为什么该Go程序挂起？

Question

I'm setting up a chain of three goroutines, each with an input an output channel. The goroutines will read from the input channel until it's closed, increment the value, send it to the output channel. However, the program below deadlocks with this output:

goroutine 'one': 1
goroutine 'two': 2
goroutine 'three': 3
goroutine 'one': 10
goroutine 'two': 11
goroutine 'one': 100
fatal error: all goroutines are asleep - deadlock!

Code:

package main

import (
  "fmt"
)

func int_channel(id string, i chan int, o chan int) {
  defer close(o)

  for x := range i {
    fmt.Printf("goroutine '%s': %d
", id, x)
    o <- x + 1
  }

  fmt.Println("done")
}

func main() {
  c0 := make(chan int)
  c1 := make(chan int)
  c2 := make(chan int)
  c3 := make(chan int)

  go int_channel("one", c0, c1)
  go int_channel("two", c1, c2)
  go int_channel("three", c2, c3)

  c0 <- 1
  c0 <- 10
  c0 <- 100
  c0 <- 1000
  c0 <- 10000
  c0 <- 100000
  close(c0)

  fmt.Println("Sent all numbers to c0")

  for x := range c3 {
    fmt.Printf("out: %d
", x)
  }
}

Answer 1

It hangs because the loop which reads from the output channel is never reached, thus the channels are not "emptyed" and once each channe has a value written into it no progress can be made and program hangs. To fix it write to the input in another goroutine, ie

func main() {
  c0 := make(chan int)
  c1 := make(chan int)
  c2 := make(chan int)
  c3 := make(chan int)

  go int_channel("one", c0, c1)
  go int_channel("two", c1, c2)
  go int_channel("three", c2, c3)

  go func(){
    c0 <- 1
    c0 <- 10
    c0 <- 100
    c0 <- 1000
    c0 <- 10000
    c0 <- 100000
    fmt.Println("Sent all numbers to c0")
    close(c0)
  }()


  for x := range c3 {
    fmt.Printf("out: %d
", x)
  }
}

IOW, when the line c0 <- 1 is executed, the value flows throught all three cannels and ends up in c3, but since the reader loop is not reached yet, it just "sits in there". Then the line c0 <- 10 is executed, and this value ends up in c2 because it can't be written into c3 - the previous value is still in there, blocking the write. And thus when the line c0 <- 100 is executed, all channels are full and no further progress can be made.