panic: send on closed channel in mirror()

[mikluko]

Description

The mirror() function in nats.go can panic with "send on closed channel" when the consumer closes mch while messages are still being received on nch.

Stack trace

panic: send on closed channel

goroutine 5677 [running]:
github.com/mikluko/peanats.(*connectionImpl).mirror.func1()
    github.com/mikluko/peanats@v0.20.2/nats.go:199 +0x4c
created by github.com/mikluko/peanats.(*connectionImpl).mirror in goroutine 5831
    github.com/mikluko/peanats@v0.20.2/nats.go:197 +0x80

Root cause

https://github.com/mikluko/peanats/blob/main/nats.go#L196-L203

func (c *connectionImpl) mirror(mch chan Msg) chan *nats.Msg {
    nch := make(chan *nats.Msg, cap(mch))
    go func() {
        for msg := range nch {
            mch <- NewMsg(msg)  // panics if mch is closed
        }
    }()
    return nch
}

The goroutine doesn't handle the case where mch is closed by the consumer.

Suggested fix

Use a done channel or recover:

func (c *connectionImpl) mirror(mch chan Msg) chan *nats.Msg {
    nch := make(chan *nats.Msg, cap(mch))
    go func() {
        defer func() {
            recover() // ignore send on closed channel
        }()
        for msg := range nch {
            mch <- NewMsg(msg)
        }
    }()
    return nch
}

Or better, use select with a done channel to allow graceful shutdown.

Version

v0.20.2