Go times out on sleep but not on busy-wait - concurrency

Question

In Go, I can use time.After to time out a sleeping function, but I can't do the same to a function that is busy-waiting (or working). The following code returns timed out after one second, and then hangs.
package main
import (
"fmt"
"time"
)
func main() {
sleepChan := make(chan int)
go sleep(sleepChan)
select {
case sleepResult := <-sleepChan:
fmt.Println(sleepResult)
case <-time.After(time.Second):
fmt.Println("timed out")
}
busyChan := make(chan int)
go busyWait(busyChan)
select {
case busyResult := <-busyChan:
fmt.Println(busyResult)
case <-time.After(time.Second):
fmt.Println("timed out")
}
}
func sleep(c chan<- int) {
time.Sleep(10 * time.Second)
c <- 0
}
func busyWait(c chan<- int) {
for {
}
c <- 0
}
Why doesn't the timeout fire in the second case, and what alternative do I need to use to interrupt working goroutines?

Answer 1

The for {} statement is an infinite loop which monopolizes a single processor. Set runtime.GOMAXPROCS to 2 or more to allow the timer to run.
For example,
package main
import (
"fmt"
"runtime"
"time"
)
func main() {
fmt.Println(runtime.GOMAXPROCS(0))
runtime.GOMAXPROCS(runtime.NumCPU())
fmt.Println(runtime.GOMAXPROCS(0))
sleepChan := make(chan int)
go sleep(sleepChan)
select {
case sleepResult := <-sleepChan:
fmt.Println(sleepResult)
case <-time.After(time.Second):
fmt.Println("timed out")
}
busyChan := make(chan int)
go busyWait(busyChan)
select {
case busyResult := <-busyChan:
fmt.Println(busyResult)
case <-time.After(time.Second):
fmt.Println("timed out")
}
}
func sleep(c chan<- int) {
time.Sleep(10 * time.Second)
c <- 0
}
func busyWait(c chan<- int) {
for {
}
c <- 0
}
Output (4 CPU processor):
1
4
timed out
timed out