt"
> "sync"
> )
>
> func do(fc chan<- int, rc chan<- string) {
> fc <- 42
> fc <- 43
> rc <- "foo"
> }
>
> func main() {
> w := 4
> r := 100000
> n := 0
> for i := 0; i < r; i++ {
> n = n + run(w)
> }
> fmt.Printf("Got %d, expected %d\n", n, 3*w*r)
> }
>
> func run(worker int) int {
> fc := make(chan int)
> rc := make(chan string)
> var wg sync.WaitGroup
> wg.Add(worker)
> for i := 0; i < worker; i++ {
> go func() {
> defer wg.Done()
> do(fc, rc)
> }()
> }
> go func() {
> wg.Wait()
> close(fc)
> close(rc)
> }()
> n := 0
> closed := 0
> for closed < 2 {
> select {
> case _, ok := <-fc:
> if ok {
> n++
> } else {
> closed++
> }
> case _, ok := <-rc:
> if ok {
> n++
> } else {
> closed++
> }
> }
> }
> return n
> }
>
> Am Di., 18. Dez. 2018 um 15:50 Uhr schrieb Ian Lance Taylor <
> i...@golang.org>:
>
>> 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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