广播员：所有goroutine都在睡眠中-死锁

Question

The code below (http://play.golang.org/p/ikUtdoKOo5) is supposed to broadcast a message to a number of clients. But it does not work and I can't figure out why.

package main

import "fmt"

type Broadcaster struct {
    Clients []Client
}

func (b *Broadcaster) Broadcast(msg string) {
    for _, c := range b.Clients {
        go func() {
            c.Inbox() <- msg
        }()
    }
}

type Client interface {
    Inbox() chan string
}

type TestClient struct {
    Messages chan string
}

func (tc TestClient) Inbox() chan string {
    return tc.Messages
}

func main() {
    client1 := TestClient{Messages: make(chan string)}
    client2 := TestClient{Messages: make(chan string)}
    broadcaster := Broadcaster{Clients: []Client{client1, client2}}

    broadcaster.Broadcast("sos")

    fmt.Printf("client1: '%s'
", <-client1.Messages)
    fmt.Printf("client2: '%s'
", <-client2.Messages)
}

Error:

go run main.go
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
main.main()
        /Users/artem/projects/gocode/src/github.com/artemave/broadcaster/main.go:36 +0x1f3

goroutine 3 [chan send]:
main.func·001()
        /Users/artem/projects/gocode/src/github.com/artemave/broadcaster/main.go:12 +0x5f
created by main.(*Broadcaster).Broadcast
        /Users/artem/projects/gocode/src/github.com/artemave/broadcaster/main.go:13 +0xcd

goroutine 4 [chan send]:
main.func·001()
        /Users/artem/projects/gocode/src/github.com/artemave/broadcaster/main.go:12 +0x5f
created by main.(*Broadcaster).Broadcast
        /Users/artem/projects/gocode/src/github.com/artemave/broadcaster/main.go:13 +0xcd

UPDATE:

The go vet tool reveals the problem:

% go vet
main.go:12: range variable c enclosed by function

Answer 1

This is a subtle bug brought on by the reassignment of c in the for-range loop. It looks a little strange, but you see this pattern in the std library in a few places:

func (b *Broadcaster) Broadcast(msg string) {
    for _, c := range b.Clients {
        c := c  // redeclare c for the closure
        go func() {
            c.Inbox() <- msg
        }()
    }
}

http://golang.org/doc/faq#closures_and_goroutines