all go routines are asleep deadlock - multithreading

Question

I'm in the process of learning how to do concurrency, and I've written this as its own app so that I can port it into a different project once it's working.
The project I'm adding it to will basically send in a RowInfo to a global QueueChannel, and then my workers should pick up this work and process it. If I queue two rows with the same ID, and one of them is currently processing by a worker, I'll remove the duplicate row from the queue (as you can see where I do my "continue" in the dispatcher).
This queueing/worker code will be running on a web server blocking on ListenAndServe, so I want it to always remain running and the workers to always remain actively looking for jobs. I don't want to have to close the channels (unless perhaps I ctrl+C'd the app or something). I suspect the error I'm getting has something to do with not closing channels because that's what a lot of the other threads mentioning this error seem to indicate, but I'm not sure how it relates to the code I have exactly.
Terminal error output:
[~/go/src/github.com/zzz/asynch]> go run main.go
fatal error: all goroutines are asleep - deadlock!
goroutine 1 [chan send]:
main.main()
/home/zzz/go/src/github.com/zzz/asynch/main.go:29 +0x14b
goroutine 5 [select]:
main.diszzzcher(0xc82001a120, 0xc82001a180, 0xc82001a1e0)
/home/zzz/go/src/github.com/zzz/asynch/main.go:42 +0x21a
created by main.main
/home/zzz/go/src/github.com/zzz/asynch/main.go:19 +0xb1
goroutine 6 [chan receive]:
main.worker(0xc82001a180, 0xc82001a1e0)
/home/zzz/go/src/github.com/zzz/asynch/main.go:55 +0x54
created by main.main
/home/zzz/go/src/github.com/zzz/asynch/main.go:24 +0xf7
goroutine 7 [chan receive]:
main.worker(0xc82001a180, 0xc82001a1e0)
/home/zzz/go/src/github.com/zzz/asynch/main.go:55 +0x54
created by main.main
/home/zzz/go/src/github.com/zzz/asynch/main.go:24 +0xf7
goroutine 8 [chan receive]:
main.worker(0xc82001a180, 0xc82001a1e0)
/home/zzz/go/src/github.com/zzz/asynch/main.go:55 +0x54
created by main.main
/home/zzz/go/src/github.com/zzz/asynch/main.go:24 +0xf7
goroutine 9 [chan receive]:
main.worker(0xc82001a180, 0xc82001a1e0)
/home/zzz/go/src/github.com/zzz/asynch/main.go:55 +0x54
created by main.main
/home/zzz/go/src/github.com/zzz/asynch/main.go:24 +0xf7
exit status 2
Code:
package main
import (
"log"
"time"
)
type RowInfo struct {
id int64
}
var QueueChan chan RowInfo
func main() {
QueueChan := make(chan RowInfo)
workerChan := make(chan RowInfo)
exitChan := make(chan int64)
go dispatcher(QueueChan, workerChan, exitChan)
// Start WorkerCount number of workers
workerCount := 4
for i := 0; i < workerCount; i++ {
go worker(workerChan, exitChan)
}
// Send test data
for i := 0; i < 12; i++ {
QueueChan <- RowInfo{id: int64(i)}
}
// Prevent app close
for {
time.Sleep(1 * time.Second)
}
}
func dispatcher(queueChan, workerChan chan RowInfo, exitChan chan int64) {
state := make(map[int64]bool)
for {
select {
case job := <-QueueChan:
if state[job.id] == true {
continue
}
workerChan <- job
case result := <-exitChan:
state[result] = false
}
}
}
func worker(workerChan chan RowInfo, exitChan chan int64) {
for job := range workerChan {
log.Printf("Doing work on job rowInfo ID: %d", job.id)
// Finish job
exitChan <- job.id
}
}
Thank you.

Answer 1

The error tells you: all goroutines are asleep, the program has deadlocked.
Now why are all your goroutines asleep? Let's check them one by one:
the worker goroutines: Wait indefinitely for new work on workerChan, will not exit until workerChan is closed, is asleep whenever it waits for new work
the dispatcher goroutine: Loops forever, selecting over two channels. Will never exit, is asleep while waiting in the select
the main goroutine: Loops forever on a time.Sleep, will never exit and be asleep most of the time
Typically, in a situation like this, you'd introduce a chan struct{} (call it closing or something like that) and inclue it in your selects. If you want to close the program, just close(closing). The select will choose the <-closing option, you return the goroutines.
You should also add a sync.WaitGroup to be notified when all your goroutines have exited.