Awesome, thanks Jan for the fast and clear response.
In fact, the for {} is an infinite loop so wg.Wait() will never be reached
and serve() will never terminate. Correct?
I guess I over-read how much this prototypical code was representative of a
real server loop. Sorry Dave!
Tom
On Thursday, August 24, 2017 at 7:41:59 PM UTC+2, Jan Mercl wrote:
>
> No, wg.Add cannot "switch" to wg.Wait, they're both in the samr goroutine,
> the go statement will be always the next one to execute after wg.Add within
> serve().
>
> On Thu, Aug 24, 2017, 19:29 Tom Payne <[email protected] <javascript:>>
> wrote:
>
>> I'm not singling out Dave Cheney here, I'd just like to check my
>> understanding of Go's resource handling and concurrency.
>>
>> In this blog post a "prototypical network server" is presented:
>>
>> https://dave.cheney.net/2017/08/20/context-isnt-for-cancellation
>>
>> Code:
>>
>> func serve(l net.Listener) error {
>> var wg sync.WaitGroup
>> var conn net.Conn
>> var err error
>> for {
>> conn, err = l.Accept()
>> if err != nil {
>> break
>> }
>> wg.Add(1)
>> go func(c net.Conn) {
>> defer wg.Done()
>> handle(c)
>> }(conn)
>> }
>> wg.Wait()
>> return err
>> }
>>
>> My understanding is that this contains a race condition. Specifically, a
>> goroutine switch can occur at the entry to wg.Add(1), switching to
>> wg.Wait() in the main goroutine. At this point, a connection has been
>> accepted, but the WaitGroup counter has not been incremented, so the serve
>> function will terminate while silently dropping a network connection (the
>> connection will be accepted, but never handled, so will probably eventually
>> time out on the client side and leak a file descriptor on the server side).
>>
>> Is this understanding correct? Furthermore, I suspect that it's
>> impossible to implement race-free graceful termination using only
>> sync.WaitGroups. Is it actually possible to do so?
>>
>> Thanks for any correction, and apologies to Dave for picking on his code.
>>
>> Cheers,
>> Tom
>>
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> --
>
> -j
>
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