Thanks, everyone, for your answers! I think I understand what
sync.WaitGroup is doing now.
Jon
On Friday, April 27, 2018 at 7:29:50 AM UTC-4, rog wrote:
>
> On 26 April 2018 at 16:29, Jon Bodner <j...@bodnerfamily.com <javascript:>>
> wrote:
> > Hello,
> >
> > We were having a discussion at work about passing around references to
> > sync.WaitGroup. I said it was a bad idea, because if a goroutine
> launched a
> > goroutine of its own and called sync.WaitGroup.Add() after a goroutine
> > called sync.WaitGroup.Wait(), but before the count dropped to 0, your
> code
> > would panic. The comments seem to support this:
> >
> > "If a WaitGroup is reused to wait for several independent sets of
> events,
> > new Add calls must happen after all previous Wait calls have returned."
> >
> > And the WaitGroup source code for Add has lines like this:
> >
> > if w != 0 && delta > 0 && v == int32(delta) {
> > panic("sync: WaitGroup misuse: Add called concurrently with Wait")
> > }
> >
> > But a co-worker wrote this code, and it runs without a panic:
> > https://play.golang.org/p/NjqK_YoqHeH
>
> FWIW This code is fine, and was part of the original design goal of
> WaitGroup. When the count is known to be non-zero (for example because
> the current goroutine has previously done an Add) it's OK to call Add
> to add more.
>
> The key in the above quote is "If a WaitGroup is reused to wait for
> several independent sets of events". If you're reusing a WaitGroup,
> this makes sense, but in the example above, the WaitGroup is not
> reused so all is good.
>
> As for the panic, others have already explained how that might happen.
>
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