runtime: wake every waiter when stopping the signal watcher

The watcher is not the only thing that sleeps on signalFutex. sleepTicks waits
on it, and so does waitForEvents. Waking a single waiter could therefore wake a
sleeping goroutine instead of the watcher: that goroutine consumes the value
with its own Swap, and the watcher is left asleep on a futex that is 0 again,
never seeing the stop flag it was told to look at — which is the thread leak
this function exists to prevent.

WakeAll is what the signal handler already uses on this futex a few lines
below, for the same reason.

Author: 0pcom

src/runtime/runtime_unix.go

@@ -457,8 +457,15 @@ func stopSignalWatcher(s uint32) {
 	// Wake it so it can observe the flag. The value bump matters as much as the
 	// wake: Wait(0) returns immediately if the futex is already non-zero, which
 	// closes the window between the store above and a watcher about to sleep.
+	//
+	// WakeAll rather than Wake because the watcher is not the only thing that
+	// sleeps on this futex: sleepTicks waits on it too, and so does
+	// waitForEvents. Waking one waiter could wake a sleeping goroutine instead
+	// — which would consume the value with its own Swap and leave the watcher
+	// asleep on a futex that is 0 again, never seeing the flag it was told to
+	// look at. The signal handler wakes this futex the same way.
 	signalFutex.Store(1)
-	signalFutex.Wake()
+	signalFutex.WakeAll()
 }
 
 // signalWatcher runs on its own thread under the threads scheduler. It blocks on