package main

import (
  "context"
  "fmt"
  "reflect"
  "time"
  "unsafe"
)

func isChanClosed(ch interface{}) bool {
  if reflect.TypeOf(ch).Kind() != reflect.Chan {
    panic("only channels!")
  }

  // get interface value pointer, from cgo_export
  // typedef struct { void *t; void *v; } GoInterface;
  // then get channel real pointer
  cptr := *(*uintptr)(unsafe.Pointer(
    unsafe.Pointer(uintptr(unsafe.Pointer(&ch)) + unsafe.Sizeof(uint(0))),
  ))

  // this function will return true if chan.closed > 0
  // see hchan on https://github.com/golang/go/blob/master/src/runtime/chan.go
  // type hchan struct {
  // qcount   uint           // total data in the queue
  // dataqsiz uint           // size of the circular queue
  // buf      unsafe.Pointer // points to an array of dataqsiz elements
  // elemsize uint16
  // closed   uint32
  // **

  cptr += unsafe.Sizeof(uint(0)) * 2
  cptr += unsafe.Sizeof(unsafe.Pointer(uintptr(0)))
  cptr += unsafe.Sizeof(uint16(0))
  return *(*uint32)(unsafe.Pointer(cptr)) > 0
}

func main() {
  c := make(chan int, 10)
  c <- 1
  c <- 2
  c <- 3

  ctx, cancel := context.WithCancel(context.Background())
  close(c)
  cancel()
  fmt.Println("whether channel is closed:", isChanClosed(c), "\n")

exit:
  for {
    select {
    case i, ok := <-c:
      fmt.Println(ok)
      if !ok {
        fmt.Println("channel closed!")
        break
      }
      fmt.Println(i)

    case <-ctx.Done():
      break exit
    }
  }
  fmt.Println("aaaaaaaa")
  time.Sleep(time.Second * 2)
}