Multiple producers, single consumer: all goroutines are asleep - deadlock - go

Question

I have been following a pattern of checking if there is anything in the channel before proceeding with work:
func consume(msg <-chan message) {
for {
if m, ok := <-msg; ok {
fmt.Println("More messages:", m)
} else {
break
}
}
}
that is based on this video. Here is my full code:
package main
import (
"fmt"
"strconv"
"strings"
"sync"
)
type message struct {
body string
code int
}
var markets []string = []string{"BTC", "ETH", "LTC"}
// produces messages into the chan
func produce(n int, market string, msg chan<- message, wg *sync.WaitGroup) {
// for i := 0; i < n; i++ {
var msgToSend = message{
body: strings.Join([]string{"market: ", market, ", #", strconv.Itoa(1)}, ""),
code: 1,
}
fmt.Println("Producing:", msgToSend)
msg <- msgToSend
// }
wg.Done()
}
func receive(msg <-chan message, wg *sync.WaitGroup) {
for {
if m, ok := <-msg; ok {
fmt.Println("Received:", m)
} else {
fmt.Println("Breaking from receiving")
break
}
}
wg.Done()
}
func main() {
wg := sync.WaitGroup{}
msgC := make(chan message, 100)
defer func() {
close(msgC)
}()
for ix, market := range markets {
wg.Add(1)
go produce(ix+1, market, msgC, &wg)
}
wg.Add(1)
go receive(msgC, &wg)
wg.Wait()
}
If you try to run it, we get the deadlock at the very end before we ever print a message that we are about to break. Which, tbh, makes sense, since the last time, when there is nothing else in the chan, we are trying to pull the value out, and so we get this error. But then this pattern isn't workable if m, ok := <- msg; ok. How do I make this code work & why do I get this deadlock error (presumably this pattern should work?).

Answer 1

Given that you do have multiple writers on a single channel, you have a bit of a challenge, because the easy way to do this in Go in general is to have a single writer on a single channel, and then have that single writer close the channel upon sending the last datum:
func produce(... args including channel) {
defer close(ch)
for stuff_to_produce {
ch <- item
}
}
This pattern has the nice property that no matter how you get out of produce, the channel gets closed, signalling the end of production.
You're not using this pattern—you deliver one channel to many goroutines, each of which can send one message—so you need to move the close (or, of course, use yet some other pattern). The simplest way to express the pattern you need is this:
func overall_produce(... args including channel ...) {
var pg sync.WaitGroup
defer close(ch)
for stuff_to_produce {
pg.Add(1)
go produceInParallel(ch, &pg) // add more args if appropriate
}
pg.Wait()
}
The pg counter accumulates active producers. Each must call pg.Done() to indicate that it is done using ch. The overall producer now waits for them all to be done, then it closes the channel on its way out.
(If you write the inner produceInParallel function as a closure, you don't need to pass ch and pg to it explicitly. You may also write overallProducer as a closure.)
Note that your single consumer's loop is probably best expressed using the for ... range construct:
func receive(msg <-chan message, wg *sync.WaitGroup) {
for m := range msg {
fmt.Println("Received:", m)
}
wg.Done()
}
(You mention an intent to add a select to the loop so that you can do some other computing if a message is not yet ready. If that code cannot be spun off into independent goroutines, you will in fact need the fancier m, ok := <-msg construct.)
Note also that the wg for receive—which may turn out to be unnecessary, depending on how you structure other things—is quite independent from the wait-group pg for the producers. While it's true that, as written, the consumer cannot be done until all the producers are done, we'd like to wait independently for the producers to be done, so that we can close the channel in the overall-producer wrapper.

Answer 2

Try this code, I have made few fixes that made it work:
package main
import (
"fmt"
"strconv"
"strings"
"sync"
)
type message struct {
body string
code int
}
var markets []string = []string{"BTC", "ETH", "LTC"}
// produces messages into the chan
func produce(n int, market string, msg chan<- message, wg *sync.WaitGroup) {
// for i := 0; i < n; i++ {
var msgToSend = message{
body: strings.Join([]string{"market: ", market, ", #", strconv.Itoa(1)}, ""),
code: 1,
}
fmt.Println("Producing:", msgToSend)
msg <- msgToSend
// }
}
func receive(msg <-chan message, wg *sync.WaitGroup) {
for {
if m, ok := <-msg; ok {
fmt.Println("Received:", m)
wg.Done()
}
}
}
func consume(msg <-chan message) {
for {
if m, ok := <-msg; ok {
fmt.Println("More messages:", m)
} else {
break
}
}
}
func main() {
wg := sync.WaitGroup{}
msgC := make(chan message, 100)
defer func() {
close(msgC)
}()
for ix, market := range markets {
wg.Add(1)
go produce(ix+1, market, msgC, &wg)
}
go receive(msgC, &wg)
wg.Wait()
fmt.Println("Breaking from receiving")
}

Answer 3

Only when main returns, you can close(msgC), but meanwhile receive is waiting for close signal, that's why DeadLock occurs. After producing messages, close the channel.
package main
import (
"fmt"
"strconv"
"strings"
"sync"
)
type message struct {
body string
code int
}
var markets []string = []string{"BTC", "ETH", "LTC"}
// produces messages into the chan
func produce(n int, market string, msg chan<- message, wg *sync.WaitGroup) {
// for i := 0; i < n; i++ {
var msgToSend = message{
body: strings.Join([]string{"market: ", market, ", #", strconv.Itoa(1)}, ""),
code: 1,
}
fmt.Println("Producing:", msgToSend)
msg <- msgToSend
// }
wg.Done()
}
func receive(msg <-chan message, wg *sync.WaitGroup) {
for {
if m, ok := <-msg; ok {
fmt.Println("Received:", m)
} else {
fmt.Println("Breaking from receiving")
break
}
}
wg.Done()
}
func main() {
wg := sync.WaitGroup{}
msgC := make(chan message, 100)
// defer func() {
// close(msgC)
// }()
for ix, market := range markets {
wg.Add(1)
go produce(ix+1, market, msgC, &wg)
}
wg.Wait() // wait for producer
close(msgC)
wg.Add(1)
go receive(msgC, &wg)
wg.Wait()
}