golang缓冲通道意外结果

Question

package main
import (
    "time"
    "runtime"
)

var c = make(chan int, 2)

func main() {
  go worker(1)
  for i := 0; i < 30; i++ {
    go func() {
            // k := i   make a local copy not make any difference
            c <- i
    }()
  }
  time.Sleep(100* time.Second)
}

func worker(id int) {
  for {
    a := <-c
    println(a, runtime.NumGoroutine())
    time.Sleep(time.Second)
  }
}

the output is unpredictable, sometimes like below.

7 9
13 29
13 28
13 27
13 26
13 25
13 24
13 23
16 22
16 21
17 20
19 19
21 18
21 17
23 16
25 15
26 14
26 13
26 12
26 11
26 10
26 9
26 8
26 7
27 6
27 5
13 4
28 3
30 2
30 2

I know sender will block if the buffer channel is full, and when channel is available the sender can continue.

1. why output is not constant output 0-29? how to make it??
2. how variable/local variable store in goroutine??
3. if lots of senders are blocked, are they waked up by FIFO order??

Answer 1

Output is not constant because different goroutines share same local variable i. If you uncomment your line and move it right before the goruoutine call, you'll see constant output 0-29. The better way is to move i variable to goroutine function arguments.

Wake up order is not specified in specs. You should consider it as a random one.