dgroup's shutdown logging is async

[LukeShu]

dgroup's shutdown logging works by watching the Context and logging when it observes that it becomes Done. Because this is async, the ordering of the shutdown log line might actually come after things caused by the shutdown. This is confusing and makes the logs hard to read.

This should be solvable by wrapping the Context in our own Context implementation that logs before propagating the cancelation to child Contexts.

[LukeShu]

This should be solvable by wrapping the Context in our own Context implementation that logs before propagating the cancelation to child Contexts.

That's no good, because it might introduce a race where

ctx, cancel := context.WithCancel(ctx)
ctx = loggingCtx{ctx}
cancel()
if ctx.Err() == nil {
    panic("this should not happen")
}

To do it right, I'd have to make cancel() block until the log line is printed. And I have no way to adjust cancel().

[LukeShu]

I suppose the solution could be to have ctx.Err() block.