Goroutines的顺序运行

Question

I am learning Golang right now, and came across some interesting tutorial online. for example this one: https://golangbot.com/channels/

in this part about goroutines, there is one example case as following:

package main

import (
    "fmt"
)

func producer(chnl chan int) {
    for i := 0; i < 10; i++ {
        fmt.Println("debugging send...", i)
        chnl <- i
    }
    close(chnl)
}
func main() {
    ch := make(chan int)
    go producer(ch)
    for {
        v, ok := <-ch
        if ok == false {
            break
        }
        fmt.Println("Received ", v, ok)
    }
}

the line fmt.Println("debugging send...", i) is added by me for debugging purpose. and the output is:

debugging send... 0
debugging send... 1
Received  0 true
Received  1 true
debugging send... 2
debugging send... 3
Received  2 true
Received  3 true
debugging send... 4
debugging send... 5
Received  4 true
Received  5 true
debugging send... 6
debugging send... 7
Received  6 true
Received  7 true
debugging send... 8
debugging send... 9
Received  8 true
Received  9 true

The output order seems interesting for me, but can't fully understand what's going on under the hood.

Answer 1

The only places where there's synchronization is the channel operations. There need not be correlation between the order of operations in the goroutines outside of those statements.

Running your program multiple times, I got the output you have most of the time but sometimes I saw something like this as well:

debugging send... 0
debugging send... 1
Received  0 true
Received  1 true
debugging send... 2
debugging send... 3
Received  2 true
Received  3 true
debugging send... 4
debugging send... 5
Received  4 true
Received  5 true
debugging send... 6
debugging send... 7
Received  6 true
debugging send... 8
Received  7 true
Received  8 true
debugging send... 9
Received  9 true

Try running this Bash shell script to run the program multiple times and compare its output:

#!/bin/bash

# c.go has your Go program
go run c.go > first.txt
cat first.txt

echo "======"
while :; do
    go run c.go > run.txt
    if ! diff -q run.txt first.txt; then
        break
    fi
done

cat run.txt

Edit: You may find https://golang.org/ref/mem interesting to read about synchronization in Go.