My Go 프로그램이 '모든 고루틴이 잠자기 상태입니다' 오류와 함께 교착 상태에 빠지는 이유는 무엇입니까?

package main

import (
    "fmt"
)

func total(ch chan int) {
    res := 0
    for iter := range ch {
        res += iter
    }
    ch <- res
}

func main() {
    ch := make(chan int)
    go total(ch)
    ch <- 1
    ch <- 2
    ch <- 3
    fmt.Println("Total is ", <-ch)
}

throw: all goroutines are asleep - deadlock!

package main

import (
    "fmt"
)

func total(in chan int, out chan int) {
    res := 0
    for iter := range in {
        res += iter
    }
    out <- res // Send result on separate channel
}

func main() {
    ch := make(chan int)
    rch := make(chan int) // New channel to receive result
    go total(ch, rch)
    ch <- 1
    ch <- 2
    ch <- 3
    close(ch) // Explicitly close channel to end loop in `total`
    result := <-rch // Wait for result on `rch`
    fmt.Println("Total is ", result)
}