Golang etcd观察者恐慌

Question

All!

I've the code below:

package main

import (
        "log"

        "github.com/coreos/go-etcd/etcd"
)

func main() {
        client := etcd.NewClient(
                []string{
                        "http://172.20.20.10:2379",
                        "http://172.20.20.11:2379",
                        "http://172.20.20.12:2379",
                },
        )
        for {
                watchChan := make(chan *etcd.Response)
                go client.Watch("/config", 0, false, watchChan, nil)

                log.Println("Waiting for an update...")
                r := <-watchChan

                log.Printf(">>> got an updated config: %s: %s
", r.Node.Key, r.Node.Value)
        }
}

BUT... When some node (for example 172.20.20.11) is going down the panic is complain regarding the invalid memory address or nil pointer dereference...

> $ ./etcd-watcher
2015/11/09 18:46:19 Waiting for an update...
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x10 pc=0x22fe]

goroutine 1 [running]:
main.main()
    /Users/Stalker/Workspace/src/snippets/etcd-watcher.go:26 +0x2be

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
     /Users/Stalker/App/Go/1.5.1/src/runtime/asm_amd64.s:1696 +0x1

goroutine 19 [runnable]:
net/http.(*persistConn).writeLoop(0xc8200c6dc0)
    /Users/Stalker/App/Go/1.5.1/src/net/http/transport.go:1009 +0x40c
created by net/http.(*Transport).dialConn
    /Users/Stalker/App/Go/1.5.1/src/net/http/transport.go:686 +0xc9d

Could someone explain me what is going on and how to make this simple example to work properly?? Thanks a lot for advice! Alex

Answer 1

The library can close the watchChanchannel. That will return a nil value in your r and then panic when you try to log r.Node. When you get a pointer back, you should check it's not nil. I would also recommend checking if the receiver channel has been closed and act accordingly.

r, open := <-watchChan
if !open {
    // channel is closed
}
if r == nil {
   // the watch channel return a nil value
}