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>

src/runtime/netpoll_stub.go

@@ -13,16 +13,23 @@ var netpollWaiters uint32
 
 var netpollStubLock mutex
 var netpollNote note
-var netpollBroken uint32
+
+// netpollBroken, protected by netpollBrokenLock, avoids a double notewakeup.
+var netpollBrokenLock mutex
+var netpollBroken bool
 
 func netpollGenericInit() {
 	atomic.Store(&netpollInited, 1)
 }
 
 func netpollBreak() {
-	if atomic.Cas(&netpollBroken, 0, 1) {
+	lock(&netpollBrokenLock)
+	broken := netpollBroken
+	netpollBroken = true
+	if !broken {
 		notewakeup(&netpollNote)
 	}
+	unlock(&netpollBrokenLock)
 }
 
 // Polls for ready network connections.
@@ -34,8 +41,12 @@ func netpoll(delay int64) gList {
 		// This lock ensures that only one goroutine tries to use
 		// the note. It should normally be completely uncontended.
 		lock(&netpollStubLock)
+
+		lock(&netpollBrokenLock)
 		noteclear(&netpollNote)
-		atomic.Store(&netpollBroken, 0)
+		netpollBroken = false
+		unlock(&netpollBrokenLock)
+
 		notetsleep(&netpollNote, delay)
 		unlock(&netpollStubLock)
 	}