runtime: avoid double notewakeup in netpoll stub code
Otherwise we can see
- goroutine 1 calls netpollBreak, the atomic.Cas succeeds, then suspends
- goroutine 2 calls noteclear, sets netpollBroken to 0
- goroutine 3 calls netpollBreak, the atomic.Cas succeeds, calls notewakeup
- goroutine 1 wakes up calls notewakeup, crashes due to double wakeup
This doesn't happen on Plan 9 because it only runs one thread at a time.
But Fuschia wants to use this code too.
Change-Id: Ib636e4f327bb15e44a2c40fd681aae9a91073a30
Reviewed-on: https://go-review.googlesource.com/c/go/+/218537
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com>